Probable observation of a supersolid helium phase

When liquid He-4 is cooled below 2.176 K, it undergoes a phase transition-Bose-Einstein condensation-and becomes a superfluid with zero viscosity(1). 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 phase(2), it has been proposed theoretically(3-5) that superflow can also occur in the solid phase of He-4. Owing to quantum mechanical fluctuations, delocalized vacancies and defects are expected to be present in crystalline solid He-4, 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 attempts(6), such a 'supersolid' phase has yet to be observed in bulk solid He-4. 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 inertia(5) 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-gas(7), liquid(1) and solid-can undergo Bose Einstein condensation.