On the specific heat of the superconductor NdBa2Cu3O6+x for different dopings

The subject of this thesis is the investigation of the specific heat of the high temperature superconductor NdBa2Cu3O6+x for different dopings (calcium, yttrium, zinc) in the temperature range from 20 K to 300 K. In the first part the different contributions to the specific heat are determined. The largest one is caused by the phonons and exceeds the contribution of the conduction electrons by a factor of 100. In the second part the specific heat of the conduction electrons is investigated and properties like the transition temperature Tc, the mean-field jump ΔC/Tc(MF), and the linear normal-state term γ are determined. The contribution of critical fluctuations to the specific heat can be well described within a 3dXY model (calculated by means of spin lattice models). The same result is achieved within the 2dXY model only by using discreet orientations of the spins. At oxygen concentrations in the region of the 60 K-Plateau a broad shoulder is observed in the specific heat at about 100 K which can be interpreted as an effect of the T2*-pseudogap. Indications to the T1*-pseudogap which opens at higher temperatures are not found. To determine the distribution of the charge carriers in the unit cell, the method of bond-valence sums is used. For all properties, obtained from the specific heat, the dependence on the hole concentration nEbene in the CuO2-planes is independent of the system. The optimum doping is at nEbene,opt = 0.24. (orig.)