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espite twenty years of research, the phase diagram of high temperature superconductors remains enigmatic. A central issue is the origin of the differences in the physical properties of these copper oxides doped to opposite sides of the superconducting region. In the overdoped regime, the material behaves as a reasonably conventional metal, with a large Fermi surface. The underdoped regime, however, is highly anomalous and appears to have no coherent Fermi surface, but only disconnected 'Fermi arcs'. We have reported the observation of quantum oscillations in the electrical resistance and in the magnetization of the underdoped YBa2Cu3O6.5 [1,2] and YBa2Cu4O8 [3] compound, establishing the existence of a coherent closed Fermi surface at low temperature in the underdoped side of the phase diagram of cuprates, once superconductivity is suppressed by a large magnetic field. The low oscillation frequency reveals a Fermi surface made of small pockets pointing to a carrier density given by p (the number of hole per Cu ion). These results are in contrast to the large frequency observed in overdoped Tl2Ba2CuO6+δ [4] pointing to a Fermi surface made of one large cylinder comprising 1+p quasiparticles. In addition, the negative sign of the Hall effect at low temperature in the underdoped side reveals that these small pockets are electron-like rather than hole-like. We propose that these electron pockets most probably arise from a reconstruction of the Fermi surface [5]. Work in collaboration with J. Levallois, C.Jaudet, B. Vignolle, D. Vignolles and A. Audouard from the LNCMP, N Doiron-Leyraud, D. LeBoeuf and L. Taillefer from the University of Sherbrooke, A. Bangura, A. Carrington and N. Hussey from the University of Bristol and R. Liang, D. Bonn, W. Hardy from the University of British Columbia. [1] N. Doiron-Leyraud et al, Nature 447, 565 (2007) [2] C. Jaudet et al, Phys. Rev. Lett 100, 187005 (2008) [3] A. Bangura et al, Phys. Rev. Lett 100, 047004 (2008) [4] B. Vignolle et al, to be published [5] D. LeBoeuf et al, Nature 450, 533 (2007) |
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