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. | |
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. | |
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. | |
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. | |
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. | |
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} } | |
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. | |
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. | |
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. | |
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. | |
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. | |
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} } | |
L. Wang; I. Gutiérrez-Lezama; C. Barreteau; N. Ubrig; E. Giannini; A. F. Morpurgo Tuning magnetotransport in a compensated semimetal at the atomic scale Journal Article Nature Communications, 6 (8892), 2015. @article{Wang2015, title = {Tuning magnetotransport in a compensated semimetal at the atomic scale}, author = {L. Wang and I. Gutiérrez-Lezama and C. Barreteau and N. Ubrig and E. Giannini and A. F. Morpurgo}, editor = {Nature}, url = {http://www.nature.com/ncomms/2015/151124/ncomms9892/full/ncomms9892.html}, doi = {10.1038/ncomms9892}, year = {2015}, date = {2015-11-24}, journal = {Nature Communications}, volume = {6}, number = {8892}, abstract = {Either in bulk form, or in atomically thin crystals, layered transition metal dichalcogenides continuously reveal new phenomena. The latest example is 1T’-WTe2, a semimetal found to exhibit the largest known magnetoresistance in the bulk, and predicted to become a topological insulator in strained monolayers. Here we show that reducing the thickness through exfoliation enables the electronic properties of WTe2 to be tuned, which allows us to identify the mechanisms responsible for the observed magnetotransport down to the atomic scale. The longitudinal resistance and the unconventional magnetic field dependence of the Hall resistance are reproduced quantitatively by a classical two-band model for crystals as thin as six monolayers, whereas a crossover to an Anderson insulator occurs for thinner crystals. Besides establishing the origin of the magnetoresistance of WTe2, our results represent a complete validation of the classical theory for two-band electron-hole transport, and indicate that atomically thin WTe2 layers remain gapless semimetals.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Either in bulk form, or in atomically thin crystals, layered transition metal dichalcogenides continuously reveal new phenomena. The latest example is 1T’-WTe2, a semimetal found to exhibit the largest known magnetoresistance in the bulk, and predicted to become a topological insulator in strained monolayers. Here we show that reducing the thickness through exfoliation enables the electronic properties of WTe2 to be tuned, which allows us to identify the mechanisms responsible for the observed magnetotransport down to the atomic scale. The longitudinal resistance and the unconventional magnetic field dependence of the Hall resistance are reproduced quantitatively by a classical two-band model for crystals as thin as six monolayers, whereas a crossover to an Anderson insulator occurs for thinner crystals. Besides establishing the origin of the magnetoresistance of WTe2, our results represent a complete validation of the classical theory for two-band electron-hole transport, and indicate that atomically thin WTe2 layers remain gapless semimetals. | |
J. Ruppen, J. Teyssier, O. E. Peil, S. Catalano, M. Gibert, J. Mravlje, J.-M. Triscone, A. Georges,; D. van der Marel Optical spectroscopy and the nature of the insulating state of rare-earth nickelates Journal Article Physical Review B, 92 (155145), 2015. @article{Ruppen2015, title = {Optical spectroscopy and the nature of the insulating state of rare-earth nickelates}, author = {J. Ruppen, J. Teyssier, O. E. Peil, S. Catalano, M. Gibert, J. Mravlje, J.-M. Triscone, A. Georges, and D. van der Marel}, url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.92.155145}, doi = {http://dx.doi.org/10.1103/PhysRevB.92.155145}, year = {2015}, date = {2015-10-27}, journal = {Physical Review B}, volume = {92}, number = {155145}, abstract = {Using a combination of spectroscopic ellipsometry and DC transport measurements, we determine the temperature dependence of the optical conductivity of NdNiO3 and SmNiO3 films. The optical spectra show the appearance of a characteristic two-peak structure in the near-infrared when the material passes from the metal to the insulator phase. Dynamical mean-field theory calculations confirm this two-peak structure and allow us to identify these spectral changes and the associated changes in the electronic structure. We demonstrate that the insulating phase in these compounds and the associated characteristic two-peak structure are due to the combined effect of bond disproportionation and Mott physics associated with half of the disproportionated sites. We also provide insights into the structure of excited states above the gap.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Using a combination of spectroscopic ellipsometry and DC transport measurements, we determine the temperature dependence of the optical conductivity of NdNiO3 and SmNiO3 films. The optical spectra show the appearance of a characteristic two-peak structure in the near-infrared when the material passes from the metal to the insulator phase. Dynamical mean-field theory calculations confirm this two-peak structure and allow us to identify these spectral changes and the associated changes in the electronic structure. We demonstrate that the insulating phase in these compounds and the associated characteristic two-peak structure are due to the combined effect of bond disproportionation and Mott physics associated with half of the disproportionated sites. We also provide insights into the structure of excited states above the gap. | |
