Temperature-dependent characteristic velocity and excitations of solid helium-4

Excitations that allow shear velocity to inhibit superflow are well known in superfluids, and typically generate temperature-dependent critical velocities that depend sensitively on the physics of these microscopic dissipative mechanisms. By contrast, the nature of the microscopic excitations associated with the low-temperature {}``supersolid'' inertial anomaly and shear stiffening of solid $^{4}$He remain unknown, and its temperature-dependent characteristic velocity curve $v_{\star}\left(T\right)$ has not yet been observed. Using a SQUID-based torsion oscillator to map the complete complex rotational susceptibility of solid $^{4}$He, we observed that the internal dissipative excitation rates obey power laws of temperature and velocity, and we acquired the full temperature-dependent characteristic velocity function $v_{\star}\left(T\right)$. We compare these observations to the predicted $v_{\star}\left(T\right)$ curves of several microscopic models, including those of a thin-film vortex unbinding and a two-level-system (TLS) momentum deficit.