The electronic contribution to the specific heat of NdBa2Cu3O6+x

U. Tutsch; P. Schweiss; H. Wühl; B. Obst; Th. Wolf

doi:10.1140/epjb/e2004-00340-5

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Condensed Matter and Complex Systems

Eur. Phys. J. B 41, 471-478 (2004)
https://doi.org/10.1140/epjb/e2004-00340-5

The electronic contribution to the specific heat of NdBa₂Cu₃O_6+x

U. Tutsch¹^a, P. Schweiss², H. Wühl¹^,3, B. Obst¹ and Th. Wolf²

¹ Forschungszentrum Karlsruhe, Institut für Technische Physik, 76021 Karlsruhe, Germany
² Forschungszentrum Karlsruhe, Institut für Festkörperphysik, 76021 Karlsruhe, Germany
³ Universität Karlsruhe, IEKP, 76128 Karlsruhe, Germany

Corresponding author: ^a ulrich.tutsch@web.de

Received: 28 October 2003
Revised: 17 September 2004
Published online: 5 November 2004

Abstract

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.