Michaël K. Tran; Julien Levallois; Ana Akrap; Jérémie Teyssier; Alexey B. Kuzmenko; Florence Levy-Bertrand; Riccardo Tediosi; Mehdi P. Brandt; Philippe Lerch; Dirk van Der Marel Versatile setup for optical spectroscopy under high pressure and low temperature Journal Article Review of Scientific Instruments, 86 (105102), 2015. @article{Tran2015, title = {Versatile setup for optical spectroscopy under high pressure and low temperature}, author = {Michaël K. Tran and Julien Levallois and Ana Akrap and Jérémie Teyssier and Alexey B. Kuzmenko and Florence Levy-Bertrand and Riccardo Tediosi and Mehdi P. Brandt and Philippe Lerch and Dirk van Der Marel}, editor = {Review of Scientific Instruments}, url = {http://dx.doi.org/10.1063/1.4931990 http://archive-ouverte.unige.ch/unige:75744}, year = {2015}, date = {2015-10-01}, journal = {Review of Scientific Instruments}, volume = {86}, number = {105102}, abstract = {We present an optical setup for spectroscopic measurements in the infrared and of Raman shift under high pressure and at low temperature. Using a membrane-driven diamond anvil cell, the pressure can be tuned in situ up to 20 GPa and the temperatures ranges from room temperature down to 18 K in transmission mode and 13 K in reflection mode. In transmission, the setup is entirely working under vacuum to reduce the water absorption features and obtain a higher spectral stability. Since the infrared throughput obtained with a thermal source is limited, the use of a synchrotron source allowed to enhance the performance, as illustrated with results obtained with various materials. The analysis of the reflectivity is adapted so that it benefits from ambient pressure data and produces quantitative optical conductivity curves that can be easily compared to the results at ambient pressure.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We present an optical setup for spectroscopic measurements in the infrared and of Raman shift under high pressure and at low temperature. Using a membrane-driven diamond anvil cell, the pressure can be tuned in situ up to 20 GPa and the temperatures ranges from room temperature down to 18 K in transmission mode and 13 K in reflection mode. In transmission, the setup is entirely working under vacuum to reduce the water absorption features and obtain a higher spectral stability. Since the infrared throughput obtained with a thermal source is limited, the use of a synchrotron source allowed to enhance the performance, as illustrated with results obtained with various materials. The analysis of the reflectivity is adapted so that it benefits from ambient pressure data and produces quantitative optical conductivity curves that can be easily compared to the results at ambient pressure. | |
D. Lançon; N. Tsyrulin; M. Böhm; R. Viennois; S. Zabihzadeh; A. Kusmartseva; E. Giannini; H. M. Rønnow Pressure induced evolution of superconductivity and magnetic hourglass dispersion in Fe1.02Te0.7Se0.3 Journal Article New Journal of Physics, 17 (4), pp. 043020, 2015. @article{Lançon2015, title = {Pressure induced evolution of superconductivity and magnetic hourglass dispersion in Fe1.02Te0.7Se0.3}, author = {D. Lançon and N. Tsyrulin and M. Böhm and R. Viennois and S. Zabihzadeh and A. Kusmartseva and E. Giannini and H. M. Rønnow}, url = {http://stacks.iop.org/1367-2630/17/i=4/a=043020}, doi = {10.1088/1367-2630/17/4/043020}, year = {2015}, date = {2015-04-14}, journal = {New Journal of Physics}, volume = {17}, number = {4}, pages = {043020}, abstract = {Iron based high temperature superconductors have several common features with superconducting cuprates, including the square lattice and the proximity to an antiferromagnetic phase. The magnetic excitation spectrum below Tc of Fe1.02Te0.7Se0.3 shows an hourglass-shaped dispersion with a resonance around the commensurate point . In a previous inelastic neutron scattering study, we showed that the hourglass-shaped dispersion is most likely a prerequisite for superconductivity, while the consequences are the opening of a gap and a shift of spectral weight. In this paper we follow the evolution of the hourglass shaped dispersion under applied pressure up to 12 kbar. Our results show that that the pressure-induced 37% increase of Tc is concomitant with a change in the magnetic excitation spectrum, with an increase of the hourglass energy by 38%.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Iron based high temperature superconductors have several common features with superconducting cuprates, including the square lattice and the proximity to an antiferromagnetic phase. The magnetic excitation spectrum below Tc of Fe1.02Te0.7Se0.3 shows an hourglass-shaped dispersion with a resonance around the commensurate point . In a previous inelastic neutron scattering study, we showed that the hourglass-shaped dispersion is most likely a prerequisite for superconductivity, while the consequences are the opening of a gap and a shift of spectral weight. In this paper we follow the evolution of the hourglass shaped dispersion under applied pressure up to 12 kbar. Our results show that that the pressure-induced 37% increase of Tc is concomitant with a change in the magnetic excitation spectrum, with an increase of the hourglass energy by 38%. | |
