Observation of the Lambda Point in the 4He-Vycor System: A Test of Hyperuniversality

Abstract

We have performed a high resolution specific heat measurement on 4He completely filling the pores of Vycor glass. Within 10mK of the superfluid transition we have found a peak in the heat capacity which is only 0.02% the size of the background. The peak can be fit with a rounded version of the ``logarithmic singularity'' observed in bulk 4He. Along with the previously observed ``2/3'' power law dependence of the superfluid density on temperature, this strongly suggests that the disorder imposed by the Vycor is irrelevant to the 3DXY superfluid phase transition. The critical amplitude of the peak, while in agreement with that found in other experiments in dilute superfluid 4He, is considerably larger than that predicted by the theory of hyperuniversality.