Aleksandra V Zabuga; Marcelle I Arrigo; Jérémie Teyssier; Sébastien R Mouchet; Kanto Nishikawa; Masafumi Matsui; Miguel Vences; Michel C Milinkovitch Translucent in air and iridescent in water: structural analysis of a salamander egg sac Journal Article Soft Matter, pp. -, 2020. @article{C9SM02151E, title = {Translucent in air and iridescent in water: structural analysis of a salamander egg sac}, author = {Aleksandra V Zabuga and Marcelle I Arrigo and Jérémie Teyssier and Sébastien R Mouchet and Kanto Nishikawa and Masafumi Matsui and Miguel Vences and Michel C Milinkovitch}, url = {http://dx.doi.org/10.1039/C9SM02151E}, doi = {10.1039/C9SM02151E}, year = {2020}, date = {2020-01-01}, journal = {Soft Matter}, pages = {-}, publisher = {The Royal Society of Chemistry}, abstract = {Females of some Asian salamanders of the genus Hynobius deposit in streams their eggs embedded in a translucent envelope called an ‘egg sac’. The edges of the envelope exhibit a spectacular blue-to-yellow iridescent glow, which instantaneously disappears when the sac is removed from water. First, our scanning electron microscopy analyses reveal that the inner surface of the 100 μm-thick envelope displays striations (length scale of about 3 μm), which are themselves covered by much smaller (190 ± 30 nm) and quasi-periodic corrugations. The latter could constitute a surface diffraction grating generating iridescence by light interference. Second, our transmission electron microscopy and focused-ion-beam scanning electron microscopy analyses show that the bulk of the egg sac wall is composed of meandering fibres with a quasi-periodic modulation of 190 ± 60 nm along the thickness of the envelope, generating a photonic crystal. Third, Fourier power analyses of 450 electron microscopy images with varying incident angles indicate that changing the surrounding medium from water to air shifts most of the backscattered power spectrum to the ultraviolet range, hence, explaining that the egg sac loses visible iridescence when removed out of the water. Fourth, the results of our photography and optical spectroscopy experiments of submerged and emerged egg sacs rule out the possibility that the iridescence is due to a thin film or a multilayer, whereas the observed non-specular response is compatible with the backscattering expected from surface diffraction gratings and volumetric photonic crystals with spatial 1D modulation. Finally, although we mention several potential biological functions of the egg sac structural colours and iridescence, we emphasise that these optical properties might be the by-products of the envelope material internal structure selected during evolution for its mechanical properties.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Females of some Asian salamanders of the genus Hynobius deposit in streams their eggs embedded in a translucent envelope called an ‘egg sac’. The edges of the envelope exhibit a spectacular blue-to-yellow iridescent glow, which instantaneously disappears when the sac is removed from water. First, our scanning electron microscopy analyses reveal that the inner surface of the 100 μm-thick envelope displays striations (length scale of about 3 μm), which are themselves covered by much smaller (190 ± 30 nm) and quasi-periodic corrugations. The latter could constitute a surface diffraction grating generating iridescence by light interference. Second, our transmission electron microscopy and focused-ion-beam scanning electron microscopy analyses show that the bulk of the egg sac wall is composed of meandering fibres with a quasi-periodic modulation of 190 ± 60 nm along the thickness of the envelope, generating a photonic crystal. Third, Fourier power analyses of 450 electron microscopy images with varying incident angles indicate that changing the surrounding medium from water to air shifts most of the backscattered power spectrum to the ultraviolet range, hence, explaining that the egg sac loses visible iridescence when removed out of the water. Fourth, the results of our photography and optical spectroscopy experiments of submerged and emerged egg sacs rule out the possibility that the iridescence is due to a thin film or a multilayer, whereas the observed non-specular response is compatible with the backscattering expected from surface diffraction gratings and volumetric photonic crystals with spatial 1D modulation. Finally, although we mention several potential biological functions of the egg sac structural colours and iridescence, we emphasise that these optical properties might be the by-products of the envelope material internal structure selected during evolution for its mechanical properties. | |
Á. Pásztor; A. Scarfato; C. Barreteau; E. Giannini; C. Renner Dimensional cross-over of the charge density wave order parameter in thin exfoliated 1T-VSe2 Journal Article 2D Materials, 4 (4), pp. 041005, 2019. @article{Pásztor2019, title = {Dimensional cross-over of the charge density wave order parameter in thin exfoliated 1T-VSe2}, author = {Á. Pásztor and A. Scarfato and C. Barreteau and E. Giannini and C. Renner}, url = {https://doi.org/10.1088%2F2053-1583%2Faa86de}, doi = {10.1088/2053-1583/aa86de}, year = {2019}, date = {2019-09-04}, journal = {2D Materials}, volume = {4}, number = {4}, pages = {041005}, abstract = {Isolating single unit-cell thin layers from the bulk matrix of layered compounds offers tremendous opportunities to design novel functional electronic materials. However, a comprehensive thickness dependence study is paramount to harness the electronic properties of such atomic foils and their stacking into synthetic heterostructures. Here we show that a dimensional crossover and quantum confinement with reducing thickness result in a striking non-monotonic evolution of the charge density wave transition temperature in VSe2. Our conclusion is drawn from a direct derivation of the local order parameter and transition temperature from the real space charge modulation amplitude imaged by scanning tunnelling microscopy. This study lifts the disagreement of previous independent transport measurements. We find that thickness can be a non-trivial tuning parameter and demonstrate the importance of considering a finite thickness range to accurately characterize its influence.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Isolating single unit-cell thin layers from the bulk matrix of layered compounds offers tremendous opportunities to design novel functional electronic materials. However, a comprehensive thickness dependence study is paramount to harness the electronic properties of such atomic foils and their stacking into synthetic heterostructures. Here we show that a dimensional crossover and quantum confinement with reducing thickness result in a striking non-monotonic evolution of the charge density wave transition temperature in VSe2. Our conclusion is drawn from a direct derivation of the local order parameter and transition temperature from the real space charge modulation amplitude imaged by scanning tunnelling microscopy. This study lifts the disagreement of previous independent transport measurements. We find that thickness can be a non-trivial tuning parameter and demonstrate the importance of considering a finite thickness range to accurately characterize its influence. | |
T. Jaouen; B. Hildebrand; M. L. Mottas; I. M. Di Giovannantonio; P. Ruffieux; M. Rumo; C. W. Nicholson; E. Razzoli; C. Barreteau; A. Ubaldini; E. Giannini; F. Vanini; H. Beck; C. Monney; P. Aebi Phase separation in the vicinity of Fermi surface hot spots Journal Article Physical Review B, 100 (7), pp. 075152, 2019. @article{Jaouen2019, title = {Phase separation in the vicinity of Fermi surface hot spots}, author = {T. Jaouen and B. Hildebrand and M. L. Mottas and I. M. Di Giovannantonio and P. Ruffieux and M. Rumo and C. W. Nicholson and E. Razzoli and C. Barreteau and A. Ubaldini and E. Giannini and F. Vanini and H. Beck and C. Monney and P. Aebi}, url = {https://link.aps.org/doi/10.1103/PhysRevB.100.075152}, doi = {10.1103/PhysRevB.100.075152}, year = {2019}, date = {2019-08-11}, journal = {Physical Review B}, volume = {100}, number = {7}, pages = {075152}, abstract = {Spatially inhomogeneous electronic states are expected to be key ingredients for the emergence of superconducting phases in quantum materials hosting charge-density waves (CDWs). Prototypical materials are transition-metal dichalcogenides (TMDCs) and among them, 1T−TiSe2 exhibiting intertwined CDW and superconducting states under Cu intercalation, pressure, or electrical gating. Although it has been recently proposed that the emergence of superconductivity relates to CDW fluctuations and the development of spatial inhomogeneities in the CDW order, the fundamental mechanism underlying such a phase separation (PS) is still missing. Using angle-resolved photoemission spectroscopy and variable-temperature scanning tunneling microscopy, we report on the phase diagram of the CDW in 1T−TiSe2 as a function of Ti self-doping, an overlooked degree of freedom inducing CDW texturing. We find an intrinsic tendency towards electronic PS in the vicinity of Fermi surface (FS) “hot spots,” i.e., locations with band crossings close to, but not at the Fermi level. We therefore demonstrate an intimate relationship between the FS topology and the emergence of spatially textured electronic phases which is expected to be generalizable to many doped CDW compounds.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Spatially inhomogeneous electronic states are expected to be key ingredients for the emergence of superconducting phases in quantum materials hosting charge-density waves (CDWs). Prototypical materials are transition-metal dichalcogenides (TMDCs) and among them, 1T−TiSe2 exhibiting intertwined CDW and superconducting states under Cu intercalation, pressure, or electrical gating. Although it has been recently proposed that the emergence of superconductivity relates to CDW fluctuations and the development of spatial inhomogeneities in the CDW order, the fundamental mechanism underlying such a phase separation (PS) is still missing. Using angle-resolved photoemission spectroscopy and variable-temperature scanning tunneling microscopy, we report on the phase diagram of the CDW in 1T−TiSe2 as a function of Ti self-doping, an overlooked degree of freedom inducing CDW texturing. We find an intrinsic tendency towards electronic PS in the vicinity of Fermi surface (FS) “hot spots,” i.e., locations with band crossings close to, but not at the Fermi level. We therefore demonstrate an intimate relationship between the FS topology and the emergence of spatially textured electronic phases which is expected to be generalizable to many doped CDW compounds. | |
D. van der Marel; F. Barantani; C. W. Rischau Possible mechanism for superconductivity in doped SrTiO3 Journal Article Physical Review Research, 1 (1), pp. 013003, 2019. @article{vanderMarel2019b, title = {Possible mechanism for superconductivity in doped SrTiO3}, author = {D. van der Marel and F. Barantani and C. W. Rischau}, url = {https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.1.013003}, doi = {https://doi.org/10.1103/PhysRevResearch.1.013003}, year = {2019}, date = {2019-08-09}, journal = {Physical Review Research}, volume = {1}, number = {1}, pages = {013003}, abstract = {The soft ferroelectric phonon in SrTiO3 observed with optical spectroscopy has an extraordinarily strong spectral weight which is much stronger than expected in the limit of a perfectly ionic compound. This “charged phonon” effect in SrTiO3 is caused by the close-to-covalent character of the Ti-O ionic bond and implies a strong coupling between the soft ferroelectric phonon and the interband transitions across the 3-eV gap of SrTiO3. We demonstrate that this coupling leads, in addition to the charged phonon effect, to a pairing interaction involving the exchange of two transverse optical phonons. This process owes its relevance to the strong electron-phonon coupling and to the fact that the interaction mediated by a single transverse optical phonon vanishes at low electron density. We use the experimental soft phonon spectral weight to calculate the strength of the biphonon mediated pairing interaction in the electron-doped material and show that it is of the correct magnitude when compared to the experimental value of the superconducting critical temperature. Biphonon exchange is therefore an important pairing mechanism at low doping, and may be the key to understanding the occurrence of superconductivity in doped SrTiO3 and other low electron density materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The soft ferroelectric phonon in SrTiO3 observed with optical spectroscopy has an extraordinarily strong spectral weight which is much stronger than expected in the limit of a perfectly ionic compound. This “charged phonon” effect in SrTiO3 is caused by the close-to-covalent character of the Ti-O ionic bond and implies a strong coupling between the soft ferroelectric phonon and the interband transitions across the 3-eV gap of SrTiO3. We demonstrate that this coupling leads, in addition to the charged phonon effect, to a pairing interaction involving the exchange of two transverse optical phonons. This process owes its relevance to the strong electron-phonon coupling and to the fact that the interaction mediated by a single transverse optical phonon vanishes at low electron density. We use the experimental soft phonon spectral weight to calculate the strength of the biphonon mediated pairing interaction in the electron-doped material and show that it is of the correct magnitude when compared to the experimental value of the superconducting critical temperature. Biphonon exchange is therefore an important pairing mechanism at low doping, and may be the key to understanding the occurrence of superconductivity in doped SrTiO3 and other low electron density materials. | |
