Abstract
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 Tc 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 Tc(p) deviates from the usual parabolic law and where quantum oscillations and a negative Hall coefficient are observed.
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