Those confirmations became big news in the world of condensed-matter physics. Most reproduced a frequency shift that could be interpreted as a manifestation of decoupled mass, consistent with the presence of a supersolid. Several research groups worldwide followed up on the work of Kim and Chan with similar torsional-oscillator experiments.

(Adapted from Physics Today, November 2004, page 23.) A cell filled with 4He frozen in porous Vycor glass and one filled only with bulk solid 4He yielded similar results. (b) Kim and Chan found a period shift that depended on temperature and the peak oscillator velocity and that was consistent with expectations based on many years of experience with the superfluid transition in liquid helium. The capacitively coupled electrodes shown in this schematic of the Kim and Chan oscillator helped to maintain the oscillation and measure its frequency. If the contents decouple from the motion, the oscillation period shifts.

(a) In a torsional-oscillator experiment, the period of the torsional oscillator is related to the moment of inertia of the sample cell and its contents and to the spring constant of the torsion rod. A sensitive torsional oscillator filled with solid helium appeared to indicate supersolid behavior in a 2004 experiment by Eunseong Kim and Moses Chan. For nearly two decades, the experiments by Greywall and by Bishop and colleagues effectively killed any enthusiasm to search for a supersolid state of matter.įigure 1. Although they saw interesting temperature dependence, they could attribute it to other, nonsuper properties of the solid and concluded that they had found no evidence for a supersolid.

Within a few years of Greywall’s investigations, David Bishop and Mikko Paalanen at Bell Labs and John Reppy at Cornell looked, per Leggett, for a shift in the period of a solid-filled torsional oscillator at some transition temperature. Greywall’s experiments, conducted at several average cell pressures, resulted in the unambiguous conclusion that pressure differences did not relax: There was no supersolid. If the solid were to transition to an unusual state in which it could flow through the channels, then an initial pressure difference between the two chambers would relax due to that flow. In one of those studies, Dennis Greywall, then at Bell Labs, put the solid in two chambers that could communicate through narrow, solid-filled channels. then a visiting scientist at the Institute for Physical Problems, who used a stack of closely placed disks at the end of a torsion rod and detected a temperature-dependent superfluid component for liquid 4He as mass between the plates decoupled from them.īy the late 1970s, experimentalists were investigating whether there was something unusual about solid 4He. The basic technique had been pioneered in the 1940s in a famous experiment by Elevter Andronikashvili, 2 2. If some of the 4He atoms were to decouple from the rest of the solid, that decoupling would manifest itself as a change in the oscillation period. Motivated by the idea of superfluid-like motion in a solid, Anthony Leggett, then at the University of Sussex, proposed a simple experiment that could put it to the test: Fill a container with solid 4He, place it at the end of a torsion rod, and measure the period of torsional oscillation as a function of temperature. The startling prediction soon aroused experimental interest.

Solid helium