Vortex Diffusivity and Core Diameter of 2D Superfluid in 4He Films on Gold and H2 Substrates

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 a ₀) have not been precisely obtained for various conditions. Here, we accurately determined the parameter D/a ₀ ² 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/a ₀ ² , on the other hand, has the same value for the coverages with the same KT temperature T _KT. This suggests that the vortex diffusions on both substrates have the largest value D～?/m in the quantum limit. The core diameter a ₀ was estimated to be the same magnitude as the de Broglie wavelength at T _KT between 0.1 and 0.9 K.