Low temperature acoustic properties of amorphous silica and the Tunneling Model

TL;DR

This study measures the low-temperature acoustic properties of amorphous silica, confirming the Tunneling Model's predictions in the linear regime and revealing unexpected behaviors in the non-linear regime below 10 mK.

Contribution

It provides experimental validation of the Tunneling Model at ultra-low temperatures and uncovers novel non-linear effects below 10 mK in amorphous silica.

Findings

Excellent agreement with the Tunneling Model in the linear regime

Identification of a cut-off energy at 6.6 mK for tunneling states

Observation of unexplained non-linear behavior below 10 mK

Abstract

Internal friction and speed of sound of a-SiO(2) was measured above 6 mK using a torsional oscillator at 90 kHz, controlling for thermal decoupling, non-linear effects, and clamping losses. Strain amplitudes e(A) = 10^{-8} mark the transition between the linear and non-linear regime. In the linear regime, excellent agreement with the Tunneling Model was observed for both the internal friction and speed of sound, with a cut-off energy of E(min) = 6.6 mK. In the non-linear regime, two different behaviors were observed. Above 10 mK the behavior was typical for non-linear harmonic oscillators, while below 10 mK a different behavior was found. Its origin is not understood.