date:ESPCI, CNRS, UPR5, F-75231 Paris 05, France; Katholieke Univ Leuven, INPAC, B-3001 Heverlee, Belgium; LNCMP, F-31400 Toulouse, France
The contribution of superconducting fluctuations to the conductivity, or paraconductivity, is studied in the underdoped regime of La2-xSrxCuO4 cuprates. A perpendicular magnetic field up to 50 T is applied to suppress the superconductivity and obtain the normal state resistivity, which is then used to calculate the paraconductivity. Surprisingly enough, it is consistent with a two-dimensional Aslamazov-Larkin regime of Gaussian fluctuations close to the critical temperature. At higher temperature, the paraconductivity exhibits a power-law decrease in temperature (as T-alpha), as was previously shown for underdoped YBa2Cu3O7-delta and Bi2Sr2CaCu2O8+delta samples. Our observations are not consistent with the existence of Kosterlitz-Thouless fluctuations. This tends to indicate that the superconducting pair amplitude is not already defined above T-C in the pseudogap state.