I. Gutierrez Lezama; A. Arora; A. Ubaldini; C. Barreteau; E. Giannini; M. Potemski; A. F. Morpurgo Indirect-to-Direct Band Gap Crossover in Few-Layer MoTe2 Journal Article Nano Letters, 15 (4), pp. 2336–2342, 2015. @article{Lezama2015, title = {Indirect-to-Direct Band Gap Crossover in Few-Layer MoTe2}, author = {I. Gutierrez Lezama and A. Arora and A. Ubaldini and C. Barreteau and E. Giannini and M. Potemski and A. F. Morpurgo}, editor = {ACS}, url = {http://pubs.acs.org/doi/full/10.1021/nl5045007}, doi = {10.1021/nl5045007}, year = {2015}, date = {2015-03-24}, journal = {Nano Letters}, volume = {15}, number = {4}, pages = {2336–2342}, abstract = {We study the evolution of the band gap structure in few-layer MoTe2 crystals, by means of low-temperature microreflectance (MR) and temperature-dependent photoluminescence (PL) measurements. The analysis of the measurements indicate that in complete analogy with other semiconducting transition metal dichalchogenides (TMDs) the dominant PL emission peaks originate from direct transitions associated with recombination of excitons and trions. When we follow the evolution of the PL intensity as a function of layer thickness, however, we observe that MoTe2 behaves differently from other semiconducting TMDs investigated earlier. Specifically, the exciton PL yield (integrated PL intensity) is identical for mono and bilayer, decreases slightly for trilayer, and it is significantly lower in the tetralayer. The analysis of this behavior and of all our experimental observations is fully consistent with mono and bilayer MoTe2 being direct band gap semiconductors with tetralayer MoTe2 being an indirect gap semiconductor and with trilayers having nearly identical direct and indirect gaps. This conclusion is different from the one reached for other recently investigated semiconducting transition metal dichalcogenides for which monolayers are found to be direct band gap semiconductors, and thicker layers have indirect band gaps that are significantly smaller (by hundreds of meV) than the direct gap. We discuss the relevance of our findings for experiments of fundamental interest and possible future device applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We study the evolution of the band gap structure in few-layer MoTe2 crystals, by means of low-temperature microreflectance (MR) and temperature-dependent photoluminescence (PL) measurements. The analysis of the measurements indicate that in complete analogy with other semiconducting transition metal dichalchogenides (TMDs) the dominant PL emission peaks originate from direct transitions associated with recombination of excitons and trions. When we follow the evolution of the PL intensity as a function of layer thickness, however, we observe that MoTe2 behaves differently from other semiconducting TMDs investigated earlier. Specifically, the exciton PL yield (integrated PL intensity) is identical for mono and bilayer, decreases slightly for trilayer, and it is significantly lower in the tetralayer. The analysis of this behavior and of all our experimental observations is fully consistent with mono and bilayer MoTe2 being direct band gap semiconductors with tetralayer MoTe2 being an indirect gap semiconductor and with trilayers having nearly identical direct and indirect gaps. This conclusion is different from the one reached for other recently investigated semiconducting transition metal dichalcogenides for which monolayers are found to be direct band gap semiconductors, and thicker layers have indirect band gaps that are significantly smaller (by hundreds of meV) than the direct gap. We discuss the relevance of our findings for experiments of fundamental interest and possible future device applications. | |
Julien Levallois, Ievgeniia O. Nedoliuk, Iris Crassee; Alexey B. Kuzmenko Magneto-optical Kramers-Kronig analysis Journal Article Review of Scientific Instruments, 86 (033906), 2015. @article{Levallois2015, title = {Magneto-optical Kramers-Kronig analysis}, author = {Julien Levallois, Ievgeniia O. Nedoliuk, Iris Crassee and Alexey B. Kuzmenko}, url = {http://scitation.aip.org/content/aip/journal/rsi/86/3/10.1063/1.4914846}, year = {2015}, date = {2015-03-17}, journal = {Review of Scientific Instruments}, volume = {86}, number = {033906}, abstract = {We describe a simple magneto-optical experiment and introduce a magneto-optical Kramers-Kronig analysis (MOKKA) that together allow extracting the complex dielectric function for left- and righthanded circular polarizations in a broad range of