Torque magnetization study of superconducting fluctuations in single-layer cuprates: new implications for the phase diagram

We have studied the superconducting fluctuations above the transition temperature by angle-dependent torque magnetization in single-layer La$_{\mathrm{2-x}}$Sr$_{\mathrm{x}}$CuO$_{\mathrm{4}}$ (LSCO), Bi$_{\mathrm{2}}$(Sr,La)$_{\mathrm{2}}$CuO$_{\mathrm{6+\delta }}$ (Bi2201), and HgBa$_{\mathrm{2}}$CuO$_{\mathrm{4+\delta }}$ (Hg1201). The latter is a more ideal compound, with a maximum $T_{\mathrm{c}}$ of 97 K, more than twice the values for LSCO and Bi2201. In all three cases, the diamagnetic signal above $T_{\mathrm{c}}$ vanishes in an unusual exponential fashion, and at a rate that is universal, despite the dramatic differences in $T_{\mathrm{c}}$ [G. Yu \textit{et al}., arXiv:1210.6942v1]. These observations suggest that anomalies observed at much higher temperatures in both LSCO and Bi2201 are not associated with superconducting fluctuations.