QCM Study on 2D Vortex in Superfluid 4He and 3He-4He Mixture Films

Abstract

Two-dimensional (2D) 4He fluid films show the Kosterlitz-Thouless (KT) transition where pairing and unpairing of the thermally excited 2D vortices play an important role. However, in the superfluid submonolayer, the vortex parameters (the diffusion constant D, the core diameter a0) have been incompletely estimated for various conditions. Here, we undertake QCM measurements for pure 4He films (on gold and H2 substrates) and 3He-4He mixture films (on gold substrate) and accurately determine the parameter D/a20 by the high frequency dependence of the superfluid onset from 20 to 180 MHz. By the comparison of the results of pure 4He film on gold and H2 substrates, the vortex diffusion in our study has the largest value D ∼ ℏ/m in the quantum limit. The core diameter a0 is estimated to be the same magnitude as the de Broglie wavelength at TKT between 0.1 and 0.9 K In terms of 3He-4He mixture films, we observe no effect of 3He on the vortex parameters up to the 3He coverage of 15.1 μmol/m2.