Amorphous solid like heat capacity of 4He fluid films adsorbed on pores

Abstract

In the nanopores of HMM-2 whose mean pore diameter is 2.7 nm, adsorbed 4He forms layers up to about two atomic layers. At low temperatures, the 4He films show properties of the Bose quantum fluid and superfluidity above about 1.4 layers. Below the coverage or above the superfluid transition temperature, 4He film is in the normal fluid state. The heat capacity of the normal fluid shows linear temperature (T) dependence. The T-linear coefficient of the heat capacity is about 5 μJ/(m2-K2). According to Andreev's idea originally developed for bulk helium liquids, the 4He fluid can be regarded as an amorphous solid with T-linear dependence of the heat capacity. Its coefficient is described as π2D0κB2/6, where D0 is a density of states and κB is the Boltzmann constant. The density of states is estimated from the density (coverage) dependence of the isosteric heat of sorption qst which was obtained from the vapor pressure data of adsorption. The estimated density of state well explains the observed magnitude of the T-linear coefficient of heat capacity.