frequencies without actually generating circularly polarized light. The experiment consists of measuring reflectivity and Kerr rotation, or alternatively transmission and Faraday rotation, at normal incidence using only standard broadband polarizers without retarders or quarter-wave plates. In a common case, where the magneto-optical rotation is small (below ∼0.2 rad), a fast measurement protocol can be realized, where the polarizers are fixed at 45◦ with respect to each other. Apart from the time-effectiveness, the advantage of this protocol is that it can be implemented at ultra-high magnetic fields and in other situations, where an in-situ polarizer rotation is difficult. Overall, the proposed technique can be regarded as a magneto-optical generalization of the conventional Kramers-Kronig analysis of reflectivity on bulk samples and the Kramers-Kronig constrained variational analysis of more complex types of spectral data. We demonstrate the application of this method to the textbook semimetals bismuth and graphite and also use it to obtain handedness-resolved magneto-absorption spectra of graphene on SiC.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We describe a simple magneto-optical experiment and introduce a magneto-optical Kramers-Kronig analysis (MOKKA) that together allow extracting the complex dielectric function for left- and righthanded circular polarizations in a broad range of frequencies without actually generating circularly polarized light. The experiment consists of measuring reflectivity and Kerr rotation, or alternatively transmission and Faraday rotation, at normal incidence using only standard broadband polarizers without retarders or quarter-wave plates. In a common case, where the magneto-optical rotation is small (below ∼0.2 rad), a fast measurement protocol can be realized, where the polarizers are fixed at 45◦ with respect to each other. Apart from the time-effectiveness, the advantage of this protocol is that it can be implemented at ultra-high magnetic fields and in other situations, where an in-situ polarizer rotation is difficult. Overall, the proposed technique can be regarded as a magneto-optical generalization of the conventional Kramers-Kronig analysis of reflectivity on bulk samples and the Kramers-Kronig constrained variational analysis of more complex types of spectral data. We demonstrate the application of this method to the textbook semimetals bismuth and graphite and also use it to obtain handedness-resolved magneto-absorption spectra of graphene on SiC. | |
Pascal Schouwink; Adrien Ramel; Enrico Giannini; Radovan Černý Flux-assisted single crystal growth and heteroepitaxy of perovskite-type mixed-metal borohydrides Journal Article CrystEngComm, 17 , pp. 2682-2689, 2015. @article{Schouwink2015, title = {Flux-assisted single crystal growth and heteroepitaxy of perovskite-type mixed-metal borohydrides}, author = {Pascal Schouwink and Adrien Ramel and Enrico Giannini and Radovan Černý}, url = {http://pubs.rsc.org/en/Content/ArticleLanding/2015/CE/C5CE00135H#!divAbstract}, doi = {10.1039/C5CE00135H}, year = {2015}, date = {2015-02-20}, journal = {CrystEngComm}, volume = {17}, pages = {2682-2689}, abstract = {Structural investigations on mixed-metal borohydrides have been the subject of powder diffraction since the discovery that hydrogen-release temperatures can be tailored by using more electronegative metals. The lack of producing suitable samples for single crystal X-ray diffraction has defined powder diffraction as the method of choice which, however, is less sensitive to the structural details of the compounds in question. Here we show how to overcome this limitation by developing a flux-assisted single crystal growth procedure to lower the melting point of mixed-metal compounds that are thermally unstable and usually decompose before melting, or are unstable in the melt. We prove the validity of this principle on a member of the recently reported perovskite-type class of borohydrides and show that the defined approach is easily generalized. Interesting structural details are revealed that stand in contrast to the results obtained from samples produced by mechano-chemistry. The differences in lattice instabilities are discussed and put into context with the discovered epitaxial relationships between rocksalt-type ABH4 and perovskite-type ACa(BH4)3 (A = alkaline metal). In this context, the preliminary results provide a valuable scheme that can be made use of when physical deposition of metal borohydrides reaches its working stage.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Structural investigations on mixed-metal borohydrides have been the subject of powder diffraction since the discovery that hydrogen-release temperatures can be tailored by using more electronegative metals. The lack of producing suitable samples for single crystal X-ray diffraction has defined powder diffraction as the method of choice which, however, is less sensitive to the structural details of the compounds in question. Here we show how to overcome this limitation by developing a flux-assisted single crystal growth procedure to lower the melting point of mixed-metal compounds that are thermally unstable and usually decompose before melting, or are unstable in the melt. We prove the validity of this principle on a member of the recently reported perovskite-type class of borohydrides and show that the defined approach is easily generalized. Interesting structural details are revealed that stand in contrast to the results obtained from samples produced by mechano-chemistry. The differences in lattice instabilities are discussed and put into context with the discovered epitaxial relationships between rocksalt-type ABH4 and perovskite-type ACa(BH4)3 (A = alkaline metal). In this context, the preliminary results provide a valuable scheme that can be made use of when physical deposition of metal borohydrides reaches its working stage. | |
