Effect of imposed superflow on the superfluid dissipation in the vicinity of the Kosterlitz-Thouless transition

We have developed a new technique for probing the Kosterlitz-Thouless transition in thin helium films. A single quartz crystal microbalance is operated at two different harmonic frequencies simultaneously. We observe the well-known dissipation peak in the vicinity of the superfluid transition, presumably associated with vortex-antivortex pairs having a size comparable to the vortex diffusion length. The dissipation peak is monitored at both harmonic frequencies and the relative displacement of the peaks associated with finite frequency effects is seen. We find, surprisingly, that driving one of the resonant modes of the crystal affects the magnitude of the dissipation peak observed using the second mode. This possibly reflects an alteration of the vortex pair distribution associated with the induced flow.