Thermodynamic and magnetic properties of oxide superconductors

Thermodynamic measurements on high Tc superconductors are of utmost importance since they reflect true bulk properties in contrast to e.g. transport, diamagnetic shielding or even Meissner effect measurements. The peculiarities of the observed specific heat anomalies near Tc appear to be particularly significant. Well ordered YBa2Cu3O7 exhibits an unmistakable second order transition (quantitatively in agreement with the measured thermodynamic critical field), the magnitude of the discontinuity ΔC (Tc), however, indicates a strong sample dependence of the effective superconducting volume fraction. Various kinds of disorder, such as minor substitutions on the copper sites or oxygen deficiency progressively lead to an apparent loss of the mean field type phase transition. These effects may be viewed as a consequence of the intrinsically short and anisotropic coherence length. The behavior of Bi2Sr2CaCu2O8-type material appears extreme in the above sense, partly due to a very low density of states, whereas T1-based compounds may be an intermediate case. An analysis of the normal state magnetic susceptibility of the present metallic oxides is still somewhat uncertain. One of the difficulties consists in estimating the effect of reduced-dimensional short range order of fluctuating copper magnetic moments. A recent study of the compound CuO helps to illustrate such situations. Finally, the question of strong or weak coupling is addressed for ideal YBa2Cu3O7