Jérémie Teyssier; Suzanne V. Saenko; Dirk van der Marel; Michel C. Milinkovitch Photonic crystals cause active colour change in chameleons Journal Article Nat Commun, 6 , 2015. @article{teyssier_photonic_2015, title = {Photonic crystals cause active colour change in chameleons}, author = {Jérémie Teyssier and Suzanne V. Saenko and Dirk van der Marel and Michel C. Milinkovitch}, url = {http://dx.doi.org/10.1038/ncomms7368}, year = {2015}, date = {2015-01-01}, journal = {Nat Commun}, volume = {6}, abstract = {Many chameleons, and panther chameleons in particular, have the remarkable ability to exhibit complex and rapid colour changes during social interactions such as male contests or courtship. It is generally interpreted that these changes are due to dispersion/aggregation of pigment-containing organelles within dermal chromatophores. Here, combining microscopy, photometric videography and photonic band-gap modelling, we show that chameleons shift colour through active tuning of a lattice of guanine nanocrystals within a superficial thick layer of dermal iridophores. In addition, we show that a deeper population of iridophores with larger crystals reflects a substantial proportion of sunlight especially in the near-infrared range. The organization of iridophores into two superposed layers constitutes an evolutionary novelty for chameleons, which allows some species to combine efficient camouflage with spectacular display, while potentially providing passive thermal protection.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Many chameleons, and panther chameleons in particular, have the remarkable ability to exhibit complex and rapid colour changes during social interactions such as male contests or courtship. It is generally interpreted that these changes are due to dispersion/aggregation of pigment-containing organelles within dermal chromatophores. Here, combining microscopy, photometric videography and photonic band-gap modelling, we show that chameleons shift colour through active tuning of a lattice of guanine nanocrystals within a superficial thick layer of dermal iridophores. In addition, we show that a deeper population of iridophores with larger crystals reflects a substantial proportion of sunlight especially in the near-infrared range. The organization of iridophores into two superposed layers constitutes an evolutionary novelty for chameleons, which allows some species to combine efficient camouflage with spectacular display, while potentially providing passive thermal protection. | |
M. Guarise; B. Dalla Piazza; H. Berger; E. Giannini; T. Schmitt; H. M. Rønnow; G. A. Sawatzky; J. van den Brink; D. Altenfeld; I. Eremin; M. Grioni Anisotropic softening of magnetic excitations along the nodal direction in superconducting cuprates Journal Article Nature Communications, 5 , 2014. @article{guarise_anisotropic_2014, title = {Anisotropic softening of magnetic excitations along the nodal direction in superconducting cuprates}, author = {M. Guarise and B. Dalla Piazza and H. Berger and E. Giannini and T. Schmitt and H. M. Rønnow and G. A. Sawatzky and J. van den Brink and D. Altenfeld and I. Eremin and M. Grioni}, url = {http://www.nature.com/ncomms/2014/141218/ncomms6760/full/ncomms6760.html}, year = {2014}, date = {2014-12-18}, urldate = {2015-02-19}, journal = {Nature Communications}, volume = {5}, abstract = {The high-Tc cuprate superconductors are close to antiferromagnetic order. Recent measurements of magnetic excitations have reported an intriguing similarity to the spin waves—magnons—of the antiferromagnetic insulating parent compounds, suggesting that magnons may survive in damped, broadened form throughout the phase diagram. Here we show by resonant inelastic X-ray scattering on Bi2Sr2CaCu2O8+δ (Bi-2212) that the analogy with spin waves is only partial. The magnon-like features collapse along the nodal direction in momentum space and exhibit a photon energy dependence markedly different from the Mott-insulating case. These observations can be naturally described by the continuum of charge and spin excitations of correlated electrons. The persistence of damped magnons could favour scenarios for superconductivity built from quasiparticles coupled to spin fluctuations. However, excitation spectra composed of particle–hole excitations suggest that superconductivity emerges from a coherent treatment of electronic spin and charge in the form of quasiparticles with very strong magnetic correlations.