3He Effect on 2D Superfluidity in 3He–4He Mixture Films on Planar Gold

Abstract

There have been a number of experiments exploring the nature of 2D superfluidity and the configuration of 3He–4He mixture films on various substrates. To date, a possible film-structure at T=0 is that of a simple layer model, 3He/superfluid 4He/solid-like 4He/substrate, in which the submonolayer superfluidity is strongly affected by the coverage of the 3He overlayer. Yet the mechanism is not been fully understood. In this paper, we report a QCM study at 60 MHz for the 3He effect on the superfluidity of mixture films on flat gold, mainly focusing on the anomalous depletion of the temperature dependence of the superfluid density σs. In the measurements, we kept the 3He coverage constant (n3= 0, 3.6, 7.2, 19.0, 57.2, or 92.8 μmol/m2) and then incrementally added 4He. We observed the evolution of the 3He effect on σs(T) with increasing 3He coverage; this depletion of σs(T) rapidly increases and then saturates near n3∼1 layer. From the analysis of the linear-temperature region in the plot of the dissipation peak temperature Tp as a function of the superfluid 4He coverage n4s and comparison with previous studies on Mylar and porous gold, we found a universal function for the strength of the 3He effect for all substrates.