NONLINEAR AC RESPONSE IN SUPERFLUID FILMS

We have studied the non-linear response of a helium film to ar? extern al AC dr ive near the Kosterlitz Thouless superfluid transition by usi ng the theory of Ambegaokar. et al. Previously we have investigated th e weakly non-linear response, corresponding to a small ratio of the di ffusion length r(D) to the length scale r(v) = h/mv(n) where v(n) is t he velocity of the substrate. Here we extend the calculation to descri be the opposite regime where the ratio r(D)/r(V) is large, The period shift and Q factor of a torsional oscillator containing the film are c alculated for the strongly non-linear regime. The expected features of the response in the intermediate regime where r(D) similar to r(V) ar e obtained by smoothly interpolating the results obtained in the two l imiting cases.