ac vortex-dependent torsional oscillation response and onset temperature T0 in solid 4He

Detailed studies of ac velocity V(ac) and T dependence of torsional oscillator responses of solid (4)He are reported. A characteristic onset temperature T(0) similar to 0.5 K is found, below which a significant V(ac)-dependent change occurs in the energy dissipation for the samples at similar to 32 bar and for one at 49 bar. A Vac dependence of the so-called nonclassical rotational inertia'' fraction also appears below similar to T(0). The log (V(ac)) linear dependence, which suggests involvement of quantized vorticies, was examined in the nonclassical rotational inertia fraction. We find a common 1/T(2) dependence for this linear slope change in all of the samples for 30 < V(ac) < 300 mu m/s. We discuss that our observation is consistent with nonlinear rotational susceptibility of the vortex fluid, proposed by Anderson above T(c) below T(0).