Disorder and the Supersolid State of SolidHe4

Abstract

We report torsional oscillator supersolid studies of highly disordered samples of solid 4 He. In an attempt to approach the amorphous or glassy state of the solid, we prepare our samples by rapid freezing from the normal phase of liquid 4 He. Less than two minutes is required for the entire process of freezing and the subsequent cooling of the sample to below 1 K. The supersolid signals observed for such samples are remarkably large, exceeding 20 % of the entire solid helium moment of inertia. These results, taken with the finding that the magnitude of the small supersolid signals observed in our earlier experiments can be reduced to an unobservable level by annealing, strongly suggest that the supersolid state exists for the disordered or glassy state of helium and is absent in high quality crystals of solid 4 He. Following the discovery by Kim and Chan (KC) [1, 2] of the supersolid or nonclassical rotational inertia (NCRI) state of bulk solid 4 He, several independent groups using the same torsional oscillator technique have confirmed the KC supersolid results. These include the Japanese groups of Shirahama et al. [3], working at Keio University, and Kubota et al. [4] at the ISSP, as well as our group [5] at Cornell University. In these early experiments, the solid samples were formed by the blocked capillary technique. In this method the fill line to the cell is first allowed to freeze ensuring that solidification in the cell occurs under a condition of constant average density. This technique is known to produce relatively disordered polycrystalline samples. The signals observed in the early experiments were small representing, at most, a few percent of the total solid helium mass. The Cornell experiments [5] also demonstrated that the supersolid signal could be substantially reduced through annealing of the sample. In some cases the annealing process appeared to eliminate the supersolid signal; i.e., it reduced the supersolid fraction below the 0.05% level of experimental detection. This signal reduction upon annealing strongly suggested that sample disorder plays an important role in supersolid phenomena. This inference is supported by more recent work by Chan's group [6] where high quality crystals were grown under conditions of constant pressure. The supersolid signals observed for these constant pressure samples were somewhat smaller in magnitude than those obtained for more disordered samples created in the same cell by the blocked capillary method.Recently, there has been a growing consensus in the theoretical community [7,8,9,10] that an ideal hcp helium crystal will not exhibit the supersolid phenomenon, but rather, some form of disorder such as vacancies, interstitials, superfluid grain boundaries [11,12], or perhaps a glassy or superglass phase [13,14] is required for the existence of the supersolid state. A summary of the current theoretical literature is given in a recent review [15]. FIG. 1: Torsional oscillator:The motion of the torsion bob is excited and detected electrostatically. A Straty...

Activated Nucleation of Vortices in a Dipole-Blockaded Supersolid Condensate

“…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. (11). The present view is, in fact, compatible with the vortex fluid model developed by Anderson [34] and also with recent experiments by Kim et al [35] in three dimensions.…”

“…While these experiments have been much reproduced [6][7][8][9], another line of experiment has been to focus on particular aspects of the helium sample, such as the annealing process or the measurement of the shear modulus, which might suggest another explanation for the apparent existence of the supersolid phase. The annealing process has been shown to dramatically reduce the superfluid fraction [10,11], while the shear modulus displays a similar dependence to the NCRI [12].…”

Annealing Effect for Supersolid Fraction in 4He

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“…Recently the observation of NCRI 1 in solid 4 He reported by Kim and Chan 2 for samples confined in Vyvor, in porous gold, and also in bulk solid. The existence of NCRI was reported by Shirahama et al 3 and by Rittner and Reppy 4 with torsional oscillator (TO) measurements.…”

Mobility transition from ballistic to diffusive transport in non-Hermitian lattices

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