QCM study of superfluid transition in ⁴He films adsorbed in SBA-15

A quartz crystal microbalance (QCM) is a useful tool to study the superfluidity of ⁴He films at very high frequencies. Our recent efforts to fabricate mesoporous silica films onto QCM have enabled us to study the noble superfluidity adsorbed in nanopores. In this paper we report results of QCM measurements for the superfluid ⁴He films in SBA-15 with one-dimensional nanopores 4.1 nm in diameter and ∼ 1 μM in length. From the ⁴He pressure isotherm at 4.2 K, a uniform layer is formed in the nanopores up to the coverage of 48 μmol/m², corresponding to ∼ 2.5 layers. The superfluid response is measured at 12 MHz for various coverages in the temperature range of 0.1 - 1.5 K. Above the onset coverage of ∼ 23 μmol/m², we observed a frequency shift accompanied by a dissipation peak due to the superfluid Kosterlitz-Thouless (KT) transition.