Abstract
We have studied the non-linear response of a helium film to an external AC drive near the Kosterlitz Thouless superfluid transition by using the theory of Ambegaokaret al. Previously we have investigated the weakly non-linear response, corresponding to a small ratio of the diffusion lengthrD to the length scalerv= ħ/mvn wherevn is the velocity of the substrate. Here we extend the calculation to describe the opposite regime where the ratiorD/rv is large. The period shift and Q factor of a torsional oscillator containing the film are calculated for the strongly non-linear regime. The expected features of the response in the intermediate regime whererD∼ rv are obtained by smoothly interpolating the results obtained in the two limiting cases.
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