Observation of Unusual Mass Transport in Solid hcpHe4

Abstract

Solid 4 He has been created off the melting curve by growth at nearly constant mass via the "blocked capillary" technique and growth from the 4 He superfluid at constant temperature. The experimental apparatus allows injection of 4 He atoms from superfluid directly into the solid. Evidence for the superfluid-like transport of mass through a sample cell filled with hcp solid 4 He off the melting curve is found. This mass flux depends on temperature and pressure. 67.80.Mg, 67.40.Hf, 67.90.+z Experiments by Kim and Chan[1,2,3,4], who studied the behavior of a torsional oscillator filled with hcp solid 4 He, showed a clear reduction in the period of the oscillator as a function of temperature at temperatures below T ≈ 250 mK. This observation was interpreted as evidence for the presence of "supersolid" behavior in hcp solid 4 He. Subsequent work in a number of laboratories has confirmed the observation of a period shift, with the interpretation of mass decoupling in most cases in the 0.05 -1 percent range, but with dramatically larger decoupling seen in quench-frozen samples in small geometries [5]. Aoki et al.[6] observed sample history dependence under some conditions. These observations and interpretations, among others, have kindled considerable interest and debate concerning solid hcp 4 He.Early measurements by Greywall [7], showed no evidence for mass flow in solid helium. Work by the Beamish group also showed no evidence for mass flow in two sets of experiments involving Vycor[8] and narrow channels [9]. Sasaki et al. [10] attempted to cause flow through solid helium on the melting curve, using a technique similar to that used by Bonfait et al. [11] (that showed no flow). Initial interpretations suggested that flow might be taking place through the solid[10], but subsequent measurements have been interpreted to conclude that the flow was instead likely carried by small liquid regions at the interface between crystal faces and the surface of the sample cell [12], which were shown to be present for helium on the melting curve. Recent work by Day and Beamish [13] showed that the shear modulus of hcp solid 4 He increased at low temperature and demonstrated a temperature and 3 He impurity dependence very similar to that shown by the torsional oscillator results. The theoretical situation is also complex, with clear analytic predictions that a supersolid cannot exist without vacancies (or interstitials) [14], numerical predictions that no vacancies exist in the ground state of hcp solid 4 He [15,16,17], and ab initio simulations that predict that in the presence of disorder the solid can demonstrate superflow [15,18,19] along imperfections. But, there are alternate points of view [20]. There has been no clear experimental evidence presented for the flow of atoms through solid hcp 4 He.We have created a new approach, related to our "sandwich"[21] design, with an important modification. The motivation was to attempt to study hcp solid 4 He at pressures off the melting curve in a way that would allow a chemical po...

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