Studies of superfluid phase transitions of helium-4 films adsorbed on mesoporous ceramic MCM-41

The superfluid transition of 4He films adsorbed on ordered mesoporous MCM-41 ceramic with 40-A-diameter, micron-length cylindrical channels are studied with a torsion oscillator technique. The coverage of the superfluid films ranges from 19.8 mumole/m2 to 26.4 mumole/m 2. For films thicker than 23.3 mumole/m2, a finite-size Kosterlitz-Thouless (KT) transition becomes apparent in the data, with considerable broadening of the KT jump of the superfluid fraction. With decreasing helium coverage, the extent of the broadening increases, indicating that the vortex core size is increasing. The effective index of refraction increases as the helium coverage is reduced toward the superfluid onset thickness, which suggests a crossover from two dimensions to one. The data is consistent with the Machta-Guyer theory of the KT transition in a cylindrical channel, but with a coverage-dependent vortex core size. In the low temperature regime, a linear decrease of superfluid fraction with temperature is observed, indicating a zero-dimensional third sound excitation.