Mise en page 1 - Laboratoire National des Champs Magnétiques ...

2009 METALS, SUPERCONDUCTORS...Thermo-electric study of Fermi surface reconstruction in YBa 2 Cu 3 O yThe Seebeck and Nernst coefficients S and ν of the high-T c superconductor YBa 2 Cu 3 O y (YBCO) were measured ina single crystal with a hole concentration p = 0.12 in magneticfields up to H = 28 T. For temperatures down to 9 K, νbecomes independent of field by H ≃ 30 T, showing that bythen the Nernst signal due to superconducting fluctuationshas become negligible. In this field-induced normal state,S/T and ν/T are both large and negative in the T → 0 limit.The magnitude of S/T is consistent with the small Fermisurface pocket previously detected via quantum oscillationsin YBCO at a similar doping and its negative sign confirmsthat the pocket is electron-like. For more information see[J. Chang et al., arXiv:0907.5039].Our Nernst measurements in YBCO reveal that the quasiparticlecontribution in cuprates can be as large as the vortexcontribution, on which most of the attention has been focuseduntil now, see for example [Yayu Wang et al., Phys.Rev. B 73, 024510 (2006)]. We expect this quasiparticlecontribution, which can be of either sign, to dominate theNernst signal well above T c , as found in the hole-dopedcuprate Eu-LSCO [O. Cyr-Choiniere et al., Nature 458, 743(2009)] and the electron-doped cuprate Pr 2−x Ce x CuO 4 [P.Li and R.L. Greene, Phys. Rev. B 76, 174512 (2007)],where quasiparticle and vortex signals have also been disentangled.The sample was an uncut, unpolished, detwinned crystal ofYBCO grown, at UBC in Vancouver (Canada), in a nonreactiveBaZrO 3 crucible from high-purity starting materials.The dopant oxygen atoms (y = 6.67) were made to orderinto an ortho-VIII superstructure, yielding a superconductingtransition temperature T c = 66.0 K. Transport propertieswere measured via gold evaporated contacts (resistance< 1 Ω), in a six-contact geometry. The thermal gradient∆T was applied along the a-axis and the field H along thec-axis. For a detailed describtion of the experimental setupsee [O. Cyr-Choiniere et al., Nature 458, 743 (2009)].The Nernst and Seebeck coefficients are plotted as a functionof magnetic field in Fig. 1. At T >80 K, the Seebeckcoefficient S is essentially field independent. At T