Superconducting fluctuation regime in the cuprates revealed by torque magnetometry

The extent of the superconducting fluctuation regime in the normal state of the cuprate superconductors has remained unclear. For the single-CuO$_{2}$-layer compounds La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO) and Bi$_{2}$(Sr,La)$_{2}$CuO$_{6+\delta }$ (Bi2201), one class of experiments indicates characteristic temperatures as high as 2-3 times $T_{c}$ at optimal doping, whereas a second class reveals superconducting fluctuations in a relatively narrow temperature range above $T_{c}$. Here we report a systematic torque magnetometry study of the superconducting fluctuation regime in three single-layer compounds, LSCO, Bi2201 and HgBa$_{2}$CuO$_{4+\delta }$. We find in all three cases that the regime of fluctuating diamagnetism is narrow and closely tracks the doping dependence of $T_{c}$, consistent with the second class of experiments [1]. The seemly controversial results can be understood if short-range phase correlations develop only in the vicinity of $T_{c}$, whereas local pair formation appears at a relatively high temperature that is universal among all single-layer cuprates. \\[4pt] [1] G. Yu, D.-D. Xia, N. Bari\v{s}ic, R.-H. He, N Kaneko, Yangmu Li, Yuan Li, T. Sasagawa, A. Shekhter, X. Zhao and M. Greven, unpublished