Defect motion in a quantum solid with spin: hcp He3

Abstract

Defect motion in solid helium has a unique quantum nature due to the large zero-point motion of helium atoms, which allows vacancies and isotopic impurities to tunnel and move ballistically. Recent shear modulus experiments showed that dislocations are also extraordinarily mobile in solid $^{4}\mathrm{He}$. The lighter isotope, $^{3}\mathrm{He}$, has even larger zero-point motion and an extra degree of freedom---nuclear spin---which can affect defect motion. We have measured the shear modulus of hcp solid $^{3}\mathrm{He}$ to probe the motion of dislocations and isotopic impurities. We observed a crossover between stiff and soft states, due to $^{4}\mathrm{He}$ impurities which immobilize dislocations at low temperatures. In contrast to solid $^{4}\mathrm{He}$, the impurities in hcp $^{3}\mathrm{He}$ act as static pinning sites because of the disordered configuration of $^{3}\mathrm{He}$ nuclear spins. In addition, we observed an unexpected dissipation that increased rapidly at low frequencies, indicating a strong interaction between nuclear spins and moving dislocations.