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The high-Tc cuprate superconductors are close to antiferromagnetic order. Recent measurements of magnetic excitations have reported an intriguing similarity to the spin waves—magnons—of the antiferromagnetic insulating parent compounds, suggesting that magnons may survive in damped, broadened form throughout the phase diagram. Here we show by resonant inelastic X-ray scattering on Bi2Sr2CaCu2O8+δ (Bi-2212) that the analogy with spin waves is only partial. The magnon-like features collapse along the nodal direction in momentum space and exhibit a photon energy dependence markedly different from the Mott-insulating case. These observations can be naturally described by the continuum of charge and spin excitations of correlated electrons. The persistence of damped magnons could favour scenarios for superconductivity built from quasiparticles coupled to spin fluctuations. However, excitation spectra composed of particle–hole excitations suggest that superconductivity emerges from a coherent treatment of electronic spin and charge in the form of quasiparticles with very strong magnetic correlations. | |
Michael Tran Possible phase transitions probed by infrared spectroscopy under high pressure: BiTeI and Li0.9Mo6O17 PhD Thesis 2014. @phdthesis{Tran2014, title = {Possible phase transitions probed by infrared spectroscopy under high pressure: BiTeI and Li0.9Mo6O17}, author = {Michael Tran}, year = {2014}, date = {2014-12-16}, keywords = {}, pubstate = {published}, tppubtype = {phdthesis} } | |
M. P. M. Dean; A. J. A. James; A. C. Walters; V. Bisogni; I. Jarrige; M. H"ucker; E. Giannini; M. Fujita; J. Pelliciari; Y. B. Huang; R. M. Konik; T. Schmitt; J. P. Hill Itinerant effects and enhanced magnetic interactions in Bi-based multilayer cuprates Journal Article Phys. Rev. B, 90 , pp. 220506, 2014. @article{PhysRevB.90.220506, title = {Itinerant effects and enhanced magnetic interactions in Bi-based multilayer cuprates}, author = {M. P. M. Dean and A. J. A. James and A. C. Walters and V. Bisogni and I. Jarrige and M. H"ucker and E. Giannini and M. Fujita and J. Pelliciari and Y. B. Huang and R. M. Konik and T. Schmitt and J. P. Hill}, url = {http://link.aps.org/doi/10.1103/PhysRevB.90.220506}, year = {2014}, date = {2014-12-04}, journal = {Phys. Rev. B}, volume = {90}, pages = {220506}, publisher = {American Physical Society}, keywords = {}, pubstate = {published}, tppubtype = {article} } | |
A Stucky; A Ubaldini; J Levallois; M K Tran; D van der Marel; E Giannini Shining light on CuO for exploring high-Tc multiferroics Journal Article Journal of Physics: Conference Series, 566 (1), pp. 012012, 2014. @article{1742-6596-566-1-012012, title = {Shining light on CuO for exploring high-Tc multiferroics}, author = {A Stucky and A Ubaldini and J Levallois and M K Tran and D van der Marel and E Giannini}, url = {http://stacks.iop.org/1742-6596/566/i=1/a=012012}, year = {2014}, date = {2014-12-01}, journal = {Journal of Physics: Conference Series}, volume = {566}, number = {1}, pages = {012012}, abstract = {Searching for a 3D multiferroic material with a strong magnetoelectric coupling and high critical temperature is a major challenge in modern condensed matter research. CuO is the building block of high-temperature superconductors and triggered a new interest when it was established as potential high temperature multiferroic. We have succeeded in growing high quality single crystals of CuO with two different methods, namely the floating zone under high oxygen pressure and the chemical vapor transport growth. The fact that we are able to grow crystals of the same compound by different techniques makes it possible to study the effect of slightly different chemical compositions, various kinds of defects and variable strain on the final properties of the compound. Optical spectroscopy has been deployed to study the optical response of cupric oxide. Thereby we achieved a deeper insight of the optical, electronic and structural properties by measuring the infrared reflectivity under a magnetic field and the Raman shift under hydrostatic high pressure.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Searching for a 3D multiferroic material with a strong magnetoelectric coupling and high critical temperature is a major challenge in modern condensed matter research. CuO is the building block of high-temperature superconductors and triggered a new interest when it was established as potential high temperature multiferroic. We have succeeded in growing high quality single crystals of CuO with two different methods, namely the floating zone under high oxygen pressure and the chemical vapor transport growth. The fact that we are able to grow crystals of the same compound by different techniques makes it possible to study the effect of slightly different chemical compositions, various kinds of defects and variable strain on the final properties of the compound. Optical spectroscopy has been deployed to study the optical response of cupric oxide. Thereby we achieved a deeper insight of the optical, electronic and structural properties by measuring the infrared reflectivity under a magnetic field and the Raman shift under hydrostatic high pressure. | |
