1. Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA

Correspondence to: M. H. W. Chan¹ Email: chan@phys.psu.edu

Abstract

When liquid ⁴He is cooled below 2.176 K, it undergoes a phase transition—Bose–Einstein condensation—and becomes a superfluid with zero viscosity¹. Once in such a state, it can flow without dissipation even through pores of atomic dimensions. Although it is intuitive to associate superflow only with the liquid phase², it has been proposed theoretically^{3, 4, 5} that superflow can also occur in the solid phase of ⁴He. Owing to quantum mechanical fluctuations, delocalized vacancies and defects are expected to be present in crystalline solid ⁴He, even in the limit of zero temperature. These zero-point vacancies can in principle allow the appearance of superfluidity in the solid^{3, 4}. However, in spite of many attempts⁶, such a 'supersolid' phase has yet to be observed in bulk solid ⁴He. Here we report torsional oscillator measurements on solid helium confined in a porous medium, a configuration that is likely to be more heavily populated with vacancies than bulk helium. We find an abrupt drop in the rotational inertia⁵ of the confined solid below a certain critical temperature. The most likely interpretation of the inertia drop is entry into the supersolid phase. If confirmed, our results show that all three states of matter—gas⁷, liquid¹ and solid—can undergo Bose–Einstein condensation.

1. Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA

Correspondence to: M. H. W. Chan¹ Email: chan@phys.psu.edu

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