Two distinct superconducting fluctuation diamagnetisms inBi2Sr2−xLaxCuO6+δ

Abstract

Superconducting fluctuations were studied through angular-dependent torque measurements on a series of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{CuO}}_{6+\ensuremath{\delta}}$ (BSLCO) single crystals. Two distinct superconducting fluctuation diamagnetisms were observed: one is the superconducting thermal fluctuation, with a boundary close to the superconducting phase boundary, while the other one, up to a temperature as high as about 180 K, showing maximum signal in the sample with hole carrier density $p=0.125$, could be due to preformed pairs. In addition, we observed linearly temperature-dependent paramagnetic torque signals in BSLCO samples, possibly a result of quantum criticality from a quantum critical point at the optimal doping.