Day / Time
Date(s) - 18/10/2016
13 h 00 min - 14 h 00 min
Tuesday, October 18th 2016 – 13h00
Probing the change of Coulomb energy through a superconducting phase transition
Michaël Tran (group of Prof. Van der Marel)
It was postulated by Leggett that the saving of the energy stored in the Coulomb interaction between the conduction electrons, and specifically the part associated with long wavelengths and mid-infrared frequencies, is the main driver of the superconducting transition in the cuprates (the “MIR scenario”). To obtain information on the Coulomb interaction, on the temperature dependence thereof, and on its variation when Cooper-pairs are formed, we performed temperature-dependent ellipsometry measurements on Bi2Sr2CaCu2O8-x single crystals: under-doped with TC = 60, 70 and 83 K, optimally doped with TC = 91 K, overdoped with TC = 81, 70 and 58 K, as well as optimally doped Bi2Sr2Ca2Cu3O10+x with TC=110 K.
Our observations are that, as the temperature drops through TC, the integral of the loss function in the range up to 2 eV displays an abrupt 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. Based on our results we arrived at the conclusion that, while the saving of Coulomb energy for small momentum is an important factor in the energy balance, it is not the only one driving the mechanism of pair formation.
Strain tuning of a Metal-Insulator Transition for SrVO3 films
Elías Ferreiro-Vila (group of Prof. Triscone)
SrVO3 is a 3d1 metal with cubic perovskite structure in bulk. A recent study by in situ photoemission spectroscopy has revealed that ultrathin SrVO3 (SVO) films undergo a thickness-driven Metal-insulator Transition (MIT) [1,2]; the ground state for 2 unit cell thick layers is of Mott-Hubbard nature, i.e. the insulatingstate seems to be driven by electronic correlations, probably enhanced by the reduced dimensionality.
Dynamical mean field theory (DMFT) calculations, performed in the Held group, show that this MIT is very sensitive to low variations of epitaxial strain and thickness .
We report the control of this MIT by epitaxial strain. For this study, high quality SVO thin films were grown by Pulsed Laser Deposition on a variety of substrates with a complete structural and morphological characterization. Transport and magnetotransport analysis indicates the combined role of electronelectron correlations and disorder-induced electron localization as the driving force for this MIT.
 M. Gu, S. A. Wolf and J. Lu, Adv. Mat. Interfaces 1, 1300126 (2014)
Z. Zhong, M. Wallerberger, J. M. Tomczak, C. Taranto, N. Parragh, A. Toschi, G. Sangiovanni and K. Held, PRL
114, 246401 (2015).
Forum Committee : M.Gibert, A.Tamai, N.Ubrig (11.10.2016)