Remarks on Solid Helium

Abstract

In this Special Issue, someof the fundamental andpractical challenges andquestions that appear to us to exist concerning the possibility of supersolidity or other unusual phenomena that may be present in solid helium are presented. The work of Kim and Chan stimulated a major rebirth of significant investigation of the properties and behavior of solid 4He. In the intervening ten or so years, a number of discoveries have been made, but a number of early questions remain, and a number of new questions have emerged in recent years. Here, a number of issues that remain outstanding are presented. This list could no doubt be increased by other scientists active in these fields, but these ideas come to mind, stimulated by the recent work reported at LT27 and at this Workshop. A major question is, of course, is there a supersolid? And, if so, is it anything like the supersolid predicted in the early 1970s to exist? The community has moved from excitement, that such state of matter exists, to very serious skepticism. The original interpretation that seemed obvious from the torsional oscillator results has been retracted, and most in the community believe that the torsional oscillator effects were due to the temperature dependence of the shear modulus. The community has not come into complete agreement on how to fully interpret double-frequency torsional oscillator experiments. So, there needs to be a clarification of the torsional oscillator work. At the moment, there appears to be no convincing evidence for supersolidity from torsional oscillator experiments. But, this needs to be definitively settled by the scientific community.