Search for supersolid helium by using a rigid torsional oscillator

Abstract

Supersolidity is a quantum state of matter in which both off-diagonal long-range order of a superfluid and crystalline order in a solid coexist simultaneously in same specimen. Its first experimental evidence had been reported by Kim and Chan through a torsional oscillator (TO) experiment. However, it was proposed that a change in a shear modulus of solid helium at low temperatures can be an alternative explanation for the TO anomaly, if the TO is not designed to be rigid. Most previous TOs were made with an improper design in which various non-linear elastic effects mimic the superfluid-like TO responses. For this study, KAIST rigid double-torus TO is constructed to reduce any elastic effects inherent to complicated viscoelastic coupling with TO structures, allowing explicit probe for a genuine supersolid transition. We investigated the frequency- and temperature-dependent TO responses containing solid helium. As a result, we did not find evidence to support the frequency-independent contribution proposed to be a property of supersolid helium. The frequency-dependent contribution which comes from the simple elastic effect of solid helium is essentially responsible for the most of responses. The majority of period drop is linearly proportional to the square of frequency, indicating that the responses observed in this TO are mostly caused by the elastic overshoot effect. We also tested the rigid TO responses under a dc rotation, motivated by the supserolid signature found in rotating solid helium. The KAIST rigid double-torus TO was attached to the RIKEN dilution refrigerator. Most of intriguing results in the previous rotation experiment are not successfully reproduced. Instead, we find that the frequency-independent period drop is suppressed by increasing rotation velocity. This superfluid-like TO responses in rotating solid helium are not able to be reconciled with a framework of elastic stiffening models. We consider several possible scenarios.