Two-level systems and mass deficit in quantum solids

TL;DR

This paper investigates how two-level systems in disordered quantum solids cause an effective mass deficit observable at high frequencies, with implications for interpreting torsional oscillator experiments.

Contribution

It demonstrates that the mass deficit effect appears only at high frequencies and vanishes at low frequencies, clarifying experimental observations in quantum solids.

Findings

Mass deficit manifests at high frequencies

Mass deficit vanishes in low-frequency limit

Experimental crossover observed in torsional oscillators

Abstract

We study a disordered quantum solid incorporating two-level systems in which a group of atoms (or a single atom) can experience coherent tunnelling between two different positions and demonstrate that an effective mass deficit induced by the presence of such objects can manifest itself only at relatively high frequencies and should vanish in the low-frequency limit. The crossover to the regime which can be associated with the appearance of an effective mass deficit has been observed in recent torsional oscillator experiments.