AC FLOW EFFECT ON A VORTEX STATE IN THIN HE-4 SUPERFLUID FILMS ON POROUS-GLASS

AC flow response of submonolayer superfluid He-4 films formed on porou s glass with 1 mu m pore diameter has been studied over more than thre e decades of the oscillation amplitude or the AC velocity V-AC, using a high Q-value torsional oscillator. As the oscillation amplitude incr eases a clear crossover has been observed, from linear regime where th e height and the shape of the dissipation peak at Tc does not depend o n the drive level, to nonlinear regime where energy dissipation peak c hanges the height in proportion to V-AC(2) at the characteristic veloc ity of the order of 0.1cm/sec. At much higher V-AC, deviation from V-A C(2) dependence as well as rapid change of the peak shape have been ob served.