Observation of Nonclassical Rotational Inertia in Bulk Solid $^4$He Confined to a Cylindrical Cavity

Recent observation of nonclassical rotational inertia (NCRI) in solid $^4$He by Kim and Chan has attracted great interest in physics of supersolid, and motivated a number of theoretical studies. However, there have been surprisingly few experiments to pursuit the nature of their observation. Thus, we have begun a torsional oscillator experiment for bulk solid $^4$He. In order to study the effects of sample geometry and crystal quality, we employ a cylindrical cell, 8 mm high and 8 mm in diameter, which is mounted on an aluminum alloy torsion rod. The resonant torsion frequency is about 1750 Hz, which is $2 \sim 5$ times the frequencies in the Kim and Chan's experiments. In a preliminary experiment we have observed an increase in the frequency below about 250 mK, indicating the existence of NCRI. The frequency shift shows a substantial driving amplitude dependence, which is also consistent with the previous observation.