Non-classical rotational inertia and shear modulus of solid helium

Abstract

Supersolid is a state of matter that has the properties of a solid and a superfluid simultaneously. In 1970, non-classical rotational inertia (NCRI) was suggested to be a consequence of supersolidity; this property was detected by Kim and Chan in 2004, who found a drop in the resonant period of a torsional oscillator. However, the exact mechanism of the phenomenon is not clear yet. This dissertation presents three experimental approaches to revealing the origin of the supersolid state. The purpose of the first experiment is to look for a 2-dimensional supersolid. In 2-dimensional helium films, the first few atomic layers, which are called the inert layer, are strongly localized and considered to be solid-like. In order to find the superfluidity in the inert layer helium films, torsional oscillators containing porous Vycor glass were used. However, no signature of NCRI was found in the inert layer films that were adsorbed on Vycor glass. The helium films adsorbed on the Vycor glass are known to be amorphous. This amorphous structure or strong interaction between helium and Vycor might have prohibited the appearance of superfluidity. Meanwhile, Day and Beamish observed a shear modulus increase in solid helium at low temperatures in 2007. This result brought a lot of interest because it showed remarkable similarities with the NCRI in the torsional oscillator experiments. Although the rotational inertia and the shear modulus are different physical quantities, they showed similarities in temperature dependence, drive amplitude dependence, frequency dependence, $^3He$ concentration dependence, hysteresis and dissipation. These similarities indicate that the two phenomena are closely related. It was suggested that the origin of the shear modulus increase was the dislocation pinning to $^3He$ impurities. To clarify the relationship between the NCRI and the shear modulus increase, two experiments have been performed. In the solid helium inside a narrow pores, it is ha...