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Nernst effect of epitaxial Y0.95Ca0.05Ba2(Cu1-xZnx)3Oy and Y0.9Ca0.1Ba2Cu3Oy films

In an influential paper, Xu et al.1 reported the existence of a sizeable Nernst signal over a broad temperature range above the superconducting transition temperature (Tc) for single crystals of La2− xSrxCuO4. The effect is particularly pronounced for underdoped samples, in the pseudogap region of the cuprate phase diagram. They argued that the quasiparticle contribution to the Nernst signal should be negligible and interpreted their findings as evidence for existence of vortex-like excitations above Tc. These Nernst effect measurements have inspired extensions to the theory of superconducting fluctuations in the cuprates. Thus, Ussishkin et al.2 calculate the contributions of the Gaussian superconducting fluctuations to the transverse thermoelectric response above Tc. They find that the Gaussian fluctuations are su&plusmn ; cient to explain the Nernst effect in optimally doped and overdoped samples whereas for the underdoped regime they suggest that stronger non-Gaussian fluctuations reduce the mean-field transition temperature. The Gaussian fluctuation model also gives good agreement with experiments on thin amorphous low-temperature superconductors.3 Recently the Nernst effect has also been used to study other correlated electron systems. We report the measurements of in-plane resistivity, magnetoresistivity, Hall coe&plusmn ; cient, and Nernst signal of slightly under- and nearly optimally-doped Y0.95Ca0.05Ba2(Cu1− x Znx)3Oy (with x = 0, 0.02, 0.04) and overdoped Y0.9Ca0.1Ba2Cu3Oy thin films grown by pulsed laser deposition. We discuss the interpretation of the Nernst results in terms of superconducting fluctuations2 and its relation to the vortex scenario proposed by Xu et al.1 We also comment on the implications of the Nernst analysis in relation to the pseudogap. The pseudogap temperature T¤, determined from the temperature dependence of resistivity, does not change significantly with Zn doping, while the onset temperature Tn of the anomalous Nernst signal above Tc, decreases sharply as the Tc does and is not related toT¤. In the magnetoresistivity data, 4 the strong T− 4 dependence of magnetoresistivity in the high temperature region, is ascribed to the normal orbital magnetoresistance (i.e the effect of the Lorentz force on the motion of quasiparticles). This suggests that superconducting fluctuations only become important below Tc + 25 K, where there are strong upward deviations from T− 4 behavior. For all samples studied the Nernst signal in this temperature region is entirely consistent with the theory of Ussishkin et al.2 for Gaussian fluctuations. ¤ Corresponding author ; email address: kivan@phy.hr 1 Z.A. Xu, N.P. Ong, Y. Wang, T. Kakeshita, and S. Uchida, Nature (London) 406, 486 (2000). 2 I. Ussishkin, S. L. Sondhi, and D. A. Huse, Phys. Rev. Lett. 89, 287001 (2002). 3 A. Pourret, H. Aubin, J. Lesueur, C. A. Marrache-Kikuchi, L. Berge, L. Dumoulin, and K. Behnia, Nature Phys. 2, 683 (2006). 4 I. Kokanovic, J. Cooper, S. Naqib, R. Islam, R.A. Chakalov, Phys. Rev. B. 73, 184509 (2006).

Nernst effect; Superconductivity; Correlated electron systems

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