Why phonon scattering in glasses is universally small at low temperatures

TL;DR

This paper offers a new theoretical explanation for the universal smallness of phonon scattering in glasses at low temperatures, aligning well with experimental data across various glass types.

Contribution

It introduces a model with a broad distribution of phonon-TLS couplings that accounts for the universal phonon scattering behavior in glasses at low temperatures.

Findings

Good agreement between theory and experiment for various glasses.

Exponential renormalization of tunneling matrix elements.

Reduction of TLS density of states due to TLS-TLS interactions.

Abstract

We present a novel view of the standard model of tunneling two level systems (TLS) to explain the puzzling universal value of a quantity, $C\sim 3\times 10^{-4}$, that characterizes phonon scattering in glasses below 1 K as reflected in thermal conductivity, ultrasonic attenuation, internal friction, and the change in sound velocity. Physical considerations lead to a broad distribution of phonon-TLS couplings that (1) exponentially renormalize tunneling matrix elements, and (2) reduce the TLS density of states through TLS-TLS interactions. We find good agreement between theory and experiment for a variety of individual glasses.