The shear modulus of solid 4 He below 200 mK exhibits an unusual increase, the characteristics of which show remarkable similarities to those of the period reduction in torsional oscillator experiments. We systematically studied the drive strain and temperature dependence of the shear modulus at low temperatures. The hysteretic behavior depends strongly on the drive and cooling history, which can be associated with the thermally assisted Granato-Lucke dislocation theory. The phase diagram of the shear modulus is constructed on the basis of the emerging hysteretic behavior.An anomalous increase in the shear modulus at low temperatures was recently observed in solid helium [1]. The anomaly exhibits temperature, frequency, and 3 He concentration dependences resembling those of the nonclassical rotational inertia (NCRI) in torsional oscillator (TO) experiments [2,3]. These remarkable similarities have attracted theoretical and experimental attention [4,5]. For instance, a dislocation vibration model for the NCRI is proposed. The temperature dependence of the period of a torsional oscillator containing solid helium is derived from the variation in the average pinning length of dislocations in solid helium [6]. Other efforts to provide a non-supersolid explanation for the TO response were also motivated by the similarities between the two phenomena [4,5,7,8]. The Cornell group recently reported that the TO motion was controlled by the same microscopic excitation that is agitated by temperature and stress independently, which they suggested can be understood within the framework of a dissipation model in which the elastic properties change at low temperatures [9]. Nevertheless, the resonant period reduction found in some earlier TO measurements, including Ref.[9], can be ascribed to the change in the elastic properties of solid helium filling a torsion rod [10]. Thus, the similar response of TO to the drive and temperature agitation can be understood in terms of the dislocation pinning mechanism.The shear modulus increase can be interpreted as the pinning of dislocations by 3 He impurities according to the Granato-Lucke (GL) dislocation theory [11]. The original GL theory considers only the unpinning of dislocations by applied stress without a thermal-fluctuationinduced unbinding mechanism: a large stress reduces the number of pinning points by detaching 3 He impurities from dislocations, thus suppressing the shear modulus. Nevertheless, the effects of stress and temperature on the shear modulus are similar, so its temperature dependence can be explained by the variation in the dislocation loop length with progressive lengthening of freely vibrating dislocation segments due to thermal evaporation of 3 He impurities [1,6,12]. Moreover, the softening of the shear modulus is not a phase transition but a crossover of the thermally activated relaxation process from a stiff (pinned) state to a relaxed (unpinned) state.The discrepancy in the effects of the drive stress and temperature appears when the hysteretic behaviors dur...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.