E Cappelluti; L Benfatto; A B Kuzmenko Infrared phonon activity and Fano interference in multilayer graphenes Journal Article Physica Scripta, T162 , pp. 014018, 2014. @article{Cappelluti2014, title = {Infrared phonon activity and Fano interference in multilayer graphenes}, author = {E Cappelluti and L Benfatto and A B Kuzmenko}, doi = {10.1088/0031-8949/2014/t162/014018}, year = {2014}, date = {2014-09-19}, journal = {Physica Scripta}, volume = {T162}, pages = {014018}, abstract = {Recent optical measurements in bilayer graphene have reported a strong dependence on phonon peak intensity, as well on the asymmetric Fano lineshape, on the charge doping and on the bandgap, tuned by gate voltage. In this paper, we show how these features can be analyzed and predicted on a microscopic quantitative level using the charge-phonon theory applied to the specific case of graphene systems. We present a phase diagram where the infrared activity of both the symmetric (Eg) and antisymmetric (Eu) phonon modes is evaluated as a function of doping and gap. We also show how a switching mechanism between these two modes can occur, governing the dominance of the optical response of one mode with respect to the other. The theory presented here can be also generalized to bulk graphite and to multilayer systems with different stacking orders, providing a useful roadmap for the characterization of graphenic systems by optical infrared means.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Recent optical measurements in bilayer graphene have reported a strong dependence on phonon peak intensity, as well on the asymmetric Fano lineshape, on the charge doping and on the bandgap, tuned by gate voltage. In this paper, we show how these features can be analyzed and predicted on a microscopic quantitative level using the charge-phonon theory applied to the specific case of graphene systems. We present a phase diagram where the infrared activity of both the symmetric (Eg) and antisymmetric (Eu) phonon modes is evaluated as a function of doping and gap. We also show how a switching mechanism between these two modes can occur, governing the dominance of the optical response of one mode with respect to the other. The theory presented here can be also generalized to bulk graphite and to multilayer systems with different stacking orders, providing a useful roadmap for the characterization of graphenic systems by optical infrared means. | |
Ana Akrap; Alberto Ubaldini; Enrico Giannini; László Forró Bi2Te(3−x)Se(x) series studied by resistivity and thermopower Journal Article EPL (Europhysics Letters), 107 (5), pp. 57008, 2014. @article{0295-5075-107-5-57008, title = {Bi2Te(3−x)Se(x) series studied by resistivity and thermopower}, author = {Ana Akrap and Alberto Ubaldini and Enrico Giannini and László Forró}, url = {http://dx.doi.org/10.1209/0295-5075/107/57008 http://archive-ouverte.unige.ch/unige:40713 http://arxiv.org/abs/1210.3901}, year = {2014}, date = {2014-09-08}, journal = {EPL (Europhysics Letters)}, volume = {107}, number = {5}, pages = {57008}, abstract = {We study the detailed temperature and composition dependence of the resistivity, ##IMG## [http://ej.iop.org/images/0295-5075/107/5/57008/epl16520ieqn1.gif] $rho(T)$ , and thermopower, S ( T ), for a series of layered bismuth chalcogenides Bi 2 Te 3− x Se x , and report the stoichiometry dependence of the optical band gap. In the resistivity of the most compensated member, Bi 2 Te 2.1 Se 0.9 , we find a low-temperature plateau whose onset temperature correlates with the high-temperature activation energy. For the whole series S ( T ) can be described by a simple model for an extrinsic semiconductor. By substituting Se for Te, the Fermi level is tuned from the valence band into the conduction band. The maximum values of S ( T ), bulk band gap as well the activation energy in the resistivity are found for ##IMG## [http://ej.iop.org/images/0295-5075/107/5/57008/epl16520ieqn2.gif] $x approx 0.9$ .}, keywords = {}, pubstate = {published}, tppubtype = {article} } We study the detailed temperature and composition dependence of the resistivity, ##IMG## [http://ej.iop.org/images/0295-5075/107/5/57008/epl16520ieqn1.gif] $rho(T)$ , and thermopower, S ( T ), for a series of layered bismuth chalcogenides Bi 2 Te 3− x Se x , and report the stoichiometry dependence of the optical band gap. In the resistivity of the most compensated member, Bi 2 Te 2.1 Se 0.9 , we find a low-temperature plateau whose onset temperature correlates with the high-temperature activation energy. For the whole series S ( T ) can be described by a simple model for an extrinsic semiconductor. By substituting Se for Te, the Fermi level is tuned from the valence band into the conduction band. The maximum values of S ( T ), bulk band gap as well the activation energy in the resistivity are found for ##IMG## [http://ej.iop.org/images/0295-5075/107/5/57008/epl16520ieqn2.gif] $x approx 0.9$ . | |
D. Stricker; J. Mravlje; C. Berthod; R. Fittipaldi; A. Vecchione; A. Georges; D. van der Marel Optical Response of Sr2RuO4 Reveals Universal Fermi-Liquid Scaling and Quasiparticles Beyond Landau Theory Journal Article Phys. Rev. Lett., 113 , pp. 087404, 2014. @article{PhysRevLett.113.087404, title = {Optical Response of Sr2RuO4 Reveals Universal Fermi-Liquid Scaling and Quasiparticles Beyond Landau Theory}, author = {D. Stricker and J. Mravlje and C. Berthod and R. Fittipaldi and A. Vecchione and A. Georges and D. van der Marel}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.113.087404 http://archive-ouverte.unige.ch/unige:39703 http://arxiv.org/abs/1403.5445}, year = {2014}, date = {2014-08-22}, journal = {Phys. Rev. Lett.}, volume = {113}, pages = {087404}, publisher = {American Physical Society}, keywords = {}, pubstate = {published}, tppubtype = {article} } | |