W Luo; M. Boselli; J M Poumirol; I Ardizzone; J Teyssier; D van der Marel; S Gariglio; J M Triscone; A B Kuzmenko High sensitivity variable-temperature infrared nanoscopy of conducting oxide interfaces Journal Article Nature Communications, 10 (1), pp. 2774, 2019. @article{Luo2019, title = {High sensitivity variable-temperature infrared nanoscopy of conducting oxide interfaces}, author = {W Luo and M. Boselli and J M Poumirol and I Ardizzone and J Teyssier and D van der Marel and S Gariglio and J M Triscone and A B Kuzmenko}, url = {https://doi.org/10.1038/s41467-019-10672-5}, doi = {10.1038/s41467-019-10672-5}, year = {2019}, date = {2019-06-24}, journal = {Nature Communications}, volume = {10}, number = {1}, pages = {2774}, abstract = {Probing the local transport properties of two-dimensional electron systems (2DES) confined at buried interfaces requires a non-invasive technique with a high spatial resolution operating in a broad temperature range. In this paper, we investigate the scattering-type scanning near field optical microscopy as a tool for studying the conducting LaAlO3/SrTiO3 interface from room temperature down to 6 K. We show that the near-field optical signal, in particular its phase component, is highly sensitive to the transport properties of the electron system present at the interface. Our modeling reveals that such sensitivity originates from the interaction of the AFM tip with coupled plasmon–phonon modes with a small penetration depth. The model allows us to quantitatively correlate changes in the optical signal with the variation of the 2DES transport properties induced by cooling and by electrostatic gating. To probe the spatial resolution of the technique, we image conducting nano-channels written in insulating heterostructures with a voltage-biased tip of an atomic force microscope.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Probing the local transport properties of two-dimensional electron systems (2DES) confined at buried interfaces requires a non-invasive technique with a high spatial resolution operating in a broad temperature range. In this paper, we investigate the scattering-type scanning near field optical microscopy as a tool for studying the conducting LaAlO3/SrTiO3 interface from room temperature down to 6 K. We show that the near-field optical signal, in particular its phase component, is highly sensitive to the transport properties of the electron system present at the interface. Our modeling reveals that such sensitivity originates from the interaction of the AFM tip with coupled plasmon–phonon modes with a small penetration depth. The model allows us to quantitatively correlate changes in the optical signal with the variation of the 2DES transport properties induced by cooling and by electrostatic gating. To probe the spatial resolution of the technique, we image conducting nano-channels written in insulating heterostructures with a voltage-biased tip of an atomic force microscope. | |
L. Thiel; Z. Wang; M. A. Tschudin; D. Rohner; I. Gutierrez-Lezama; N. Ubrig; M. Gibertini; E. Giannini; A. F. Morpurgo; P. Maletinsky Probing magnetism in 2D materials at the nanoscale with single-spin microscopy Journal Article Science, 364 (6444), pp. 973-976, 2019. @article{Thiel2019, title = {Probing magnetism in 2D materials at the nanoscale with single-spin microscopy}, author = {L. Thiel and Z. Wang and M. A. Tschudin and D. Rohner and I. Gutierrez-Lezama and N. Ubrig and M. Gibertini and E. Giannini and A. F. Morpurgo and P. Maletinsky}, url = {https://science.sciencemag.org/content/364/6444/973}, doi = {10.1126/science.aav6926}, year = {2019}, date = {2019-06-07}, journal = {Science}, volume = {364}, number = {6444}, pages = {973-976}, abstract = {The discovery of ferromagnetism in two-dimensional (2D) van der Waals (vdW) crystals has generated widespread interest. Making further progress in this area requires quantitative knowledge of the magnetic properties of vdW magnets at the nanoscale. We used scanning single-spin magnetometry based on diamond nitrogen-vacancy centers to image the magnetization, localized defects, and magnetic domains of atomically thin crystals of the vdW magnet chromium(III) iodide (CrI3). We determined the magnetization of CrI3 monolayers to be ≈16 Bohr magnetons per square nanometer, with comparable values in samples with odd numbers of layers; however, the magnetization vanishes when the number of layers is even. We also found that structural modifications can induce switching between ferromagnetic and antiferromagnetic interlayer ordering. These results demonstrate the benefit of using single-spin scanning magnetometry to study the magnetism of 2D vdW magnets.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The discovery of ferromagnetism in two-dimensional (2D) van der Waals (vdW) crystals has generated widespread interest. Making further progress in this area requires quantitative knowledge of the magnetic properties of vdW magnets at the nanoscale. We used scanning single-spin magnetometry based on diamond nitrogen-vacancy centers to image the magnetization, localized defects, and magnetic domains of atomically thin crystals of the vdW magnet chromium(III) iodide (CrI3). We determined the magnetization of CrI3 monolayers to be ≈16 Bohr magnetons per square nanometer, with comparable values in samples with odd numbers of layers; however, the magnetization vanishes when the number of layers is even. We also found that structural modifications can induce switching between ferromagnetic and antiferromagnetic interlayer ordering. These results demonstrate the benefit of using single-spin scanning magnetometry to study the magnetism of 2D vdW magnets. | |
M. Spera; A. Scarfato; E. Giannini; C. Renner Energy-dependent spatial texturing of charge order in 1T-CuxTiSe2 Journal Article Physical Review B, 99 (15), pp. 155133, 2019. @article{Spera2019, title = {Energy-dependent spatial texturing of charge order in 1T-CuxTiSe2}, author = {M. Spera and A. Scarfato and E. Giannini and C. Renner}, year = {2019}, date = {2019-04-18}, journal = {Physical Review B}, volume = {99}, number = {15}, pages = {155133}, abstract = {We report a detailed study of the microscopic effects of Cu intercalation on the charge density wave (CDW) in 1T−CuxTiSe2. Scanning tunneling microscopy and spectroscopy reveal a unique, Cu-driven spatial texturing of the charge-ordered phase, with the appearance of energy-dependent CDW patches and sharp π-phase shift domain walls (πDWs). The energy and doping dependencies of the patchwork are directly linked to the inhomogeneous potential landscape due to the Cu intercalants. They imply a CDW gap with unusual features, including a large amplitude, the opening below the Fermi level, and a shift to higher binding energy with electron doping. Unlike the patchwork, the πDWs occur independently of the intercalated Cu distribution. They remain atomically sharp throughout the investigated phase diagram and occur in both superconducting and nonsuperconducting specimens. These results provide unique atomic-scale insight into the CDW ground state, questioning the existence of incommensurate CDW domain walls and contributing to understanding its formation mechanism and interplay with superconductivity.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We report a detailed study of the microscopic effects of Cu intercalation on the charge density wave (CDW) in 1T−CuxTiSe2. Scanning tunneling microscopy and spectroscopy reveal a unique, Cu-driven spatial texturing of the charge-ordered phase, with the appearance of energy-dependent CDW patches and sharp π-phase shift domain walls (πDWs). The energy and doping dependencies of the patchwork are directly linked to the inhomogeneous potential landscape due to the Cu intercalants. They imply a CDW gap with unusual features, including a large amplitude, the opening below the Fermi level, and a shift to higher binding energy with electron doping. Unlike the patchwork, the πDWs occur independently of the intercalated Cu distribution. They remain atomically sharp throughout the investigated phase diagram and occur in both superconducting and nonsuperconducting specimens. These results provide unique atomic-scale insight into the CDW ground state, questioning the existence of incommensurate CDW domain walls and contributing to understanding its formation mechanism and interplay with superconductivity. | |
M.-L. Mottas; T. Jaouen; B. Hildebrand; M. Rumo; F. Vanini; E. Razzoli; E. Giannini; C. Barreteau; D. R. Bowler; C. Monney; H. Beck; and P. Aebi Semimetal-to-semiconductor transition and charge-density-wave suppression in 1T−TiSe2−xSx single crystals Journal Article Physical Review B, 99 (15), pp. 155103, 2019. @article{Mottas2019, title = {Semimetal-to-semiconductor transition and charge-density-wave suppression in 1T−TiSe2−xSx single crystals}, author = {M.-L. Mottas and T. Jaouen and B. Hildebrand and M. Rumo and F. Vanini and E. Razzoli and E. Giannini and C. Barreteau and D. R. Bowler and C. Monney and H. Beck and and P. Aebi}, url = {https://link.aps.org/doi/10.1103/PhysRevB.99.155103}, doi = {10.1103/PhysRevB.99.155103}, year = {2019}, date = {2019-04-01}, journal = {Physical Review B}, volume = {99}, number = {15}, pages = {155103}, abstract = {The transition-metal dichalcogenide 1T−TiSe2 is a quasi-two-dimensional layered material with a phase transition towards a commensurate charge-density wave (CDW) at a critical temperature Tc≈200K. The relationship between the origin of the CDW instability and the semimetallic or semiconducting character of the normal state, i.e., with the nonreconstructed Fermi-surface topology, remains elusive. By combining angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations, we investigate 1T−TiSe2−xSx single crystals. Using STM, we first show that the long-range phase-coherent CDW state is stable against S substitutions with concentrations, at least, up to x=0.34. The ARPES measurements then reveal a slow but continuous decrease in the overlap between the electron and the hole (e−h) bands of the semimetallic normal state well reproduced by DFT and related to slight reductions of both the CDW order parameter and Tc. Our DFT calculations further predict a semimetal-to-semiconductor transition of the normal state at a higher critical S concentration of xc=0.9±0.1 that coincides with a suppressed CDW state in TiSeS as measured with STM. Finally, we rationalize the x dependence of the e−h band overlap in terms of isovalent substitution-induced competing chemical pressure and charge localization effects. Our study highlights the key role of the e−h band overlap for the CDW instability.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The transition-metal dichalcogenide 1T−TiSe2 is a quasi-two-dimensional layered material with a phase transition towards a commensurate charge-density wave (CDW) at a critical temperature Tc≈200K. The relationship between the origin of the CDW instability and the semimetallic or semiconducting character of the normal state, i.e., with the nonreconstructed Fermi-surface topology, remains elusive. By combining angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations, we investigate 1T−TiSe2−xSx single crystals. Using STM, we first show that the long-range phase-coherent CDW state is stable against S substitutions with concentrations, at least, up to x=0.34. The ARPES measurements then reveal a slow but continuous decrease in the overlap between the electron and the hole (e−h) bands of the semimetallic normal state well reproduced by DFT and related to slight reductions of both the CDW order parameter and Tc. Our DFT calculations further predict a semimetal-to-semiconductor transition of the normal state at a higher critical S concentration of xc=0.9±0.1 that coincides with a suppressed CDW state in TiSeS as measured with STM. Finally, we rationalize the x dependence of the e−h band overlap in terms of isovalent substitution-induced competing chemical pressure and charge localization effects. Our study highlights the key role of the e−h band overlap for the CDW instability. | |
O. Shcherban; L. Akselrud; E. Giannini; R. Gladyshevskii Refinement of the Modulated Structures of Pb-Free and Pb-Doped Bi-2223 HTSC Journal Article Acta Physica Polonica A, 133 (4), pp. 1027-1029, 2019. @article{Shcherban2019, title = {Refinement of the Modulated Structures of Pb-Free and Pb-Doped Bi-2223 HTSC}, author = {O. Shcherban and L. Akselrud and E. Giannini and R. Gladyshevskii}, url = {http://psjd.icm.edu.pl/psjd/element/bwmeta1.element.bwnjournal-article-appv133n4p62kz}, year = {2019}, date = {2019-04-01}, journal = {Acta Physica Polonica A}, volume = {133}, number = {4}, pages = {1027-1029}, abstract = {The incommensurate modulated structures of Pb-free and Pb-doped Bi-2223 phases were refined on single-crystal X-ray diffraction data. }, keywords = {}, pubstate = {published}, tppubtype = {article} } The incommensurate modulated structures of Pb-free and Pb-doped Bi-2223 phases were refined on single-crystal X-ray diffraction data. | |
Dirk van der Marel Time to respond (Commentary) Online Journal Club for Condensed Matter Physics 2019. @online{vanderMarel2019, title = {Time to respond (Commentary)}, author = {Dirk van der Marel}, url = {https://www.condmatjclub.org/uploads/2019/01/JCCM_January_2019_01.pdf}, year = {2019}, date = {2019-01-01}, organization = {Journal Club for Condensed Matter Physics}, abstract = {Dirk van der Marel writes on some fascinating new technical developments on wideband spectroscopy far far below the diffraction limit and developments following from it including transient spectroscopy.}, keywords = {}, pubstate = {published}, tppubtype = {online} } Dirk van der Marel writes on some fascinating new technical developments on wideband spectroscopy far far below the diffraction limit and developments following from it including transient spectroscopy. | |
