Two distinct superconducting fluctuation diamagnetisms in Bi2Sr2-xLaxCuO6+delta

Superconducting fluctuations were studied through angular-dependent torque measurements on a series of Bi2Sr2-x LaxCuO6+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.