Changes in the in- and out-of-plane magnetic susceptibility of YBCO crystals with temperature and hole content

We report new magnetic susceptibility and anisotropy data for single crystals of YBa2Cu3O6+x from x = 0.38 to 0.95. We argue that the temperature (T) dependence of the magnetic anisotropy in the normal state well above T-c arises from a combination of the pseudogap and substantial g-factor anisotropy. With this interpretation we obtain onset temperatures for superconducting fluctuations over a wide range of x. For hole concentrations p < 0.14 there are two extra contributions to the magnetic susceptibility and its anisotropy. We analyse these in terms of changes in the Van Vleck paramagnetism plus a significant paramagnetic contribution from an electron pocket at low fields in the region 0.084 < p < 0.135. This is where the superconducting transition temperature T-c(p) deviates from the usual parabolic law and where quantum oscillations and a negative Hall coefficient are observed. Copyright (C) EPLA, 2012