Surprising behavior of the superfluid fraction for 4He and 3He-4He mixtures in 18.5 nm channels

We have obtained the superfluid fraction of (4) He and mixtures confined in channels 0.0185 mum x 1.08 mum x infinity. We compare this with film data L x infinity x infinity, and data for channels 0.0483 mum x 3.0 pm x infinity. The behavior of the data in the smallest channels is quite different from what one might expect for a planar film of a given thickness. The transition is shifted to a lower temperature than expected; and, the overall behavior of p(s) does not follow the trend observed with other confined films. This suggests that the lateral dimension plays a significant role in the behavior. We believe our observations are consistent with the recent proposal of Sobnack and Kusmartsev about a new mechanism for vortex unbinding in 2D films. (C) 2003 Elsevier Science B.V. All rights reserved.