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Gaël Grissonnanche - Université de Sherbrooke, Québec

Direct measurement of the upper critical field Hc2 in a cuprate superconductor

The upper critical field Hc2 in a superconductor is a fundamental measure of the pairing strength. Among other things, it gives access to the condensation energy of the superconducting state. In cuprate superconductors, there is no agreement on the amplitude of
Hc2, because of their complex vortex state. Indeed, the presence of an extended vortex-liquid phase in cuprates makes resistivity an uncertain probe of Hc2 in these materials. In this seminar, I will present the first direct measurement of Hc2 in a cuprate superconductor, using thermal conductivity measurements on YBCO and Tl-2201 in magnetic fields up to 45 Tesla. The data reveal that there is no vortex-liquid phase in the zero-temperature limit. As a result, we can use resistivity in the T=0 limit to obtain Hc2. We have performed extensive resistivity measurements up to 80 T to map out the doping dependence of Hc2 in YBCO and Tl-2201, covering the entire phase diagram. The resulting H-p phase diagram immediately reveals the role of phase competition in shaping the superconducting phase of hole-doped cuprates.