Nonuniversality and strongly interacting two-level systems in glasses at low temperatures

M Schechter,P Nalbach,A L Burin

doi:10.1088/1367-2630/aac930

Abstract

Recent experimental results showing atypical nonlinear absorption and marked deviations from well known universality in the low temperature acoustic and dielectric losses in amorphous solids prove the need for improving the understanding of the nature of two-level systems (TLSs) in these materials. Here we suggest the study of TLSs focused on their properties which are nonuniversal. Our theoretical analysis shows that the standard tunneling model and the recently suggested two-TLS model provide markedly different predictions for the experimental outcome of these studies. Our results may be directly tested in disordered lattices, e.g KBr:CN, where there is ample theoretical support for the validity of the two-TLS model, as well as in amorphous solids. Verification of our results in the latter will significantly enhance understanding of the nature of TLSs in amorphous solids, and the ability to manipulate them and reduce their destructive effect in various cutting edge applications including superconducting qubits.

Highlights

Amorphous solids, and many disordered lattices, show remarkable universality in their thermodynamic, dielectric and acoustic properties at low temperatures
Recent experimental results showing untypical nonlinear absorption and marked deviations from well known universality in the low temperature acoustic and dielectric losses in amorphous solids prove the need for improving the understanding of the nature of two-level systems (TLSs) in these materials
The predictive power of the Two-TLS model lies in the domination of nonuniversal low-energy properties by the S-TLSs, despite their scarcity at the relevant energies. It is the focus of this paper to obtain and propose within the Two-TLSs model experimental observables which go beyond the Standard Tunneling Model” (STM), and provide a clear signature of Two-TLSs physics

Summary

Introduction

Many disordered lattices, show remarkable universality in their thermodynamic, dielectric and acoustic properties at low temperatures. The Two-TLS model derives a relation between the single TLS DOS of the τ -TLSs and their strain interaction strength, which results in the small and universal value of the tunneling strength This result suggests that the origin of the low temperature universality, as well as the energy scale of 3 K determining the universal regime, are consequences of the smallness and universality of deviations from local inversion symmetry in strongly disordered and amorphous materials. The predictive power of the Two-TLS model lies in the domination of nonuniversal low-energy properties by the S-TLSs, despite their scarcity at the relevant energies It is the focus of this paper to obtain and propose within the Two-TLSs model experimental observables which go beyond the STM, and provide a clear signature of Two-TLSs physics. Nonuniversality and strongly interacting two-level systems in glasses at low temperatures

Single TLS DOS in the Two-TLS model

Spectral diffusion and nonlinear acoustic absorption

Nonequilibruim absorption

The dipole gap of the S and τ TLSs

Discussion