I. Cucchi; I. Gutierrez-Lezama; E. Cappelli; S. M. Walker; F. Y. Bruno; G. Tenasini; L. Wang; N. Ubrig; C. Barreteau; E. Giannini; M. Gibertini; A. Tamai; A. F. Morpurgo; F. Baumberger Microfocus Laser–Angle-Resolved Photoemission on Encapsulated Mono-, Bi-, and Few-Layer 1T′-WTe2 Journal Article Nano Letters, 19 (1), pp. 554-560, 2018. @article{Cucchi2018, title = {Microfocus Laser–Angle-Resolved Photoemission on Encapsulated Mono-, Bi-, and Few-Layer 1T′-WTe2}, author = {I. Cucchi and I. Gutierrez-Lezama and E. Cappelli and S. M. Walker and F. Y. Bruno and G. Tenasini and L. Wang and N. Ubrig and C. Barreteau and E. Giannini and M. Gibertini and A. Tamai and A. F. Morpurgo and F. Baumberger}, doi = {https://doi.org/10.1021/acs.nanolett.8b04534}, year = {2018}, date = {2018-12-20}, journal = {Nano Letters}, volume = {19}, number = {1}, pages = {554-560}, abstract = {Two-dimensional crystals of semi-metallic van der Waals materials hold much potential for the realization of novel phases, as exemplified by the recent discoveries of a polar metal in few-layer 1T′-WTe2 and of a quantum spin Hall state in monolayers of the same material. Understanding these phases is particularly challenging because little is known from experiments about the momentum space electronic structure of ultrathin crystals. Here, we report direct electronic structure measurements of exfoliated mono-, bi-, and few-layer 1T′-WTe2 by laser-based microfocus angle-resolved photoemission. This is achieved by encapsulating with monolayer graphene a flake of WTe2 comprising regions of different thickness. Our data support the recent identification of a quantum spin Hall state in monolayer 1T′-WTe2 and reveal strong signatures of the broken inversion symmetry in the bilayer. We finally discuss the sensitivity of encapsulated samples to contaminants following exposure to ambient atmosphere.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional crystals of semi-metallic van der Waals materials hold much potential for the realization of novel phases, as exemplified by the recent discoveries of a polar metal in few-layer 1T′-WTe2 and of a quantum spin Hall state in monolayers of the same material. Understanding these phases is particularly challenging because little is known from experiments about the momentum space electronic structure of ultrathin crystals. Here, we report direct electronic structure measurements of exfoliated mono-, bi-, and few-layer 1T′-WTe2 by laser-based microfocus angle-resolved photoemission. This is achieved by encapsulating with monolayer graphene a flake of WTe2 comprising regions of different thickness. Our data support the recent identification of a quantum spin Hall state in monolayer 1T′-WTe2 and reveal strong signatures of the broken inversion symmetry in the bilayer. We finally discuss the sensitivity of encapsulated samples to contaminants following exposure to ambient atmosphere. | |
S Borroni; J Teyssier; P Piekarz; A B Kuzmenko; A M Oleifmmode acuteselse śfi; J Lorenzana; F Carbone Light scattering from the critical modes of the Verwey transition in magnetite Journal Article Phys. Rev. B, 98 , pp. 184301, 2018. @article{PhysRevB.98.184301, title = {Light scattering from the critical modes of the Verwey transition in magnetite}, author = {S Borroni and J Teyssier and P Piekarz and A B Kuzmenko and A M Oleifmmode acute{s}else śfi{} and J Lorenzana and F Carbone}, url = {https://link.aps.org/doi/10.1103/PhysRevB.98.184301}, doi = {10.1103/PhysRevB.98.184301}, year = {2018}, date = {2018-11-01}, journal = {Phys. Rev. B}, volume = {98}, pages = {184301}, publisher = {American Physical Society}, keywords = {}, pubstate = {published}, tppubtype = {article} } | |
Alsu Gazizulina; Diana Lucia Quintero-Castro; Dirk Wulferding; Jeremie Teyssier; Karel Prokes; Fabiano Yokaichiya; Andreas Schilling Tuning the orbital-lattice fluctuations in the mixed spin-dimer system Ba3-xSrxCr2O8 Journal Article Phys. Rev. B, 98 , pp. 144115, 2018. @article{PhysRevB.98.144115, title = {Tuning the orbital-lattice fluctuations in the mixed spin-dimer system Ba3-xSrxCr2O8}, author = {Alsu Gazizulina and Diana Lucia Quintero-Castro and Dirk Wulferding and Jeremie Teyssier and Karel Prokes and Fabiano Yokaichiya and Andreas Schilling}, url = {https://link.aps.org/doi/10.1103/PhysRevB.98.144115}, doi = {10.1103/PhysRevB.98.144115}, year = {2018}, date = {2018-10-01}, journal = {Phys. Rev. B}, volume = {98}, pages = {144115}, publisher = {American Physical Society}, keywords = {}, pubstate = {published}, tppubtype = {article} } | |
K Wang; N Bachar; J Teyssier; W Luo; C W Rischau; G Scheerer; A de la Torre; R S Perry; F Baumberger; D van der Marel Mott transition and collective charge pinning in electron doped Sr2IrO4 Journal Article Phys. Rev. B, 98 , pp. 045107, 2018. @article{PhysRevB.98.045107, title = {Mott transition and collective charge pinning in electron doped Sr2IrO4}, author = {K Wang and N Bachar and J Teyssier and W Luo and C W Rischau and G Scheerer and A de la Torre and R S Perry and F Baumberger and D van der Marel}, url = {https://link.aps.org/doi/10.1103/PhysRevB.98.045107}, doi = {10.1103/PhysRevB.98.045107}, year = {2018}, date = {2018-07-01}, journal = {Phys. Rev. B}, volume = {98}, pages = {045107}, publisher = {American Physical Society}, abstract = {We studied the in-plane dynamic and static charge conductivity of electron doped Sr2IrO4 using optical spectroscopy and DC transport measurements. The optical conductivity indicates that the pristine material is an indirect semiconductor with a direct Mott gap of 0.55 eV. Upon substitution of 2% La per formula unit the Mott gap is suppressed except in a small fraction of the material (15%) where the gap survives, and overall the material remains insulating. Instead of a zero energy mode (or Drude peak) we observe a soft collective mode (SCM) with a broad maximum at 40meV. Doping to 10% increases the strength of the SCM, and a zero-energy mode occurs together with metallic DC conductivity. Further increase of the La substitution doesn't change the spectral weight integral up to 3 eV. It does however result in a transfer of the SCM spectral weight to the zero-energy mode, with a corresponding reduction of the DC resistivity for all temperatures from 4 to 300 K. The presence of a zero-energy mode signals that at least part of the Fermi surface remains ungapped at low temperatures, whereas the SCM appears to be caused by pinning a collective frozen state involving part of the doped electrons.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We studied the in-plane dynamic and static charge conductivity of electron doped Sr2IrO4 using optical spectroscopy and DC transport measurements. The optical conductivity indicates that the pristine material is an indirect semiconductor with a direct Mott gap of 0.55 eV. Upon substitution of 2% La per formula unit the Mott gap is suppressed except in a small fraction of the material (15%) where the gap survives, and overall the material remains insulating. Instead of a zero energy mode (or Drude peak) we observe a soft collective mode (SCM) with a broad maximum at 40meV. Doping to 10% increases the strength of the SCM, and a zero-energy mode occurs together with metallic DC conductivity. Further increase of the La substitution doesn't change the spectral weight integral up to 3 eV. It does however result in a transfer of the SCM spectral weight to the zero-energy mode, with a corresponding reduction of the DC resistivity for all temperatures from 4 to 300 K. The presence of a zero-energy mode signals that at least part of the Fermi surface remains ungapped at low temperatures, whereas the SCM appears to be caused by pinning a collective frozen state involving part of the doped electrons. | |
Z. Wang; I. Gutiérrez-Lezama; N. Ubrig; M. Kroner; M. Gibertini; T. Taniguchi; K. Watanabe; A. Imamoğlu; E. Giannini; A. F. Morpurgo Very large tunneling magnetoresistance in layered magnetic semiconductor CrI3 Journal Article Nature Communications, 9 (1), pp. 2516, 2018. @article{Wang2018, title = {Very large tunneling magnetoresistance in layered magnetic semiconductor CrI3}, author = {Z. Wang and I. Gutiérrez-Lezama and N. Ubrig and M. Kroner and M. Gibertini and T. Taniguchi and K. Watanabe and A. Imamoğlu and E. Giannini and A. F. Morpurgo}, doi = {10.1038/s41467-018-04953-8}, year = {2018}, date = {2018-06-28}, journal = {Nature Communications}, volume = {9}, number = {1}, pages = {2516}, abstract = {Magnetic layered van der Waals crystals are an emerging class of materials giving access to new physical phenomena, as illustrated by the recent observation of 2D ferromagnetism in Cr2Ge2Te6 and CrI3. Of particular interest in semiconductors is the interplay between magnetism and transport, which has remained unexplored. Here we report magneto-transport measurements on exfoliated CrI3 crystals. We find that tunneling conduction in the direction perpendicular to the crystalline planes exhibits a magnetoresistance as large as 10,000%. The evolution of the magnetoresistance with magnetic field and temperature reveals that the phenomenon originates from multiple transitions to different magnetic states, whose possible microscopic nature is discussed on the basis of all existing experimental observations. This observed dependence of the conductance of a tunnel barrier on its magnetic state is a phenomenon that demonstrates the presence of a strong coupling between transport and magnetism in magnetic van der Waals semiconductors.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Magnetic layered van der Waals crystals are an emerging class of materials giving access to new physical phenomena, as illustrated by the recent observation of 2D ferromagnetism in Cr2Ge2Te6 and CrI3. Of particular interest in semiconductors is the interplay between magnetism and transport, which has remained unexplored. Here we report magneto-transport measurements on exfoliated CrI3 crystals. We find that tunneling conduction in the direction perpendicular to the crystalline planes exhibits a magnetoresistance as large as 10,000%. The evolution of the magnetoresistance with magnetic field and temperature reveals that the phenomenon originates from multiple transitions to different magnetic states, whose possible microscopic nature is discussed on the basis of all existing experimental observations. This observed dependence of the conductance of a tunnel barrier on its magnetic state is a phenomenon that demonstrates the presence of a strong coupling between transport and magnetism in magnetic van der Waals semiconductors. | |
Tamagnone Michele; Slipchenko Tetiana M.; Moldovan Clara; Liu Peter Q.; Centeno Alba; Hasani Hamed; Zurutuza Amaia; Ionescu Adrian M.; Martin-Moreno Luis; Faist Jérôme; Mosig Juan R.; Kuzmenko Alexey B.; Poumirol Jean-Marie Magnetoplasmonic enhancement of Faraday rotation in patterned graphene metasurfaces Journal Article Phys. Rev. B, 97 (24), pp. 241410, 2018. @article{Michele2018, title = {Magnetoplasmonic enhancement of Faraday rotation in patterned graphene metasurfaces}, author = {Tamagnone Michele and Slipchenko Tetiana M. and Moldovan Clara and Liu Peter Q. and Centeno Alba and Hasani Hamed and Zurutuza Amaia and Ionescu Adrian M. and Martin-Moreno Luis and Faist Jérôme and Mosig Juan R. and Kuzmenko Alexey B. and Poumirol Jean-Marie}, url = {https://link.aps.org/doi/10.1103/PhysRevB.97.241410}, doi = {10.1103/PhysRevB.97.241410}, year = {2018}, date = {2018-06-26}, journal = {Phys. Rev. B}, volume = {97}, number = {24}, pages = {241410}, abstract = {Faraday rotation is a fundamental property present in all nonreciprocal optical elements. In the THz range, graphene displays strong Faraday rotation; unfortunately, it is limited to frequencies below the cyclotron resonance. Here, we show experimentally that in specifically designed metasurfaces, magnetoplasmons can be used to circumvent this limitation. We find excellent agreement between theory and experiment and provide physical insights and predictions on these phenomena. Finally, we demonstrate strong tunability in these metasurfaces using electric and magnetic field biasing.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Faraday rotation is a fundamental property present in all nonreciprocal optical elements. In the THz range, graphene displays strong Faraday rotation; unfortunately, it is limited to frequencies below the cyclotron resonance. Here, we show experimentally that in specifically designed metasurfaces, magnetoplasmons can be used to circumvent this limitation. We find excellent agreement between theory and experiment and provide physical insights and predictions on these phenomena. Finally, we demonstrate strong tunability in these metasurfaces using electric and magnetic field biasing. | |
Liang Yang; Jing Tian; Henry Giddens; Jean-Marie Poumirol; JingBo Wu; Alexey B. Kuzmenko; Yang Hao Magnetically tunable graphene-based reflector under linear polarized incidence at room temperature Journal Article Applied Physics Letters, 112 (15), pp. 151103, 2018. @article{Yang2018, title = {Magnetically tunable graphene-based reflector under linear polarized incidence at room temperature}, author = {Liang Yang and Jing Tian and Henry Giddens and Jean-Marie Poumirol and JingBo Wu and Alexey B. Kuzmenko and Yang Hao}, url = {https://aip.scitation.org/doi/abs/10.1063/1.5022774}, doi = {10.1063/1.5022774}, year = {2018}, date = {2018-06-13}, journal = {Applied Physics Letters}, volume = {112}, number = {15}, pages = {151103}, abstract = {At the terahertz spectrum, the 2D material graphene has diagonal and Hall conductivities in the presence of a magnetic field. These peculiar properties provide graphene-based structures with a magnetically tunable response to electromagnetic waves. In this work, the absolute reflection intensity was measured for a graphene-based reflector illuminated by linearly polarized incident waves at room temperature, which demonstrated the intensity modulation depth (IMD) under different magnetostatic biases by up to 15%. Experimental data were fitted and analyzed by a modified equivalent circuit model. In addition, as an important phenomenon of the graphene gyrotropic response, Kerr rotation is discussed according to results achieved from full-wave simulations. It is concluded that the IMD is reduced for the best Kerr rotation in the proposed graphene-based reflector.}, keywords = {}, pubstate = {published}, tppubtype = {article} } At the terahertz spectrum, the 2D material graphene has diagonal and Hall conductivities in the presence of a magnetic field. These peculiar properties provide graphene-based structures with a magnetically tunable response to electromagnetic waves. In this work, the absolute reflection intensity was measured for a graphene-based reflector illuminated by linearly polarized incident waves at room temperature, which demonstrated the intensity modulation depth (IMD) under different magnetostatic biases by up to 15%. Experimental data were fitted and analyzed by a modified equivalent circuit model. In addition, as an important phenomenon of the graphene gyrotropic response, Kerr rotation is discussed according to results achieved from full-wave simulations. It is concluded that the IMD is reduced for the best Kerr rotation in the proposed graphene-based reflector. | |
