Dimensionality of the superfluid transition inHe4films adsorbed on 500-ÅAl2O3powder

Abstract

The dimensionality of the superfluid transition in thin ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films adsorbed on 500-Å-diameter ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ powder has been investigated with use of third-sound measurements. A sharp drop in $\frac{〈{\rho }_s〉}{\rho }$ is observed at film thicknesses just below the critical Kosterlitz-Thouless value, indicating that the transition remains two dimensional for this pore size above 1.2 K. The sound signal can be observed to very low values of the superfluid density because the measured attenuation near onset is much smaller than that observed previously on flat substrates.