Damping in high-frequency metallic nanomechanical resonators

TL;DR

This study investigates damping mechanisms in high-frequency aluminum nanomechanical resonators at very low temperatures, revealing anomalous behaviors linked to two-level systems and phonon interactions.

Contribution

It identifies a crossover in damping behavior at 1 K and attributes low-temperature anomalies to long-lived two-level systems interacting with phonons.

Findings

Logarithmic frequency dependence below 1 K

Linear damping dependence below 1 K

Long relaxation times of two-level systems

Abstract

We have studied damping in polycrystalline Al nanomechanical resonators by measuring the temperature dependence of their resonance frequency and quality factor over a temperature range of 0.1 - 4 K. Two regimes are clearly distinguished with a crossover temperature of 1 K. Below 1 K we observe a logarithmic temperature dependence of the frequency and linear dependence of damping that cannot be explained by the existing standard models. We attribute these phenomena to the effect of the two-level systems characterized by the unexpectedly long (at least two orders of magnitude longer) relaxation times and discuss possible microscopic models for such systems. We conclude that the dynamics of the two-level systems is dominated by their interaction with one-dimensional phonon modes of the resonators.