Non-ohmic electrical transport properties above the critical temperature in optimally and underdoped superconducting YBa₂Cu₃O_6+x

The resistivity and the Hall effect as a function of current density and temperature in the normal state of very thin films of YBa₂Cu₃O_6+x are measured with a fast pulsed-current technique. The in-plane longitudinal and transverse (Hall) conductivities are reduced in intense current densities up to several . In optimally-doped samples, this non-ohmic effect is limited to temperatures below 100 K. In a moderately underdoped sample, however, the non-linearity extends to the temperature range from T_c =53 K to similar to 150 K. The non-linear part of the Hall conductivity does not scale with a critical exponent, thus excluding classical amplitude fluctuations of the superconducting order parameter as possible origin.