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 trans ition in thin helium films. A single quartz crystal microbalance is operate d at two different harmonic frequencies simultaneously. We observe the well -known dissipation peak in the vicinity of the superfluid transition, presu mably associated with vortex-antivortex pairs having a size comparable to t he vortex diffusion length. The dissipation peak is monitored at both harmo nic frequencies and the relative displacement of the peaks associated with finite frequency effects is seen. We find, surprisingly, that driving one o f the resonant modes of the crystal affects the magnitude of the dissipatio n peak observed using the second mode. This possibly reflects an alteration of the vortex pair distribution associated with the induced flow. (C) 2000 Elsevier Science B.V. All rights reserved.