Thermal history of solid 4He under oscillation

We have studied the thermal history of the resonant frequency of a torsional oscillator containing solid He-4. We find that the magnitude of the frequency shift that occurs below similar to 100 mK is multivalued in the low-temperature limit, depending strongly on how the state is prepared. This result can be qualitatively explained in terms of the motion and pinning of quantized vortices within the sample. Several aspects of the data are also consistent with the response of dislocation lines to oscillating stress fields imposed on the solid. However, studies of solid helium in porous media, several control experiments, and the magnitude of the frequency shift found in this and other experiments all indicate that the most appropriate interpretation of the torsional oscillator results is the existence of a supersolid He-4 phase.