D. Matera; M. Bonura; C. Barth; A. Fete; R. Černý; E. Giannini; C. Senatore Rapid synthesis of MgB2 by inductive heating Journal Article IEEE Transactions on Applied Superconductivity, 28 (4), pp. 1-5, 2018. @article{Matera2018, title = {Rapid synthesis of MgB2 by inductive heating}, author = {D. Matera and M. Bonura and C. Barth and A. Fete and R. Černý and E. Giannini and C. Senatore}, doi = {10.1109/TASC.2018.2798299}, year = {2018}, date = {2018-06-10}, journal = {IEEE Transactions on Applied Superconductivity}, volume = {28}, number = {4}, pages = {1-5}, abstract = {The effect of a rapid internal magnesium diffusion synthesis on the electrical, magnetic, and microstructural properties of binary MgB 2 bulk samples has been investigated. We have used a laboratory-made induction furnace to reduce the reaction time, typically of many hours, to few minutes. We report a comparison between two samples reacted at the same temperature with the induction furnace and with a commercial resistance oven. We have observed that the critical temperature and the width of the superconducting transition are not affected by the heat treatment. The induction heating leads to an increase of the critical current density at high fields, correlated with an enhancement of the upper critical field. This paper exposes the potentiality of the proposed fast process, which allows tuning the parameters of the heat treatment in ranges that would not be accessible with traditional ovens. The technique can be scaled for the preparation of wires and powders.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The effect of a rapid internal magnesium diffusion synthesis on the electrical, magnetic, and microstructural properties of binary MgB 2 bulk samples has been investigated. We have used a laboratory-made induction furnace to reduce the reaction time, typically of many hours, to few minutes. We report a comparison between two samples reacted at the same temperature with the induction furnace and with a commercial resistance oven. We have observed that the critical temperature and the width of the superconducting transition are not affected by the heat treatment. The induction heating leads to an increase of the critical current density at high fields, correlated with an enhancement of the upper critical field. This paper exposes the potentiality of the proposed fast process, which allows tuning the parameters of the heat treatment in ranges that would not be accessible with traditional ovens. The technique can be scaled for the preparation of wires and powders. | |
B. Hildebrand, T. Jaouen, M.-L. Mottas, G. Monney, C. Barreteau, E. Giannini, D. R. Bowler, P. Aebi: “”, Local Real-Space View of the Achiral 1T-TiSe2 2 x 2 x 2 Charge Density Wave Journal Article Physical Review Letters, 120 (13), pp. 136404, 2018. @article{Hildebrand2018, title = {Local Real-Space View of the Achiral 1T-TiSe2 2 x 2 x 2 Charge Density Wave}, author = {B. Hildebrand, T. Jaouen, M.-L. Mottas, G. Monney, C. Barreteau, E. Giannini, D. R. Bowler, P. Aebi: “”,}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.120.136404}, doi = {10.1103/PhysRevLett.120.136404}, year = {2018}, date = {2018-03-30}, journal = {Physical Review Letters}, volume = {120}, number = {13}, pages = {136404}, abstract = {The transition metal dichalcogenide 1T−TiSe2-two-dimensional layered material undergoing a commensurate 2×2×2 charge density wave (CDW) transition with a weak periodic lattice distortion (PLD) below ≈200 K. Scanning tunneling microscopy (STM) combined with intentionally introduced interstitial Ti atoms allows us to go beyond the usual spatial resolution of STM and to intimately probe the three-dimensional character of the PLD. Furthermore, the inversion-symmetric achiral nature of the CDW in the z direction is revealed, contradicting the claimed existence of helical CDW stacking and associated chiral order. This study paves the way to a simultaneous real-space probing of both charge and structural reconstructions in CDW compounds}, keywords = {}, pubstate = {published}, tppubtype = {article} } The transition metal dichalcogenide 1T−TiSe2-two-dimensional layered material undergoing a commensurate 2×2×2 charge density wave (CDW) transition with a weak periodic lattice distortion (PLD) below ≈200 K. Scanning tunneling microscopy (STM) combined with intentionally introduced interstitial Ti atoms allows us to go beyond the usual spatial resolution of STM and to intimately probe the three-dimensional character of the PLD. Furthermore, the inversion-symmetric achiral nature of the CDW in the z direction is revealed, contradicting the claimed existence of helical CDW stacking and associated chiral order. This study paves the way to a simultaneous real-space probing of both charge and structural reconstructions in CDW compounds | |
G Gäumann; I Crassee; N Numan; M Tamagnone; J R Mosig; J-M Poumirol; J-P Wolf; T Feurer Nonlinear THz spectroscopy and simulation of gated graphene Journal Article Journal of Physics Communications, 2 (6), pp. 065016, 2018. @article{2399-6528-2-6-065016, title = {Nonlinear THz spectroscopy and simulation of gated graphene}, author = {G Gäumann and I Crassee and N Numan and M Tamagnone and J R Mosig and J-M Poumirol and J-P Wolf and T Feurer}, url = {http://stacks.iop.org/2399-6528/2/i=6/a=065016}, year = {2018}, date = {2018-01-01}, journal = {Journal of Physics Communications}, volume = {2}, number = {6}, pages = {065016}, abstract = {We studied the nonlinear optical properties of single layer graphene using high terahertz (THz) fields. With the use of a back gate and cooling down the sample to cryogenic temperatures we are able to spectrally probe the nonlinear THz properties of intrinsic to highly doped graphene. The carrier density strongly affects the nonlinear properties of graphene; in the low doping and high THz field regime, an increase of the transmission on the order of 4% is found in our experiments. At higher doping levels we observe a larger relative nonlinear response: the larger the doping in the single layer the larger the relative field induced increase in transmission becomes. In all experiments, the THz field is opposing the effect of the gate, but field effects are never larger than the doping effects. We use the thermodynamical model for a hot electron gas also used by Mics et al (2015 Nat. Commun. 6 7655) to simulate our data and study the effects of doping on the nonlinear properties of single layer graphene. We find that the highest carrier temperatures are obtained in low doped graphene. The model shows a good qualitative agreement with our data for high doping levels. Nevertheless our results demonstrate the limitation of the model for low doping levels. Our results are a road map for further explorations for the control of nonlinear light–matter interaction and functionalization of graphene layers in active THz devices in which carrier temperature and saturable absorption play a role.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We studied the nonlinear optical properties of single layer graphene using high terahertz (THz) fields. With the use of a back gate and cooling down the sample to cryogenic temperatures we are able to spectrally probe the nonlinear THz properties of intrinsic to highly doped graphene. The carrier density strongly affects the nonlinear properties of graphene; in the low doping and high THz field regime, an increase of the transmission on the order of 4% is found in our experiments. At higher doping levels we observe a larger relative nonlinear response: the larger the doping in the single layer the larger the relative field induced increase in transmission becomes. In all experiments, the THz field is opposing the effect of the gate, but field effects are never larger than the doping effects. We use the thermodynamical model for a hot electron gas also used by Mics et al (2015 Nat. Commun. 6 7655) to simulate our data and study the effects of doping on the nonlinear properties of single layer graphene. We find that the highest carrier temperatures are obtained in low doped graphene. The model shows a good qualitative agreement with our data for high doping levels. Nevertheless our results demonstrate the limitation of the model for low doping levels. Our results are a road map for further explorations for the control of nonlinear light–matter interaction and functionalization of graphene layers in active THz devices in which carrier temperature and saturable absorption play a role. | |
D. van der Marel Order on command Journal Article Physics, 10 (127), 2017. @article{vanderMarel2017, title = {Order on command}, author = {D. van der Marel}, url = {https://physics.aps.org/articles/v10/127}, doi = {10.1103/Physics.10.127}, year = {2017}, date = {2017-11-27}, journal = {Physics}, volume = {10}, number = {127}, abstract = {A current of electrons with aligned spins can be used to modify magnetic order and superconductivity in an iron-based superconductor. }, keywords = {}, pubstate = {published}, tppubtype = {article} } A current of electrons with aligned spins can be used to modify magnetic order and superconductivity in an iron-based superconductor. | |
D. Valentinis; S. Gariglio; A. Fête; J.-M. Triscone; C. Berthod; D. van der Marel Modulation of the superconducting critical temperature due to quantum confinement at the LaAlO3/SrTiO3 interface Journal Article Physical Review B, 96 , pp. 094518, 2017. @article{Valentinis2017, title = {Modulation of the superconducting critical temperature due to quantum confinement at the LaAlO3/SrTiO3 interface}, author = {D. Valentinis and S. Gariglio and A. Fête and J.-M. Triscone and C. Berthod and D. van der Marel}, editor = {APS}, url = {https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.094518}, doi = {10.1103/PhysRevB.96.094518}, year = {2017}, date = {2017-09-18}, journal = {Physical Review B}, volume = {96}, pages = {094518}, abstract = {Superconductivity develops in bulk doped SrTiO3 and at the LaAlO3/SrTiO3 interface with a dome-shaped density dependence of the critical temperature Tc, despite different dimensionalities and geometries. We propose that the Tc dome of LaAlO3/SrTiO3 is a shape resonance due to quantum confinement of superconducting bulk SrTiO3. We substantiate this interpretation by comparing the exact solutions of a three-dimensional and quasi-two-dimensional two-band BCS gap equation. This comparison highlights the role of heavy bands for Tc in both geometries. For bulk SrTiO3, we extract the density dependence of the pairing interaction from the fit to experimental data. We apply quantum confinement in a square potential well of finite depth and calculate Tc in the confined configuration. We compare the calculated Tc to transport experiments and provide an explanation as to why the optimal Tc’s are so close to each other in two-dimensional interfaces and the three-dimensional bulk material. }, keywords = {}, pubstate = {published}, tppubtype = {article} } Superconductivity develops in bulk doped SrTiO3 and at the LaAlO3/SrTiO3 interface with a dome-shaped density dependence of the critical temperature Tc, despite different dimensionalities and geometries. We propose that the Tc dome of LaAlO3/SrTiO3 is a shape resonance due to quantum confinement of superconducting bulk SrTiO3. We substantiate this interpretation by comparing the exact solutions of a three-dimensional and quasi-two-dimensional two-band BCS gap equation. This comparison highlights the role of heavy bands for Tc in both geometries. For bulk SrTiO3, we extract the density dependence of the pairing interaction from the fit to experimental data. We apply quantum confinement in a square potential well of finite depth and calculate Tc in the confined configuration. We compare the calculated Tc to transport experiments and provide an explanation as to why the optimal Tc’s are so close to each other in two-dimensional interfaces and the three-dimensional bulk material. | |
Borroni, S.; Baldini, E.; Katukuri, V. M.; Mann, A.; Parlinski, K.; Legut, D.; Arrell, C.; van Mourik, F.; Teyssier, J.; Kozlowski, A.; Piekarz, P.; Yazyev, O. V.; Oleifmmode, A. M.; Lorenzana, J.; Carbone, F. Coherent generation of symmetry-forbidden phonons by light-induced electron-phonon interactions in magnetite Journal Article Phys. Rev. B, 96 , pp. 104308, 2017. @article{PhysRevB.96.104308, title = {Coherent generation of symmetry-forbidden phonons by light-induced electron-phonon interactions in magnetite}, author = {Borroni, S. and Baldini, E. and Katukuri, V. M. and Mann, A. and Parlinski, K. and Legut, D. and Arrell, C. and van Mourik, F. and Teyssier, J. and Kozlowski, A. and Piekarz, P. and Yazyev, O. V. and Oleifmmode, A. M. and Lorenzana, J. and Carbone, F.}, url = {https://link.aps.org/doi/10.1103/PhysRevB.96.104308}, doi = {10.1103/PhysRevB.96.104308}, year = {2017}, date = {2017-09-01}, journal = {Phys. Rev. B}, volume = {96}, pages = {104308}, publisher = {American Physical Society}, keywords = {}, pubstate = {published}, tppubtype = {article} } | |
Baldini E.; Mann A.; Benfatto L.; Cappelluti E.; Acocella A.; Silkin V. M.; Eremeev S. V.; Kuzmenko A. B.; Borroni S.; Tan T.; Xi X. X.; Zerbetto F.; Merlin R.; Carbone F. Real-Time Observation of Phonon-Mediated σ-π Interband Scattering in MgB2 Journal Article Phys. Rev. Lett., 119 (9), pp. 097002, 2017. @article{E.2017, title = {Real-Time Observation of Phonon-Mediated σ-π Interband Scattering in MgB2}, author = {Baldini E. and Mann A. and Benfatto L. and Cappelluti E. and Acocella A. and Silkin V. M. and Eremeev S. V. and Kuzmenko A. B. and Borroni S. and Tan T. and Xi X. X. and Zerbetto F. and Merlin R. and Carbone F.}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.119.097002}, doi = {10.1103/PhysRevLett.119.097002}, year = {2017}, date = {2017-08-31}, journal = {Phys. Rev. Lett.}, volume = {119}, number = {9}, pages = {097002}, keywords = {}, pubstate = {published}, tppubtype = {article} } | |
