Vortex Fluid State below an Onset Temperature T_0 of Solid 4He

Detailed studies of AC velocity Vac and temperature dependence of torsional oscillator responses of solid 4He are reported. A characteristic onset temperature T0 about 0.5 K is found, below which a significant Vac dependent change occurs in the energy dissipation for the sample at 32 bar. A Vac dependence of the non-classical rotational inertia fraction, NCRIF also appears below about T0. This value of T0 excludes the possible explanation of supersolid by liquid superfluidity in grain boundaries or other liquid related origins. The log(Vac) linear dependence was found in NCRIF. Furthermore, this linear slope changes in proportion to 1/T 2 for 40 < Vac < 400 micro-m/s, then crosses over to about 1/T for larger Vac. We discuss properties of the vortex fluid state proposed by Anderson above Tc, below T0.