Onset of phase correlations in YBa2Cu3O{7-x} as determined from reversible magnetization measurements

Abstract

Isofield magnetization curves are obtained and analyzed for three single crystals of YBa2Cu3O{7-x}, ranging from optimally doped to very underdoped, as well as the BCS superconductor Nb, in the presence of magnetic fields applied both parallel and perpendicular to the $ab$ planes. Near Tc, the magnetization exhibits a temperature dependence \sqrt{M} [Ta(H)-T]^m. In accordance with recent theories, we associated Ta(H) with the onset of coherent phase fluctuations of the superconducting order parameter. For Nb and optimally doped YBaCuO, Ta(H) is essentially identical to the mean-field transition line Tc(H). The fitting exponent m=0.5 takes its mean-field value for Nb, and varies just slightly from 0.5 for optimally doped YBaCuO. However, underdoped YBCO samples exhibit anomalous behavior, with Ta(H)>Tc for H applied parallel to the c axis, suggesting that the magnetization is probing a region of temperatures above Tc where phase correlations persist. In this region, the fitting exponent falls in the range 0.5 < m < 0.8 for H\parallel c, compared with m~0. for $H\parallel ab planes. The results are interpreted in terms of an anisotropic pairing symmetry of the order parameter: d-wave along the ab planes and s-wave along the c axis.

5 pages, 4 figures

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• Experimental observation of high field diamagnetic fluctuations in Niobium
• Experimental observation of two-dimensional fluctuation magnetization in the vicinity of T_c for low values of the magnetic field in deoxygenated YBa_2Cu_3O_{7-x}
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• Study of Nb${}_{0.18}$Re${}_{0.82}$ non-centrosymmetric superconductor in the normal and superconducting states
• Study of diamagnetic fluctuations in YBa_2Cu_3O_{7-x} and Bi_2Sr_2CaCu_2O_{8+x}: Possible observation of phase correlations persisting above T_c
• Non-centro-symmetric superconductors Li2Pd3B and Li2(Pd0.8Pt0.2)3B: amplitude and phase fluctuations analysis of the experimental magnetization data
• Magnetic phase diagram of the layered superconductor Bi{2+x}Sr{2-x}CuO{6+δ} (Bi2201) with Tc~7 K