Ignacio Gutiérrez Lezama; Alberto Ubaldini; Maria Longobardi; Enrico Giannini; Christoph Renner; Alexey B Kuzmenko; Alberto F Morpurgo Surface transport and band gap structure of exfoliated 2H-MoTe2 crystals Journal Article 2D Materials, 1 (2), pp. 021002, 2014. @article{2053-1583-1-2-021002, title = {Surface transport and band gap structure of exfoliated 2H-MoTe2 crystals}, author = {Ignacio Gutiérrez Lezama and Alberto Ubaldini and Maria Longobardi and Enrico Giannini and Christoph Renner and Alexey B Kuzmenko and Alberto F Morpurgo}, url = {http://stacks.iop.org/2053-1583/1/i=2/a=021002 http://arxiv.org/abs/1407.1219}, year = {2014}, date = {2014-08-06}, journal = {2D Materials}, volume = {1}, number = {2}, pages = {021002}, abstract = {Semiconducting transition metal dichalcogenides (TMDs) have emerged as materials that can be used to realize two-dimensional (2D) crystals possessing rather unique transport and optical properties. Most research has so far focused on sulfur and selenium compounds, while tellurium-based materials have attracted little attention so far. As a first step in the investigation of Te-based semiconducting TMDs in this context, we have studied MoTe 2 crystals with thicknesses above 4 nm, focusing on surface transport and a quantitative determination of the gap structure. Using ionic-liquid gated transistors, we show that ambipolar transport at the surface of the material is reproducibly achieved, with hole and electron mobility values between 10 and 30 cm 2 V −1 s −1 at room temperature. The gap structure is determined through three different techniques: ionic-liquid gated transistors and scanning tunneling spectroscopy, which allow the measurement of the indirect gap ( E ind ), and optical transmission spectroscopy on crystals of different thickness, which enables the determination of both the direct ( E dir ) and the indirect gap. We find that at room temperature E ind = 0.88 eV and E dir = 1.02 eV. Our results suggest that thin MoTe 2 layers may exhibit a transition to a direct gap before mono-layer thickness. They should also drastically extend the range of direct gaps accessible in 2D semiconducting TMDs.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Semiconducting transition metal dichalcogenides (TMDs) have emerged as materials that can be used to realize two-dimensional (2D) crystals possessing rather unique transport and optical properties. Most research has so far focused on sulfur and selenium compounds, while tellurium-based materials have attracted little attention so far. As a first step in the investigation of Te-based semiconducting TMDs in this context, we have studied MoTe 2 crystals with thicknesses above 4 nm, focusing on surface transport and a quantitative determination of the gap structure. Using ionic-liquid gated transistors, we show that ambipolar transport at the surface of the material is reproducibly achieved, with hole and electron mobility values between 10 and 30 cm 2 V −1 s −1 at room temperature. The gap structure is determined through three different techniques: ionic-liquid gated transistors and scanning tunneling spectroscopy, which allow the measurement of the indirect gap ( E ind ), and optical transmission spectroscopy on crystals of different thickness, which enables the determination of both the direct ( E dir ) and the indirect gap. We find that at room temperature E ind = 0.88 eV and E dir = 1.02 eV. Our results suggest that thin MoTe 2 layers may exhibit a transition to a direct gap before mono-layer thickness. They should also drastically extend the range of direct gaps accessible in 2D semiconducting TMDs. | |
Davide Forcella; Jan Zaanen; Davide Valentinis; Dirk van der Marel Electromagnetic properties of viscous charged fluids Journal Article Phys. Rev. B, 90 , pp. 035143, 2014. @article{PhysRevB.90.035143, title = {Electromagnetic properties of viscous charged fluids}, author = {Davide Forcella and Jan Zaanen and Davide Valentinis and Dirk van der Marel}, url = {http://link.aps.org/doi/10.1103/PhysRevB.90.035143 http://archive-ouverte.unige.ch/unige:39150 http://arxiv.org/abs/1406.1356}, year = {2014}, date = {2014-07-30}, journal = {Phys. Rev. B}, volume = {90}, pages = {035143}, publisher = {American Physical Society}, keywords = {}, pubstate = {published}, tppubtype = {article} } | |
Dirk van der Marel; George Albert Sawatzky An optical twist for triplet superconductors Journal Article Science, 345 (6193), pp. 138-139, 2014. @article{vanderMarel11072014, title = {An optical twist for triplet superconductors}, author = {Dirk van der Marel and George Albert Sawatzky}, url = {http://www.sciencemag.org/content/345/6193/138.short}, year = {2014}, date = {2014-07-11}, journal = {Science}, volume = {345}, number = {6193}, pages = {138-139}, keywords = {}, pubstate = {published}, tppubtype = {article} } |