Ubrig Nicolas; Jo Sanghyun; Philippi Marc; Costanzo Davide; Berger Helmuth; Kuzmenko Alexey B; Morpurgo Alberto F Microscopic Origin of the Valley Hall Effect in Transition Metal Dichalcogenides Revealed by Wavelength-Dependent Mapping Journal Article Nano Letters, 17 (9), pp. 5719-5725, 2017, ISSN: 1530-6984. @article{Nicolas2017, title = {Microscopic Origin of the Valley Hall Effect in Transition Metal Dichalcogenides Revealed by Wavelength-Dependent Mapping}, author = {Ubrig Nicolas and Jo Sanghyun and Philippi Marc and Costanzo Davide and Berger Helmuth and Kuzmenko Alexey B and Morpurgo Alberto F}, url = {https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b02666}, doi = {10.1021/acs.nanolett.7b02666}, issn = {1530-6984}, year = {2017}, date = {2017-08-27}, journal = {Nano Letters}, volume = {17}, number = {9}, pages = {5719-5725}, abstract = {The band structure of many semiconducting monolayer transition metal dichalcogenides (TMDs) possesses two degenerate valleys with equal and opposite Berry curvature. It has been predicted that, when illuminated with circularly polarized light, interband transitions generate an unbalanced nonequilibrium population of electrons and holes in these valleys, resulting in a finite Hall voltage at zero magnetic field when a current flows through the system. This is the so-called valley Hall effect that has recently been observed experimentally. Here, we show that this effect is mediated by photogenerated neutral excitons and charged trions and not by interband transitions generating independent electrons and holes. We further demonstrate an experimental strategy, based on wavelength dependent spatial mapping of the Hall voltage, which allows the exciton and trion contributions to the valley Hall effect to be discriminated in the measurement. These results represent a significant step forward in our understanding of the microscopic origin of photoinduced valley Hall effect in semiconducting transition metal dichalcogenides and demonstrate experimentally that composite quasi-particles, such as trions, can also possess a finite Berry curvature.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The band structure of many semiconducting monolayer transition metal dichalcogenides (TMDs) possesses two degenerate valleys with equal and opposite Berry curvature. It has been predicted that, when illuminated with circularly polarized light, interband transitions generate an unbalanced nonequilibrium population of electrons and holes in these valleys, resulting in a finite Hall voltage at zero magnetic field when a current flows through the system. This is the so-called valley Hall effect that has recently been observed experimentally. Here, we show that this effect is mediated by photogenerated neutral excitons and charged trions and not by interband transitions generating independent electrons and holes. We further demonstrate an experimental strategy, based on wavelength dependent spatial mapping of the Hall voltage, which allows the exciton and trion contributions to the valley Hall effect to be discriminated in the measurement. These results represent a significant step forward in our understanding of the microscopic origin of photoinduced valley Hall effect in semiconducting transition metal dichalcogenides and demonstrate experimentally that composite quasi-particles, such as trions, can also possess a finite Berry curvature. | |
A. Tamai; Q. S. Wu; I. Cucchi; F. Y. Bruno; S. Riccò; T. K. Kim; M. Hoesch; C. Barreteau; E. Giannini; C. Besnard; A. A. Soluyanov; F. Baumberger Fermi Arcs and Their Topological Character in the Candidate Type-II Weyl Semimetal MoTe2 Journal Article Physical Review X, 6 (3), pp. 031021, 2017. @article{Tamai2017, title = {Fermi Arcs and Their Topological Character in the Candidate Type-II Weyl Semimetal MoTe2}, author = {A. Tamai and Q. S. Wu and I. Cucchi and F. Y. Bruno and S. Riccò and T. K. Kim and M. Hoesch and C. Barreteau and E. Giannini and C. Besnard and A. A. Soluyanov and F. Baumberger}, doi = {10.1103/PhysRevX.6.031021}, year = {2017}, date = {2017-08-17}, journal = {Physical Review X}, volume = {6}, number = {3}, pages = {031021}, abstract = {We report a combined experimental and theoretical study of the candidate type-II Weyl semimetal MoTe2. Using laser-based angle-resolved photoemission, we resolve multiple distinct Fermi arcs on the inequivalent top and bottom (001) surfaces. All surface states observed experimentally are reproduced by an electronic structure calculation for the experimental crystal structure that predicts a topological Weyl semimetal state with eight type-II Weyl points. We further use systematic electronic structure calculations simulating different Weyl point arrangements to discuss the robustness of the identified Weyl semimetal state and the topological character of Fermi arcs in MoTe2.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We report a combined experimental and theoretical study of the candidate type-II Weyl semimetal MoTe2. Using laser-based angle-resolved photoemission, we resolve multiple distinct Fermi arcs on the inequivalent top and bottom (001) surfaces. All surface states observed experimentally are reproduced by an electronic structure calculation for the experimental crystal structure that predicts a topological Weyl semimetal state with eight type-II Weyl points. We further use systematic electronic structure calculations simulating different Weyl point arrangements to discuss the robustness of the identified Weyl semimetal state and the topological character of Fermi arcs in MoTe2. | |
Ruppen, J.; Teyssier, J.; Ardizzone, I.; Peil, O. E.; Catalano, S.; Gibert, M.; Triscone, J.-M.; Georges, A.; van der Marel, D. Impact of antiferromagnetism on the optical properties of rare-earth nickelates Journal Article Phys. Rev. B, 96 , pp. 045120, 2017. @article{PhysRevB.96.045120, title = {Impact of antiferromagnetism on the optical properties of rare-earth nickelates}, author = {Ruppen, J. and Teyssier, J. and Ardizzone, I. and Peil, O. E. and Catalano, S. and Gibert, M. and Triscone, J.-M. and Georges, A. and van der Marel, D.}, url = {https://link.aps.org/doi/10.1103/PhysRevB.96.045120}, doi = {10.1103/PhysRevB.96.045120}, year = {2017}, date = {2017-07-01}, journal = {Phys. Rev. B}, volume = {96}, pages = {045120}, publisher = {American Physical Society}, keywords = {}, pubstate = {published}, tppubtype = {article} } | |
D. Matera; M. Bonura; E. Giannini; C. Senatore Electrical Connectivity in MgB2: The Role of Precursors and Processing Routes in Controlling Voids and Detrimental Secondary Phases C Journal Article IEEE Transactions on Applied Superconductivity, 27 (4), pp. 1-6, 2017. @article{Matera2017, title = {Electrical Connectivity in MgB2: The Role of Precursors and Processing Routes in Controlling Voids and Detrimental Secondary Phases C}, author = {D. Matera and M. Bonura and E. Giannini and C. Senatore}, doi = {10.1109/TASC.2017.2654545}, year = {2017}, date = {2017-06-01}, journal = {IEEE Transactions on Applied Superconductivity}, volume = {27}, number = {4}, pages = {1-6}, keywords = {}, pubstate = {published}, tppubtype = {article} } | |
Manisha Chhikara; Iaroslav Gaponenko; Patrycja Paruch; Alexey B Kuzmenko Effect of uniaxial strain on the optical Drude scattering in graphene Journal Article 2D Materials, 4 (2), pp. 025081, 2017. @article{Chhikara2017, title = {Effect of uniaxial strain on the optical Drude scattering in graphene}, author = {Manisha Chhikara and Iaroslav Gaponenko and Patrycja Paruch and Alexey B Kuzmenko}, url = {https://doi.org/10.1088%2F2053-1583%2Faa6c10}, doi = {10.1088/2053-1583/aa6c10}, year = {2017}, date = {2017-04-28}, journal = {2D Materials}, volume = {4}, number = {2}, pages = {025081}, abstract = {Graphene is a mechanically robust 2D material promising for flexible optoelectronic applications. However, its electromagnetic properties under strain are experimentally poorly understood. Here we present the far-infrared transmission spectra of large-area chemical-vapor deposited monolayer graphene on a polyethylene terephthalate substrate subjected to uniaxial strain. The effective strain value is calibrated using the Raman spectroscopy and corrected for a relaxation of wrinkles and folds seen directly by atomic-force microscopy. We find that while the Drude weight and the Fermi level remain constant, the scattering rate increases by more than 10% per 1% of applied strain, showing a high level of reproducibility during strain cycling. As a result, the electronic mobility and optical absorption of graphene at terahertz and lower frequencies appear to also be sensitive to strain, which opens pathways to control these key parameters mechanically. We suggest that such a functionality can be potentially used in flexible optoelectronic and microelectromechanical systems based on graphene. By combining our findings with existing theoretical models, we discuss the possible mechanisms of strain-controlled Drude scattering.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Graphene is a mechanically robust 2D material promising for flexible optoelectronic applications. However, its electromagnetic properties under strain are experimentally poorly understood. Here we present the far-infrared transmission spectra of large-area chemical-vapor deposited monolayer graphene on a polyethylene terephthalate substrate subjected to uniaxial strain. The effective strain value is calibrated using the Raman spectroscopy and corrected for a relaxation of wrinkles and folds seen directly by atomic-force microscopy. We find that while the Drude weight and the Fermi level remain constant, the scattering rate increases by more than 10% per 1% of applied strain, showing a high level of reproducibility during strain cycling. As a result, the electronic mobility and optical absorption of graphene at terahertz and lower frequencies appear to also be sensitive to strain, which opens pathways to control these key parameters mechanically. We suggest that such a functionality can be potentially used in flexible optoelectronic and microelectromechanical systems based on graphene. By combining our findings with existing theoretical models, we discuss the possible mechanisms of strain-controlled Drude scattering. | |
B. Hildebrand; T. Jaoue; C. Didiot; E. Razzoli; G. Monney; M.-L. Mottas, F. Vanini; C. Barreteau; A. Ubaldini; E. Giannini; H. Berger; D. R. Bowler; P. Aebi Local resilience of the 1T-TiSe2 charge density wave to Ti self-doping Journal Article Physical Review B, 95 (8), pp. 081104, 2017. @article{Hildebrand2017, title = {Local resilience of the 1T-TiSe2 charge density wave to Ti self-doping}, author = {B. Hildebrand and T. Jaoue and C. Didiot and E. Razzoli and G. Monney and M.-L. Mottas, F. Vanini and C. Barreteau and A. Ubaldini and E. Giannini and H. Berger and D. R. Bowler and P. Aebi}, doi = {10.1103/PhysRevB.95.081104}, year = {2017}, date = {2017-02-02}, journal = {Physical Review B}, volume = {95}, number = {8}, pages = {081104}, keywords = {}, pubstate = {published}, tppubtype = {article} } | |
J.-M. Poumirol; P. Q. Liu; T. M. Slipchenko; A. Y. Nikitin; L. Martin-Moreno; J. Faist; A. B. Kuzmenko Electrically controlled terahertz magneto-optical phenomena in continuous and patterned graphene Journal Article Nature Communications, 8 (14626), 2017. @article{Poumirol2017, title = {Electrically controlled terahertz magneto-optical phenomena in continuous and patterned graphene}, author = {J.-M. Poumirol and P. Q. Liu and T. M. Slipchenko and A. Y. Nikitin and L. Martin-Moreno and J. Faist and A. B. Kuzmenko}, editor = {Nature}, url = {http://www.nature.com/articles/ncomms14626}, doi = {10.1038/ncomms14626}, year = {2017}, date = {2017-01-18}, journal = {Nature Communications}, volume = {8}, number = {14626}, abstract = {The magnetic circular dichroism and the Faraday rotation are the fundamental phenomena of great practical importance arising from the breaking of the time reversal symmetry by a magnetic field. In most materials, the strength and the sign of these effects can be only controlled by the field value and its orientation. Furthermore, the terahertz range is lacking materials having the ability to affect the polarization state of the light in a non-reciprocal manner. Here we demonstrate, using broadband terahertz magneto-electro-optical spectroscopy, that in graphene both the magnetic circular dichroism and the Faraday rotation can be modulated in intensity, tuned in frequency and, importantly, inverted using only electrostatic doping at a fixed magnetic field. In addition, we observe strong magneto-plasmonic resonances in a patterned array of graphene antidots, which potentially allows exploiting these magneto-optical phenomena in a broad THz range.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The magnetic circular dichroism and the Faraday rotation are the fundamental phenomena of great practical importance arising from the breaking of the time reversal symmetry by a magnetic field. In most materials, the strength and the sign of these effects can be only controlled by the field value and its orientation. Furthermore, the terahertz range is lacking materials having the ability to affect the polarization state of the light in a non-reciprocal manner. Here we demonstrate, using broadband terahertz magneto-electro-optical spectroscopy, that in graphene both the magnetic circular dichroism and the Faraday rotation can be modulated in intensity, tuned in frequency and, importantly, inverted using only electrostatic doping at a fixed magnetic field. In addition, we observe strong magneto-plasmonic resonances in a patterned array of graphene antidots, which potentially allows exploiting these magneto-optical phenomena in a broad THz range. | |
