Evidence for a Superglass State in Solid ⁴ He

<jats:title>Glasslike Supersolid</jats:title> <jats:p> Recent experiments with solid helium confined to the ring of a torsional oscillator at extremely low temperatures have been interpreted in terms of an exotic supersolid phase—a crystalline solid that somehow flows like a superfluid. However, behavioral differences between samples have raised many questions (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="601" related-article-type="in-this-issue" vol="324" xlink:href="10.1126/science.1172973">Saunders</jats:related-article> </jats:bold> ). <jats:bold> Hunt <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="632" related-article-type="in-this-issue" vol="324" xlink:href="10.1126/science.1169512">632</jats:related-article> ) present a comprehensive study of the relaxation dynamics of the torsional oscillator system as a function of time and temperature. The data provides evidence for a “supersolid glass,” where glassy behavior of crystal dislocations and superfluidity can coexist. In a separate theoretical study, <jats:bold>Anderson</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="631" related-article-type="in-this-issue" vol="324" xlink:href="10.1126/science.1169456">631</jats:related-article> ) argues that supersolidity ought to be a ground state for all bose solids, but that defects in the sample may mask the supersolid signature. </jats:p>