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

TL;DR

This study investigates phase correlations in YBa2Cu3O7-x using magnetization measurements, revealing different behaviors in optimally doped and underdoped samples, and suggesting anisotropic pairing symmetry in the superconductor.

Contribution

It provides experimental evidence linking phase fluctuations to the onset of superconductivity and highlights anisotropic pairing symmetry in YBa2Cu3O7-x.

Findings

Ta(H) coincides with Tc(H) in optimally doped samples.

Underdoped YBCO shows phase correlations above Tc for H//c.

Anisotropic pairing symmetry: d-wave in ab planes, s-wave along c axis.

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.