Sign change of the out-of-plane magnetoresistance in oxygen-deficient YBCO

The out-of-plane magnetoresistance in optimally doped (Tc approximate to 90 K) and oxygen-deficient (T-c approximate to 58 K) thin films of YBa2Cu3O7- delta grown on vicinal SrTiO3 substrates was investigated. The magnetoresis tance of both compounds changes sign above the critical temperature, The re sults are compared to theoretical predictions for the magnetic-field suppre ssion of superconducting order parameter fluctuations in anisotropic materi als, It is suggested that the competition of Aslamazov-Larkin and density-o f-states fluctuations, which contribute with different sign to the magnetor esistance, are responsible for the observed sign change.