The two-dimensional (2D) 4He fluid films show the Kosterlitz-Thouless (KT) transition where pairing and unpairing of the 2D vortices play an important role. However, the vortex properties (the diffusion constant D, the core diameter a0) have not been precisely obtained for various conditions. Here, we accurately determined the parameter D/a02 by the high frequency dependence of the superfluid onset up to 180 MHz for the submonolayer 4He fluid films adsorbed on gold and H2 (3.3 layers) preplated on gold, respectively. The superfluid onset coverage changes from 1.6 (gold) to 0.5 layers (H2), which clearly indicates the large difference of the adsorption potential. The parameter D/a02, on the other hand, has the same value for the coverages with the same KT temperature TKT. This suggests that the vortex diffusions on both substrates have the largest value D∼ℏ/m in the quantum limit. The core diameter a0 was estimated to be the same magnitude as the de Broglie wavelength at TKT between 0.1 and 0.9 K.
KT transition 4He film
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