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We analyze recent torque measurements of the magnetization $M_d$ vs. magneticfield $H$ in optimally doped YBa$_2$Cu$_3$O$_{7-y}$ (OPT YBCO) to argue againsta recent proposal by Rey et al that the magnetization results above $T_c$ areconsistent with Gaussian fluctuations. We find that, despite its stronginterlayer coupling, OPT YBCO displays an anomalous non-monotonic dependence of$M_d$ on $H$ which represents direct evidence for the locking of the pairwavefunction phase $\theta_n$ at $T_c$ and the subsequent unlocking by arelatively weak $H$. These unusual features characterize the unusual nature ofthe transition to the Meissner state in cuprate superconductors. They areabsent in low-$T_c$ superconductors to our knowledge. We also stress theimportance of the vortex liquid state, as well as the profiles of the meltingfield $H_m(T)$ 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 themagnetization 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 isshown to be invalid in OPT YBCO.

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