FAST FLUX MOTION IN YBA2CU3OX FILMS IN AN AC MAGNETIC-FIELD ACTIVATEDBY LASER-HEATING

Infrared pulsed laser irradiation has been used to generate a fast pho tosignal from epitaxial YBa2Cu3Ox superconducting films when a small a lternating magnetic field is applied in the film plane. No bias curren t was applied, and no signals were observable in static magnetic field s. The photosignals, 40 ns in duration and tenths of a millivolt in am plitude, are obtained below T(c), with alternating magnetic fields up to 3200 A/m oscillating from 20 to 150 Hz. The laser pulses had a dura tion of 80 ns, with an absorbed fluence of 1 mJ/cm2 at a wavelength of 10.6 mum. The signal is interpreted in terms of flux motion thermally activated by the laser heating. The magnetic force due to the alterna ting magnetic field exceeds the pinning force following the absorption of the radiation and generates the fast flux motion observed. The sig nal dependencies on temperature, magnetic-field amplitude, and absorbe d fluence, as well as ac-susceptibility measurements, confirm this exp lanation. It is shown that this thermally activated motion, generated at j-j(c), is faster than an Anderson-Kim creep. We propose that a the rmally activated flux-avalanche model based on the self-organized-crit icality theory may describe the photosignal observed.