I. Crassee; F. Borondics; M. K. Tran; G. Autès; A. Magrez; P. Bugnon; H. Berger; J. Teyssier; O. V. Yazyev; M. Orlita; A. Akrap BiTeCl and BiTeBr: A comparative high-pressure optical study Journal Article Physical Review B, 95 , pp. 045201, 2017. @article{Crassee2017, title = {BiTeCl and BiTeBr: A comparative high-pressure optical study}, author = {I. Crassee and F. Borondics and M. K. Tran and G. Autès and A. Magrez and P. Bugnon and H. Berger and J. Teyssier and O. V. Yazyev and M. Orlita and A. Akrap}, editor = {APS}, url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.045201}, doi = {10.1103/PhysRevB.95.045201}, year = {2017}, date = {2017-01-10}, journal = {Physical Review B}, volume = {95}, pages = {045201}, abstract = {We here report a detailed high-pressure infrared transmission study of BiTeCl and BiTeBr. We follow the evolution of two band transitions: the optical excitation β between two Rashba-split conduction bands, and the absorption γ across the band gap. In the low-pressure range, p<4 GPa, for both compounds β is approximately constant with pressure and γ decreases, in agreement with band structure calculations. In BiTeCl, a clear pressure-induced phase transition at 6 GPa leads to a different ground state. For BiTeBr, the pressure evolution is more subtle, and we discuss the possibility of closing and reopening of the band gap. Our data is consistent with a potential Weyl phase in BiTeBr at 5–6 GPa, followed by the onset of a structural phase transition above 7 GPa.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We here report a detailed high-pressure infrared transmission study of BiTeCl and BiTeBr. We follow the evolution of two band transitions: the optical excitation β between two Rashba-split conduction bands, and the absorption γ across the band gap. In the low-pressure range, p<4 GPa, for both compounds β is approximately constant with pressure and γ decreases, in agreement with band structure calculations. In BiTeCl, a clear pressure-induced phase transition at 6 GPa leads to a different ground state. For BiTeBr, the pressure evolution is more subtle, and we discuss the possibility of closing and reopening of the band gap. Our data is consistent with a potential Weyl phase in BiTeBr at 5–6 GPa, followed by the onset of a structural phase transition above 7 GPa. | |
A.M. Novello; M. Spera; A. Scarfato; A. Ubaldini; E. Giannini; D.R. Bowler; C. Renner Stripe and short range order in the charge density wave of 1T-CuxTiSe2 Journal Article Physical Review Letters, 118 (1), pp. 017002, 2017. @article{Novello2017, title = {Stripe and short range order in the charge density wave of 1T-CuxTiSe2}, author = {A.M. Novello and M. Spera and A. Scarfato and A. Ubaldini and E. Giannini and D.R. Bowler and C. Renner}, doi = {10.1103/PhysRevLett.118.017002}, year = {2017}, date = {2017-01-03}, journal = {Physical Review Letters}, volume = {118}, number = {1}, pages = {017002}, abstract = {We study the impact of Cu intercalation on the charge density wave (CDW) in 1T−CuxTiSe2 by scanning tunneling microscopy and spectroscopy. Cu atoms, identified through density functional theory modeling, are found to intercalate randomly on the octahedral site in the van der Waals gap and to dope delocalized electrons near the Fermi level. While the CDW modulation period does not depend on Cu content, we observe the formation of charge stripe domains at low Cu content (x<0.02) and a breaking up of the commensurate order into 2×2 domains at higher Cu content. The latter shrink with increasing Cu concentration and tend to be phase shifted. These findings invalidate a proposed excitonic pairing as the primary CDW formation mechanism in this material. }, keywords = {}, pubstate = {published}, tppubtype = {article} } We study the impact of Cu intercalation on the charge density wave (CDW) in 1T−CuxTiSe2 by scanning tunneling microscopy and spectroscopy. Cu atoms, identified through density functional theory modeling, are found to intercalate randomly on the octahedral site in the van der Waals gap and to dope delocalized electrons near the Fermi level. While the CDW modulation period does not depend on Cu content, we observe the formation of charge stripe domains at low Cu content (x<0.02) and a breaking up of the commensurate order into 2×2 domains at higher Cu content. The latter shrink with increasing Cu concentration and tend to be phase shifted. These findings invalidate a proposed excitonic pairing as the primary CDW formation mechanism in this material. | |
N. Bachar; D. Stricker; S. Muleady; K. Wang; J. A. Mydosh; Y. K. Huang; D. van der Marel Detailed optical spectroscopy of hybridization gap and hidden-order transition in high-quality URu2Si2 single crystals Journal Article Physical Review B, 94 , pp. 235101, 2016. @article{Bachar2016, title = {Detailed optical spectroscopy of hybridization gap and hidden-order transition in high-quality URu2Si2 single crystals}, author = {N. Bachar and D. Stricker and S. Muleady and K. Wang and J. A. Mydosh and Y. K. Huang and D. van der Marel}, editor = {APS}, url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.94.235101}, doi = {10.1103/PhysRevB.94.235101}, year = {2016}, date = {2016-12-01}, journal = {Physical Review B}, volume = {94}, pages = {235101}, abstract = {We present a detailed temperature and frequency dependence of the optical conductivity measured on clean high-quality single crystals of URu2Si2 of ac− and ab-plane surfaces. Our data demonstrate the itinerant character of the narrow 5f bands, becoming progressively coherent as the temperature is lowered below a crossover temperature T∗∼75 K. T∗ is higher than in previous reports as a result of a different sample preparation, which minimizes residual strain. We furthermore present the density-response (energy-loss) function of this compound, and determine the energies of the heavy-fermion plasmons with a− and c-axis polarization. Our observation of a suppression of optical conductivity below 50 meV along both the a and c axes, along with a heavy-fermion plasmon at 18 meV, points toward the emergence of a band of coherent charge carriers crossing the Fermi energy and the emergence of a hybridization gap on part of the Fermi surface. The evolution towards coherent itinerant states is accelerated below the hidden order temperature THO=17.5 K. In the hidden order phase the low-frequency optical conductivity shows a single gap at ∼6.5 meV, which closes at THO.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We present a detailed temperature and frequency dependence of the optical conductivity measured on clean high-quality single crystals of URu2Si2 of ac− and ab-plane surfaces. Our data demonstrate the itinerant character of the narrow 5f bands, becoming progressively coherent as the temperature is lowered below a crossover temperature T∗∼75 K. T∗ is higher than in previous reports as a result of a different sample preparation, which minimizes residual strain. We furthermore present the density-response (energy-loss) function of this compound, and determine the energies of the heavy-fermion plasmons with a− and c-axis polarization. Our observation of a suppression of optical conductivity below 50 meV along both the a and c axes, along with a heavy-fermion plasmon at 18 meV, points toward the emergence of a band of coherent charge carriers crossing the Fermi energy and the emergence of a hybridization gap on part of the Fermi surface. The evolution towards coherent itinerant states is accelerated below the hidden order temperature THO=17.5 K. In the hidden order phase the low-frequency optical conductivity shows a single gap at ∼6.5 meV, which closes at THO. | |
A. Stucky; G. W. Scheerer; Z. Ren; D. Jaccard; J.-M. Poumirol; C. Barreteau; E. Giannini; D. van der Marel Isotope effect in superconducting n-doped SrTiO3 Journal Article Scientific Reports, 6 (37582), 2016. @article{Stucky2016, title = {Isotope effect in superconducting n-doped SrTiO3}, author = {A. Stucky and G. W. Scheerer and Z. Ren and D. Jaccard and J.-M. Poumirol and C. Barreteau and E. Giannini and D. van der Marel}, editor = {Nature}, url = {http://www.nature.com/articles/srep37582}, doi = {10.1038/srep37582}, year = {2016}, date = {2016-11-28}, journal = {Scientific Reports}, volume = {6}, number = {37582}, abstract = {We report the influence on the superconducting critical temperature Tc in doped SrTiO3 of the substitution of the natural 16O atoms by the heavier isotope 18O. We observe that for a wide range of doping this substitution causes a strong (~50%) enhancement of Tc. Also the magnetic critical field Hc2 is increased by a factor ~2. Such a strong impact on Tc and Hc2, with a sign opposite to conventional superconductors, is unprecedented. The observed effect could be the consequence of strong coupling of the doped electrons to lattice vibrations (phonons), a notion which finds support in numerous optical and photo-emission studies. The unusually large size of the observed isotope effect supports a recent model for superconductivity in these materials based on strong coupling to the ferroelectric soft modes of SrTiO3.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We report the influence on the superconducting critical temperature Tc in doped SrTiO3 of the substitution of the natural 16O atoms by the heavier isotope 18O. We observe that for a wide range of doping this substitution causes a strong (~50%) enhancement of Tc. Also the magnetic critical field Hc2 is increased by a factor ~2. Such a strong impact on Tc and Hc2, with a sign opposite to conventional superconductors, is unprecedented. The observed effect could be the consequence of strong coupling of the doped electrons to lattice vibrations (phonons), a notion which finds support in numerous optical and photo-emission studies. The unusually large size of the observed isotope effect supports a recent model for superconductivity in these materials based on strong coupling to the ferroelectric soft modes of SrTiO3. | |
L. Wang; I. Gutiérrez-Lezama; C. Barreteau; D.-K. Ki; E. Giannini; A. F. Morpurgo Direct observation of a long-range field effect from gate tuning of nonlocal conductivity Journal Article Physical Review Letters, 117 (17), pp. 176601, 2016. @article{Wang2016, title = {Direct observation of a long-range field effect from gate tuning of nonlocal conductivity}, author = {L. Wang and I. Gutiérrez-Lezama and C. Barreteau and D.-K. Ki and E. Giannini and A. F. Morpurgo}, doi = {10.1103/PhysRevLett.117.176601}, year = {2016}, date = {2016-10-19}, journal = {Physical Review Letters}, volume = {117}, number = {17}, pages = {176601}, abstract = {We report the direct observation of a long-range field effect in WTe2 devices, leading to large gate-induced changes of transport through crystals much thicker than the electrostatic screening length. The phenomenon—which manifests itself very differently from the conventional field effect—originates from the nonlocal nature of transport in the devices that are thinner than the carrier mean free path. We reproduce theoretically the gate dependence of the measured classical and quantum magnetotransport, and show that the phenomenon is caused by the gate tuning of the bulk carrier mobility by changing the scattering at the surface. Our results demonstrate experimentally the possibility to gate tune the electronic properties deep in the interior of conducting materials, avoiding limitations imposed by electrostatic screening.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We report the direct observation of a long-range field effect in WTe2 devices, leading to large gate-induced changes of transport through crystals much thicker than the electrostatic screening length. The phenomenon—which manifests itself very differently from the conventional field effect—originates from the nonlocal nature of transport in the devices that are thinner than the carrier mean free path. We reproduce theoretically the gate dependence of the measured classical and quantum magnetotransport, and show that the phenomenon is caused by the gate tuning of the bulk carrier mobility by changing the scattering at the surface. Our results demonstrate experimentally the possibility to gate tune the electronic properties deep in the interior of conducting materials, avoiding limitations imposed by electrostatic screening. | |
A. Akrap; M. Hakl; S. Tchoumakov; I. Crassee; J. Kuba; M. O. Goerbig; C. C. Homes; O. Caha; J. Novák; F. Teppe; W. Desrat; S. Koohpayeh; L. Wu; N. P. Armitage; A. Nateprov; E. Arushanov; Q. D. Gibson; R. J. Cava; D. van der Marel; B. A. Piot; C. Faugeras; G. Martinez; M. Potemski; M. Orlita Magneto-Optical Signature of Massless Kane Electrons in Cd3As2 Journal Article Physical Review Letters, 117 , pp. 136401, 2016. @article{Akrap2016, title = {Magneto-Optical Signature of Massless Kane Electrons in Cd3As2}, author = {A. Akrap and M. Hakl and S. Tchoumakov and I. Crassee and J. Kuba and M. O. Goerbig and C. C. Homes and O. Caha and J. Novák and F. Teppe and W. Desrat and S. Koohpayeh and L. Wu and N. P. Armitage and A. Nateprov and E. Arushanov and Q. D. Gibson and R. J. Cava and D. van der Marel and B. A. Piot and C. Faugeras and G. Martinez and M. Potemski and M. Orlita}, editor = {APS}, url = {http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.117.136401}, doi = {10.1103/PhysRevLett.117.136401}, year = {2016}, date = {2016-09-21}, journal = {Physical Review Letters}, volume = {117}, pages = {136401}, abstract = {We report on optical reflectivity experiments performed on Cd3As2 over a broad range of photon energies and magnetic fields. The observed response clearly indicates the presence of 3D massless charge carriers. The specific cyclotron resonance absorption in the quantum limit implies that we are probing massless Kane electrons rather than symmetry-protected 3D Dirac particles. The latter may appear at a smaller energy scale and are not directly observed in our infrared experiments.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We report on optical reflectivity experiments performed on Cd3As2 over a broad range of photon energies and magnetic fields. The observed response clearly indicates the presence of 3D massless charge carriers. The specific cyclotron resonance absorption in the quantum limit implies that we are probing massless Kane electrons rather than symmetry-protected 3D Dirac particles. The latter may appear at a smaller energy scale and are not directly observed in our infrared experiments. | |
