Possible vortex fluid to supersolid transition in solid $^4$He below about 75 mK
A detailed torsional oscillator (TO) study on a stable solid 4He sample at 49 bar with To = 0.5K, is reported to T below the dissipation peak at Tp . We find hysteretic behavior starting below Tc = about 75 mK, in period shift, as well as in dissipation, with changes of AC excitation amplitude Vac . The derived difference of non-linear rotational susceptibility DeltaNLRS(T)hys across the hysteresis loop under a systematic condition is analyzed as a function of Vac and T . We propose that DeltaN LRS(T)hys is proportional to non- classical rortational inertia fraction, NCRIF, and is actually the supersolid density rho_ss of the 3D supersolid state below Tc . rho_ss changes linearly with T down to about 60 mK and then increases much more steeply, and approaching a finite value towards T = 0. We find an AC velocity of 40 micrometer/s beyond which the hysteresis starts at T < Tc and a critical AC velocity, about 1 cm/s, above which rho_ss is completely destroyed. We discuss also the coherence length of the supersolid.