The electronic contribution to the specific heat of NdBa $\mathsf{_2}\mathsf{Cu_3}\mathsf{O_{6 + x}}$

From measurements of the specific heat of $\mathrm{NdBa_2Cu_3O_{6 + \mathnormal{x}}}$ in the temperature range between 20 K and 300 K the electronic contribution C e (T)/T has been derived. The results depend strongly on the assumptions made for the normal-state reference, especially the phonon contribution. Taking into account entropy conservation between the superconductor and a hypothetical normal-state reference, we found a temperature independent electronic contribution of this normal-state reference without any sign of a pseudogap for both optimum doped and underdoped samples. For oxygen concentrations between x=0.79 and x=0.89 a broad hump in C e (T)/T is observed around 120 K, which we ascribe to pair formation above T c . The dependence of the hole concentration n h in the copper oxide planes on the oxygen concentration x in the copper oxide chains was calculated by means of bond-valence sums. We found that the optimum doping of the copper oxide planes is n h,opt =0.24 for $\mathrm{\mathnormal{R}Ba_2Cu_3O_{6 + \mathnormal{x}}}$ (R=Nd, Y) irrespective of the element on the rare-earth site. Copyright Springer-Verlag Berlin/Heidelberg 2004