Phase fluctuations versus Gaussian fluctuations in optimally-doped YBa$_2$Cu$_3$O$_7$

TL;DR

This paper investigates the nature of magnetization fluctuations in optimally doped YBa₂Cu₃O₇, providing evidence against Gaussian fluctuation models and highlighting the role of phase locking and vortex states in the transition to superconductivity.

Contribution

It demonstrates that the magnetization behavior in OPT YBCO cannot be explained by Gaussian fluctuations, emphasizing the importance of phase locking and vortex dynamics in the transition.

Findings

Non-monotonic magnetization dependence on magnetic field

Evidence for phase locking and unlocking of the pair wavefunction

Inaccessibility of the upper critical field near T_c

Abstract

We analyze recent torque measurements of the magnetization $M_d$ vs. magnetic field $H$ in optimally doped YBa$_2$Cu$_3$O$_{7-y}$ (OPT YBCO) to argue against a recent proposal by Rey et al that the magnetization results above $T_c$ are consistent with Gaussian fluctuations. We find that, despite its strong interlayer coupling, OPT YBCO displays an anomalous non-monotonic dependence of $M_d$ on $H$ which represents direct evidence for the locking of the pair wavefunction phase $\theta_n$ at $T_c$ and the subsequent unlocking by a relatively weak $H$. These unusual features characterize the unusual nature of the transition to the Meissner state in cuprate superconductors. They are absent in low-$T_c$ superconductors to our knowledge. We also stress the importance of the vortex liquid state, as well as the profiles of the melting field $H_m(T)$ and the upper critical field curve $H_{c2}(T)$ in the $T$-$H$ plane. Contrary to the claims of Rey et al, we show that the curves of the magnetization and the Nernst signal illustrate the inaccessibility of the $H_{c2}$ line near $T_c$. The prediction of the $H_{c2}$ line by Rey et al is shown to be invalid in OPT YBCO.