F. Y. Bruno; A. Tamai; Q. S. Wu; I. Cucchi; C. Barreteau; A. de la Torre; S. McKeown Walker; S. Riccò; Z. Wang; T. K. Kim; M. Hoesch; M. Shi; N. C. Plumb; E. Giannini; A. A. Soluyanov; F. Baumberger Observation of large topologically trivial Fermi arcs in the candidate type-II Weyl semimetal WTe2 Journal Article Physical Review B, 94 (12), pp. 121112, 2016. @article{Bruno2016, title = {Observation of large topologically trivial Fermi arcs in the candidate type-II Weyl semimetal WTe2}, author = {F. Y. Bruno and A. Tamai and Q. S. Wu and I. Cucchi and C. Barreteau and A. de la Torre and S. McKeown Walker and S. Riccò and Z. Wang and T. K. Kim and M. Hoesch and M. Shi and N. C. Plumb and E. Giannini and A. A. Soluyanov and F. Baumberger}, doi = {10.1103/PhysRevB.94.121112}, year = {2016}, date = {2016-09-14}, journal = {Physical Review B}, volume = {94}, number = {12}, pages = {121112}, keywords = {}, pubstate = {published}, tppubtype = {article} } | |
D. van der Marel Optical properties of correlated electrons Book Chapter van den E. Pavarini E. Koch, Brink J; Editors, Sawatzky G A (Ed.): 6 , Chapter 11, Forschungszentrum Jülich , Forschungszentrum Jülich, Zentralbibliothek, Verlag D-52425 Jülich (DE) , 2016, ISBN: 978-3-95806-159-0. @inbook{vanderMarel2016b, title = {Optical properties of correlated electrons}, author = {D. van der Marel}, editor = {E. Pavarini, E. Koch, J. van den Brink and G. A. Sawatzky Editors }, url = {https://www.cond-mat.de/events/correl16/manuscripts/correl16.pdf}, isbn = {978-3-95806-159-0}, year = {2016}, date = {2016-09-12}, volume = {6}, publisher = {Forschungszentrum Jülich }, address = {Forschungszentrum Jülich, Zentralbibliothek, Verlag D-52425 Jülich (DE) }, chapter = {11}, series = {Modeling and Simulation}, keywords = {}, pubstate = {published}, tppubtype = {inbook} } | |
J. Levallois; M. K. Tran; D. Pouliot; C. N. Presura; L. H. Greene; J. N. Eckstein; J. Uccelli; E. Giannin; G. D. Gu; A. J. Leggett; D. van der Marel Temperature-Dependent Ellipsometry Measurements of Partial Coulomb Energy in Superconducting Cuprates Journal Article Phys. Rev. X, 6 , pp. 031027, 2016. @article{PhysRevX.6.031027, title = {Temperature-Dependent Ellipsometry Measurements of Partial Coulomb Energy in Superconducting Cuprates}, author = {J. Levallois and M. K. Tran and D. Pouliot and C. N. Presura and L. H. Greene and J. N. Eckstein and J. Uccelli and E. Giannin and G. D. Gu and A. J. Leggett and D. van der Marel}, editor = {APS}, url = {http://link.aps.org/doi/10.1103/PhysRevX.6.031027 http://archive-ouverte.unige.ch/unige:87479}, doi = {10.1103/PhysRevX.6.031027}, year = {2016}, date = {2016-08-24}, journal = {Phys. Rev. X}, volume = {6}, pages = {031027}, publisher = {American Physical Society}, abstract = {We performed an experimental study of the temperature and doping dependence of the energy-loss function of the bilayer and trilayer bismuth cuprates family. The primary aim is to obtain information on the energy stored in the Coulomb interaction between the conduction electrons, on the temperature dependence thereof, and on the change of Coulomb interaction when Cooper pairs are formed. We performed temperature-dependent ellipsometry measurements on several Bi2Sr2CaCu2O8−x single crystals: underdoped with Tc=60, 70, and 83 K; optimally doped with Tc=91 K; overdoped with Tc=84, 81, 70, and 58 K; as well as optimally doped Bi2Sr2Ca2Cu3O10+x with Tc=110 K. Our first observation is that, as the temperature drops through Tc, the loss function in the range up to 2 eV displays a change of temperature dependence as compared to the temperature dependence in the normal state. This effect at—or close to—Tc depends strongly on doping, with a sign change for weak overdoping. The size of the observed change in Coulomb energy, using an extrapolation with reasonable assumptions about its q dependence, is about the same size as the condensation energy that has been measured in these compounds. Our results therefore lend support to the notion that the Coulomb energy is an important factor for stabilizing the superconducting phase. Because of the restriction to small momentum, our observations do not exclude a possible significant contribution to the condensation energy of the Coulomb energy associated with the region of q around (π,π).}, keywords = {}, pubstate = {published}, tppubtype = {article} } We performed an experimental study of the temperature and doping dependence of the energy-loss function of the bilayer and trilayer bismuth cuprates family. The primary aim is to obtain information on the energy stored in the Coulomb interaction between the conduction electrons, on the temperature dependence thereof, and on the change of Coulomb interaction when Cooper pairs are formed. We performed temperature-dependent ellipsometry measurements on several Bi2Sr2CaCu2O8−x single crystals: underdoped with Tc=60, 70, and 83 K; optimally doped with Tc=91 K; overdoped with Tc=84, 81, 70, and 58 K; as well as optimally doped Bi2Sr2Ca2Cu3O10+x with Tc=110 K. Our first observation is that, as the temperature drops through Tc, the loss function in the range up to 2 eV displays a change of temperature dependence as compared to the temperature dependence in the normal state. This effect at—or close to—Tc depends strongly on doping, with a sign change for weak overdoping. The size of the observed change in Coulomb energy, using an extrapolation with reasonable assumptions about its q dependence, is about the same size as the condensation energy that has been measured in these compounds. Our results therefore lend support to the notion that the Coulomb energy is an important factor for stabilizing the superconducting phase. Because of the restriction to small momentum, our observations do not exclude a possible significant contribution to the condensation energy of the Coulomb energy associated with the region of q around (π,π). | |
D. Valentinis; D. van der Marel; C. Berthod Rise and fall of shape resonances in thin films of BCS superconductors Journal Article Physical Review B, 94 (054516), 2016. @article{Valentinis2016b, title = {Rise and fall of shape resonances in thin films of BCS superconductors}, author = {D. Valentinis and D. van der Marel and C. Berthod}, editor = {APS}, url = {https://journals.aps.org/prb/abstract/10.1103/PhysRevB.94.054516}, doi = {10.1103/PhysRevB.94.054516}, year = {2016}, date = {2016-08-23}, journal = {Physical Review B}, volume = {94}, number = {054516}, abstract = {The confinement of a superconductor in a thin film changes its Fermi-level density of states and is expected to change its critical temperature Tc. Previous calculations have reported large discontinuities of Tc when the chemical potential coincides with a subband edge. By solving the BCS gap equation exactly, we show that such discontinuities are artifacts and that Tc is a continuous function of the film thickness. We also find that Tc is reduced in thin films compared with the bulk if the confinement potential is lower than a critical value, while for stronger confinement Tc increases with decreasing film thickness, reaches a maximum, and eventually drops to zero. Our numerical results are supported by several exact solutions. We finally interpret experimental data for ultrathin lead thin films in terms of a thickness-dependent effective mass.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The confinement of a superconductor in a thin film changes its Fermi-level density of states and is expected to change its critical temperature Tc. Previous calculations have reported large discontinuities of Tc when the chemical potential coincides with a subband edge. By solving the BCS gap equation exactly, we show that such discontinuities are artifacts and that Tc is a continuous function of the film thickness. We also find that Tc is reduced in thin films compared with the bulk if the confinement potential is lower than a critical value, while for stronger confinement Tc increases with decreasing film thickness, reaches a maximum, and eventually drops to zero. Our numerical results are supported by several exact solutions. We finally interpret experimental data for ultrathin lead thin films in terms of a thickness-dependent effective mass. | |
C. W. Rischau; A. Ubaldini; E. Giannini; C. J. van der Beek Charge puddles in a completely compensated topological insulator Journal Article New Journal of Physics, 18 (7), pp. 073024, 2016. @article{Rischau2016, title = {Charge puddles in a completely compensated topological insulator}, author = {C. W. Rischau and A. Ubaldini and E. Giannini and C. J. van der Beek}, url = {https://doi.org/10.1088%2F1367-2630%2F18%2F7%2F073024}, doi = {10.1088/1367-2630/18/7/073024}, year = {2016}, date = {2016-07-12}, journal = {New Journal of Physics}, volume = {18}, number = {7}, pages = {073024}, abstract = {Compensation of intrinsic charges is widely used to reduce the bulk conductivity of 3D topological insulators (TIs). Here we use low temperature electron irradiation-induced defects paired with in situ electrical transport measurements to fine-tune the degree of compensation in Bi2Te3. The coexistence of electrons and holes at the point of optimal compensation can only be explained by bulk carriers forming charge puddles. These need to be considered to understand the electric transport in compensated TI samples, irrespective of the method of compensation.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Compensation of intrinsic charges is widely used to reduce the bulk conductivity of 3D topological insulators (TIs). Here we use low temperature electron irradiation-induced defects paired with in situ electrical transport measurements to fine-tune the degree of compensation in Bi2Te3. The coexistence of electrons and holes at the point of optimal compensation can only be explained by bulk carriers forming charge puddles. These need to be considered to understand the electric transport in compensated TI samples, irrespective of the method of compensation. | |
Pieter J. de Visser; Julien Levallois; Michaël K. Tran; Jean-Marie Poumirol; Ievgeniia O. Nedoliuk; Jérémie Teyssier; Ctirad Uher; Dirk van der Marel; Alexey B. Kuzmenko Suppressed Magnetic Circular Dichroism and Valley-Selective Magnetoabsorption due to the Effective Mass Anisotropy in Bismuth Journal Article Physical Review Letters, 117 (017402), 2016. @article{deVisser2016, title = {Suppressed Magnetic Circular Dichroism and Valley-Selective Magnetoabsorption due to the Effective Mass Anisotropy in Bismuth}, author = {Pieter J. de Visser and Julien Levallois and Michaël K. Tran and Jean-Marie Poumirol and Ievgeniia O. Nedoliuk and Jérémie Teyssier and Ctirad Uher and Dirk van der Marel and Alexey B. Kuzmenko}, editor = {APS}, url = {http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.117.017402}, doi = {http://dx.doi.org/10.1103/PhysRevLett.117.017402}, year = {2016}, date = {2016-06-29}, journal = {Physical Review Letters}, volume = {117}, number = {017402}, abstract = {We measure the far-infrared reflectivity and Kerr angle spectra on a high-quality crystal of pure semimetallic bismuth as a function of magnetic field, from which we extract the conductivity for left- and right-handed circular polarizations. The high spectral resolution allows us to separate the intraband Landau level transitions for electrons and holes. The hole transition exhibits 100% magnetic circular dichroism; it appears only for one polarization as expected for a circular cyclotron orbit. However, the dichroism for electron transitions is reduced to only 13±1%, which is quantitatively explained by the large effective mass anisotropy of the electron pockets of the Fermi surface. This observation is a signature of the mismatch between the metric experienced by the photons and the electrons. It allows for a contactless measurement of the effective mass anisotropy and provides a direction towards valley polarized magnetooptical pumping with elliptically polarized light.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We measure the far-infrared reflectivity and Kerr angle spectra on a high-quality crystal of pure semimetallic bismuth as a function of magnetic field, from which we extract the conductivity for left- and right-handed circular polarizations. The high spectral resolution allows us to separate the intraband Landau level transitions for electrons and holes. The hole transition exhibits 100% magnetic circular dichroism; it appears only for one polarization as expected for a circular cyclotron orbit. However, the dichroism for electron transitions is reduced to only 13±1%, which is quantitatively explained by the large effective mass anisotropy of the electron pockets of the Fermi surface. This observation is a signature of the mismatch between the metric experienced by the photons and the electrons. It allows for a contactless measurement of the effective mass anisotropy and provides a direction towards valley polarized magnetooptical pumping with elliptically polarized light. | |
D. van der Marel Quantum-matter physics: Quasiparticles on a collision course Journal Article Nature, 533 , pp. 186–187, 2016, ISSN: ISSN: 0028-0836. @article{vanderMarel2016, title = {Quantum-matter physics: Quasiparticles on a collision course}, author = {D. van der Marel}, editor = {Nature}, url = {http://www.nature.com/nature/journal/v533/n7602/full/533186a.html}, doi = {10.1038/533186a}, issn = {ISSN: 0028-0836}, year = {2016}, date = {2016-05-12}, journal = {Nature}, volume = {533}, pages = {186–187}, abstract = {Emergent quanta of momentum and charge, called quasiparticles, govern many of the properties of materials. The development of a quasiparticle collider promises to reveal fundamental insights into these peculiar entities.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Emergent quanta of momentum and charge, called quasiparticles, govern many of the properties of materials. The development of a quasiparticle collider promises to reveal fundamental insights into these peculiar entities. | |
