Nernst effect and disorder in the normal state of high-T_c cuprates

F. Rullier-Albenque¹, R. Tourbot¹, H. Alloul², P. Lejay³, D. Colson¹, A. Forget¹

1 SPEC, Orme des Merisiers, CEA Saclay, 91191 Gif-sur-Yvette cedex (France)
2 Physique des Solides, UMR 8502, CNRS, Universite Paris-Sud 91405, Orsay (France)
3 CRTBT, CNRS, BP 166X, 38042 Grenoble cedex (France)

A large Nernst signal has been recently observed in the non-superconducting state of hole doped cuprates, at temperatures well above the critical temperature Tc. This effect which is particularly pronounced in underdoped samples has been interpreted as evidence for significant superconducting fluctuations surviving up to much higher temperatures than Tc, which has been tentatively connected to the pseudogap [1]. This makes the Nernst effect as a very sensitive test to probe superconducting fluctuations in samples in which Tc has been depressed by introduction of disorder. We will present here new results of Nernst effect measurements in disordered single crystals of optimally doped YBCO7 and underdoped YBCO6.6 . Electron irradiation at low temperature was used to introduce point defects in a controlled manner in the CuO2 planes of these cuprates. These defects are found analogous to Zn substitution on the copper planar site [2]. In both compounds, we observe that the Nernst signal persists above the critical temperature Tc,irr and ends a few degrees (10-20K) above Tc0 of the pure samples. This result fits very well into a scenario short range pairing correlations survive in the normal state of irradiated samples up to Tc0 without long range phase coherence. This is in agreement with our observation of a quasi linear decrease of Tc with defect concentration which cannot be solely explained by pair-breaking theories and rather points to a significant role of phase fluctuations of the order parameter [3]. Moreover, we do not detect a Nernst signal in the normal state of YBCO6.6, which re-opens the question of the relationship between pseudogap and superconductivity in underdoped cuprates. Our results also suggest that "intrinsic" defects might play a large role in the occurrence of the anomalous Nernst effect observed in the normal state of low-Tc cuprates such as LaSrCuO or Bi-2201. [1] Y. Wang et al, Phys. Rev. B 64, 224519 (2001); Phys. Rev. Lett. 88, 257003 (2002); Science 299, 86 (2003) [2] A. Legris, F. Rullier-Albenque et al, J. Phys. I France 3,1605 (1993). [3] F. Rullier-Albenque H. Alloul and R. Tourbot, Phys. Rev. Lett. 91, 047001 (2003)