Nonlinear and hysteretic ultrasound propagation in solid He4 : Dynamics of dislocation lines and pinning impurities

Abstract

We report on the measurements of 9.6 MHz ultrasound propagation down to 15 mK in polycrystalline quantum solid 4 He containing 0.3 and 20 ppm 3 He impurities. The attenuation and speed of ultrasound are strongly affected by the dislocation vibration. The observed increase in attenuation from 1.2 K to a peak near 0.3 K is independent of drive amplitude and reflects crossover from overdamped to underdamped oscillation of dislocations pinned at network nodes. Below 0.3 K, amplitude-dependent and hysteretic variations are observed in both attenuation and speed. The attenuation decreases from the peak at 0.3 K to a very small constant value below 70 mK at sufficiently low drive amplitudes of ultrasound, while it remains a high value down to 15mK at the highest drive amplitude. The behaviors at low drive amplitudes can be well described by the effects of the thermal pinning and unpinning of dislocations by the impurities. The binding energy between a dislocation line and a 3 He atom is estimated to be 0.35 K. The nonlinear and hysteretic behaviors at intermediate drive amplitudes are analyzed in terms of stress-induced unpinning which may occur catastrophically within a network dislocation segment. The relaxation time for pinning at 15 mK is very short (< 4 s), while more than 1,000 s is required for unpinning.

Anomalously Small Excitation Gaps as a Precursor of Dislocation Core Superfluidity in Solid Helium-4

“…More recently the "anomalous" absorption peak at T = 0.7K peak has also been found in Ref. [27]. The origin of such a peak is unknown.…”

“…[26]). More recent studies using ultrasound have reported smaller values-ranging from 0.18K to 0.35K [27] for the 3 He binding energy. There are also reports of the activation behavior of the intrinsic origin -unrelated to 3 He impurities.…”

“…Acknowledgments. We thank I. Iwasa for useful discussions of the anomalous absorption peak [27]. This work was supported by the National Science Foundation under the grants DMR-1720251, DMR-2032136 and DMR-2032077.…”

Characterization of Dislocations in hcp $$^4\hbox {He}$$ by Torsional Oscillator and Thermal Conductivity Measurements

“…This noninvasive method allows to characterize the distribution of dislocation lengths as was done for polycrystals [1,50,82] and single crystals [2]. As far as we are aware, it was so far only possible to apply it to solid helium-thanks to the short relaxation time of 3 He impurities [48] and high mobility of dislocations.…”

“…The response of hcp 4 He , with various 3 He concentrations, to an AC shear stress has been investigated previously by a range of techniques at different frequencies: shear in a torsional oscillator (TO) (200 Hz-2 kHz) [39], transverse sound (1 Hz-100 kHz) [2,16,[40][41][42][43][44][45][46] and ultrasound [47][48][49][50] , scattering of thermal phonons (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24) [4,[51][52][53][54][55][56]. Most can be explained in terms of interaction with vibrating dislocations [39,49].…”