Interplay of Rotational, Relaxational, and Shear Dynamics in Solid ⁴ He

Abstract

Using a high-sensitivity torsional oscillator (TO) technique, we mapped the rotational and relaxational dynamics of solid helium-4 ((4)He) throughout the parameter range of the proposed supersolidity. We found evidence that the same microscopic excitations controlling the torsional oscillator motions are generated independently by thermal and mechanical stimulation. Moreover, a measure for the relaxation times of these excitations diverges smoothly without any indication for a critical temperature or critical velocity of a supersolid transition. Finally, we demonstrated that the combined temperature-velocity dependence of the TO response is indistinguishable from the combined temperature-strain dependence of the solid's shear modulus. This implies that the rotational responses of solid (4)He attributed to supersolidity are associated with generation of the same microscopic excitations as those produced by direct shear strain.

Thermal conductivity and torsional oscillations of solid 4He

“…On the other hand, we found that the thermal conductivity is insensitive to the amplitude of oscillations. As was noted earlier [25,26], the dependences of the TO resonant frequency on amplitude V look basically identical to the temperature dependences Fig. 5), our exponent γ ≈ 0.71 is substantially different.…”

“…Softenning of solid helium caused by large-amplitude oscillations is then associated with stress-induced breakaway of 3 He impurities from dislocations. The related nonlinearities, hysteresis and long relaxation times [26] would indicate strong deviations from thermodynamic equilibrium, discussed in the next paragraph. The increase in the phonon mean free path upon either adding 3 He or annealing can be explained by reducing the number of dislocation segments capable of vibrating at frequencies of thermal phonons.…”

Activated Nucleation of Vortices in a Dipole-Blockaded Supersolid Condensate

“…It is observed that the data are consistent with a picture of a thermal activation process of superfluid vortices through a nucleation barrier. This result is in contradiction with the interpretation of the experimental data of [13] where the self-similar v and T dependence of the superfluid fraction was understood to be proof of a coupling between elasticity and the NCRI. We also tried to find a self-similar relation, but there was less clear evidence of a correlation between the critical velocity and temperature than that provided by Eq.…”

“…Related to this, recent experimental results on supersolid helium have suggested a strong interplay between rotation and relaxation [13]. There, the supersolid fraction decreases with increasing temperature and rotation frequency in a specific way, even if in this case the role of the torsion rod is questioned [12,14].…”

“…As such, the Rydberg excited atoms can give rise to supersolid behavior directly in a BEC and can then potentially provide useful insights into the nature of the supersolid phase in solid helium. In this framework, we will show that thermal activation of superfluid vortices is the physical mechanism governing the superfluid dependence on the temperature and the imposed velocity, in contrast with the self-similar structure found in [13]. Because of its condensate structure, and contrary to the case of 4 He, the dynamics of dipole-blockaded Rydberg atoms can be investigated within the mean-field approximation leading to a nonlocal Gross-Pitaevskii (GP) equation that describes the 2D (x, y) Bose gas in terms of the classical complex wave function c ðx; tÞ in the zero temperature limit,…”