M. Tamagnone; C. Moldovan; J.-M. Poumirol; A. B. Kuzmenko; A. M. Ionescu and J. R. Mosig; J. Perruisseau-Carrier Near optimal graphene terahertz non-reciprocal isolator Journal Article Nature Communications, 7 (11216), 2016. @article{Tamagnone2016, title = {Near optimal graphene terahertz non-reciprocal isolator}, author = {M. Tamagnone and C. Moldovan and J.-M. Poumirol and A. B. Kuzmenko and A. M. Ionescu and J. R. Mosig and J. Perruisseau-Carrier}, editor = {Nature}, url = {http://www.nature.com/ncomms/2016/160406/ncomms11216/abs/ncomms11216.html}, doi = {10.1038/ncomms11216}, year = {2016}, date = {2016-04-06}, journal = {Nature Communications}, volume = {7}, number = {11216}, abstract = {Isolators, or optical diodes, are devices enabling unidirectional light propagation by using non-reciprocal optical materials, namely materials able to break Lorentz reciprocity. The realization of isolators at terahertz frequencies is a very important open challenge made difficult by the intrinsically lossy propagation of terahertz radiation in current non-reciprocal materials. Here we report the design, fabrication and measurement of a terahertz non-reciprocal isolator for circularly polarized waves based on magnetostatically biased monolayer graphene, operating in reflection. The device exploits the non-reciprocal optical conductivity of graphene and, in spite of its simple design, it exhibits almost 20 dB of isolation and only 7.5 dB of insertion loss at 2.9 THz. Operation with linearly polarized light can be achieved using quarter-wave plates as polarization converters. These results demonstrate the superiority of graphene with respect to currently used terahertz non-reciprocal materials and pave the way to a novel class of optimal non-reciprocal devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Isolators, or optical diodes, are devices enabling unidirectional light propagation by using non-reciprocal optical materials, namely materials able to break Lorentz reciprocity. The realization of isolators at terahertz frequencies is a very important open challenge made difficult by the intrinsically lossy propagation of terahertz radiation in current non-reciprocal materials. Here we report the design, fabrication and measurement of a terahertz non-reciprocal isolator for circularly polarized waves based on magnetostatically biased monolayer graphene, operating in reflection. The device exploits the non-reciprocal optical conductivity of graphene and, in spite of its simple design, it exhibits almost 20 dB of isolation and only 7.5 dB of insertion loss at 2.9 THz. Operation with linearly polarized light can be achieved using quarter-wave plates as polarization converters. These results demonstrate the superiority of graphene with respect to currently used terahertz non-reciprocal materials and pave the way to a novel class of optimal non-reciprocal devices. | |
J. Teyssier; E. Giannini; A. Stucky; R. Černý; M. V. Eremin; D. van der Marel Jahn-Teller induced nematic orbital order in tetragonal Sr2VO4 Journal Article Physical Review B, 93 (125138), 2016. @article{Teyssier2016, title = {Jahn-Teller induced nematic orbital order in tetragonal Sr2VO4}, author = {J. Teyssier and E. Giannini and A. Stucky and R. Černý and M. V. Eremin and D. van der Marel}, editor = {APS}, url = {http://journals.aps.org/prb/pdf/10.1103/PhysRevB.93.125138}, doi = {10.1103/PhysRevB.93.125138}, year = {2016}, date = {2016-03-24}, journal = {Physical Review B}, volume = {93}, number = {125138}, abstract = {Using high resolution x-ray diffraction (XRD) on high purity powders, we resolved the structure and ab symmetry of the intriguing compound Sr2VO4 from room temperature down to 20 K to an unprecedented level of accuracy. Upon cooling, this set of data unambiguously reveals a second-order phase transition lowering the symmetry from tetragonal to orthorhombic at a temperature Tc2=136 K. The observation of an orthorhombic distortion of the ab plane is attributed to nematic phase formation supported by local Jahn-Teller (JT) dynamical instability. At TN=105 K, spins order and at Tc1=100 K the tetragonal structure is recovered with an elongated c axis.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Using high resolution x-ray diffraction (XRD) on high purity powders, we resolved the structure and ab symmetry of the intriguing compound Sr2VO4 from room temperature down to 20 K to an unprecedented level of accuracy. Upon cooling, this set of data unambiguously reveals a second-order phase transition lowering the symmetry from tetragonal to orthorhombic at a temperature Tc2=136 K. The observation of an orthorhombic distortion of the ab plane is attributed to nematic phase formation supported by local Jahn-Teller (JT) dynamical instability. At TN=105 K, spins order and at Tc1=100 K the tetragonal structure is recovered with an elongated c axis. | |
C. Barreteau; B. Michon; C. Besnard; E. Giannini High-pressure melt growth and transport properties of SiP, SiAs, GeP, and GeAs 2D layered semiconductors Journal Article Journal of Crystal Growth, 443 , pp. 75-80, 2016. @article{Barreteau2016, title = {High-pressure melt growth and transport properties of SiP, SiAs, GeP, and GeAs 2D layered semiconductors}, author = {C. Barreteau and B. Michon and C. Besnard and E. Giannini}, editor = {Elsevier}, url = {http://www.sciencedirect.com/science/article/pii/S0022024816300914?np=y}, doi = {10.1016/j.jcrysgro.2016.03.019}, year = {2016}, date = {2016-03-07}, journal = {Journal of Crystal Growth}, volume = {443}, pages = {75-80}, abstract = {Silicon and Germanium monopnictides SiP, SiAs, GeP and GeAs form a family of 2D layered semiconductors. We have succeeded in growing bulk single crystals of these compounds by melt-growth under high pressure (0.5-1 GPa) in a cubic anvil hot press. Large (mm-size), shiny, micaceous crystals of GeP, GeAs and SiAs were obtained, and could be exfoliated into 2D flakes. Small and brittle crystals of SiP were yielded by this method. High-pressure sintered polycrystalline SiP and GeAs have also been successfully used as a precursor in the Chemical Vapor Transport growth of these crystals in the presence of I2 as a transport agent. All compounds are found to crystallize in the expected layered structure and do not undergo any structural transition at low temperature, as shown by Raman spectroscopy down to T=5 K. All materials exhibit a semiconducting behavior. The electrical resistivity of GeP, GeAs and SiAs is found to depend on temperature following a 2D-Variable Range Hopping conduction mechanism. The availability of bulk crystals of these compounds opens new perspectives in the field of 2D semiconducting materials for device applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Silicon and Germanium monopnictides SiP, SiAs, GeP and GeAs form a family of 2D layered semiconductors. We have succeeded in growing bulk single crystals of these compounds by melt-growth under high pressure (0.5-1 GPa) in a cubic anvil hot press. Large (mm-size), shiny, micaceous crystals of GeP, GeAs and SiAs were obtained, and could be exfoliated into 2D flakes. Small and brittle crystals of SiP were yielded by this method. High-pressure sintered polycrystalline SiP and GeAs have also been successfully used as a precursor in the Chemical Vapor Transport growth of these crystals in the presence of I2 as a transport agent. All compounds are found to crystallize in the expected layered structure and do not undergo any structural transition at low temperature, as shown by Raman spectroscopy down to T=5 K. All materials exhibit a semiconducting behavior. The electrical resistivity of GeP, GeAs and SiAs is found to depend on temperature following a 2D-Variable Range Hopping conduction mechanism. The availability of bulk crystals of these compounds opens new perspectives in the field of 2D semiconducting materials for device applications. | |
S. A. Klimin, A. B. Kuzmenko, M. A. Kashchenko,; M. N. Popova Infrared study of lattice dynamics and spin-phonon and electron-phonon interactions in multiferroic TbFe3(BO3)4 and GdFe3(BO3)4 Journal Article http://journals.aps.org/prb/abstract/10.1103/PhysRevB.93.054304, 93 (054304), 2016. @article{Klimin2016, title = {Infrared study of lattice dynamics and spin-phonon and electron-phonon interactions in multiferroic TbFe3(BO3)4 and GdFe3(BO3)4}, author = {S. A. Klimin, A. B. Kuzmenko, M. A. Kashchenko, and M. N. Popova}, url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.93.054304}, doi = {http://dx.doi.org/10.1103/PhysRevB.93.054304}, year = {2016}, date = {2016-02-23}, journal = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.93.054304}, volume = {93}, number = {054304}, abstract = {We present a comparative far-infrared reflection spectroscopy study of phonons, phase transitions, spin-phonon, and electron-phonon interactions in isostructural multiferroic iron borates of gadolinium and terbium. The behavior of phonon modes registered in a wide temperature range is consistent with a weak first-order structural phase transition [Ts=143 for GdFe3(BO3)4 and 200 K for TbFe3(BO3)4] from a high-symmetry high-temperature R32 structure into a low-symmetry low-temperature P3121 one. The temperature dependences of frequencies, oscillator strengths, and damping constants of some low-frequency modes reveal an appreciable lattice anharmonicity. Peculiarities in the phonon mode behavior in both compounds at the temperature of an antiferromagnetic ordering [TN=32K for GdFe3(BO3)4 and 40 K for TbFe3(BO3)4] evidence the spin-phonon interaction. In the energy range of phonons, GdFe3(BO3)4 has no electronic levels, but TbFe3(BO3)4 possesses several. We observe an onset of new bands in the excitation spectrum of TbFe3(BO3)4 due to a resonance interaction between a lattice phonon and 4f electronic crystal-field (CF) excitations of Tb3+. This interaction causes delocalization of the CF excitations, their Davydov splitting, and formation of coupled electron-phonon modes.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We present a comparative far-infrared reflection spectroscopy study of phonons, phase transitions, spin-phonon, and electron-phonon interactions in isostructural multiferroic iron borates of gadolinium and terbium. The behavior of phonon modes registered in a wide temperature range is consistent with a weak first-order structural phase transition [Ts=143 for GdFe3(BO3)4 and 200 K for TbFe3(BO3)4] from a high-symmetry high-temperature R32 structure into a low-symmetry low-temperature P3121 one. The temperature dependences of frequencies, oscillator strengths, and damping constants of some low-frequency modes reveal an appreciable lattice anharmonicity. Peculiarities in the phonon mode behavior in both compounds at the temperature of an antiferromagnetic ordering [TN=32K for GdFe3(BO3)4 and 40 K for TbFe3(BO3)4] evidence the spin-phonon interaction. In the energy range of phonons, GdFe3(BO3)4 has no electronic levels, but TbFe3(BO3)4 possesses several. We observe an onset of new bands in the excitation spectrum of TbFe3(BO3)4 due to a resonance interaction between a lattice phonon and 4f electronic crystal-field (CF) excitations of Tb3+. This interaction causes delocalization of the CF excitations, their Davydov splitting, and formation of coupled electron-phonon modes. | |
D. Valentinis; D. van der Marel; C. Berthod BCS superconductivity near the band edge: exact results for one and several bands Journal Article Physical Review B, 94 (2), pp. 024511, 2016. @article{Valentinis2016, title = {BCS superconductivity near the band edge: exact results for one and several bands}, author = {D. Valentinis and D. van der Marel and C. Berthod}, editor = {APS}, url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.94.024511}, doi = {http://dx.doi.org/10.1103/PhysRevB.94.024511}, year = {2016}, date = {2016-01-18}, journal = {Physical Review B}, volume = {94}, number = {2}, pages = {024511}, abstract = {We revisit the problem of a BCS superconductor in the regime where the Fermi energy is smaller than the Debye energy. This regime is relevant for low-density superconductors such as SrTiO3 that are not in the BEC limit, as well as in the problem of "shape resonances" associated with the confinement of a three-dimensional superconductor. While the problem is not new, exact results were lacking in the low-density limit. In two dimensions, we find that the initial rise of the pairing temperature Tc at low density n is nonanalytic and faster than any power of n. In three dimensions, we also find that Tc is nonanalytic, but starts with zero slope at weak coupling and infinite slope at strong coupling. Self-consistent treatment of the chemical potential and energy dependence of the density of states are crucial ingredients to obtain these results. We also present exact results for multi-band systems and confirm our analytical expressions by numerical simulations.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We revisit the problem of a BCS superconductor in the regime where the Fermi energy is smaller than the Debye energy. This regime is relevant for low-density superconductors such as SrTiO3 that are not in the BEC limit, as well as in the problem of "shape resonances" associated with the confinement of a three-dimensional superconductor. While the problem is not new, exact results were lacking in the low-density limit. In two dimensions, we find that the initial rise of the pairing temperature Tc at low density n is nonanalytic and faster than any power of n. In three dimensions, we also find that Tc is nonanalytic, but starts with zero slope at weak coupling and infinite slope at strong coupling. Self-consistent treatment of the chemical potential and energy dependence of the density of states are crucial ingredients to obtain these results. We also present exact results for multi-band systems and confirm our analytical expressions by numerical simulations. | |
Damien Stricker The nature of Fermi-liquids, an optical perspective PhD Thesis 2015. @phdthesis{Stricker2015, title = {The nature of Fermi-liquids, an optical perspective}, author = {Damien Stricker}, year = {2015}, date = {2015-12-09}, keywords = {}, pubstate = {published}, tppubtype = {phdthesis} } |