A Torsional Oscillator Study of Solid Helium

A compound two-mode torsional oscillator is used to investigate the `supersolid' signature in polycrystaline 4He. We observe a change in the resonant period of both torsion modes below 200mK, accompanied by dissipation peaks, consistent with the original observation of Kim and Chan. By careful analysis of the sensitivity of the two modes of the torsional oscillator we are able to account for contributions from changes in the shear modulus of the 4He sample proposed by Beamish et al and Maris, and the viscoelastic behaviour discussed by Yoo and Dorsey. We find that a zero-frequency period shift remains unaccounted for, corresponding to a velocity-dependant decoupling of 0.14% of the sample with a critical velocity of ~100 µm/s. We examine the results of Aoki et al and apply the same corrections to their work, finding a decoupling of 0.017% Extensive repair work performed on the cryostat during the experiment is also described.