Does the boson peak survive in an ultrathin oxide glass?

TL;DR

This study demonstrates the existence of the boson peak in ultrathin alumina glass surfaces, showing it scales with particle size and is surface-originated, confirmed by neutron spectroscopy and molecular dynamics simulations.

Contribution

First direct measurement of the boson peak in nanoscale alumina glasses, revealing its surface origin and size-dependent behavior through neutron spectroscopy and simulations.

Findings

Boson peak observed at 2.8 meV in alumina nanoparticles.

Peak intensity inversely scales with particle size and oxide fraction.

Surface origin of the boson peak confirmed by simulations.

Abstract

Bulk glasses exhibit extra vibrational modes at low energies, known as the boson peak. The microscopic dynamics in nanoscale alumina impact the performance of qubits and other superconducting devices, however the existence of the boson peak in these glasses has not been previously measured. Here we report neutron spectroscopy on Al/Al$_2$O$_{3-x}$ nanoparticles consisting of spherical metallic cores from 20 to 1000 nm surrounded by a 3.5 nm thick alumina glass. An intense low-energy peak is observed at $\omega_{BP}$ = 2.8 $\pm$ 0.6 meV for highly oxidised particles, concurrent with an excess in the density of states. The intensity of the peak scales inversely with particle size and oxide fraction indicating a surface origin, and is red-shifted by 3 meV with respect to the van-Hove singularity of $\gamma$-phase Al$_2$O$_{3-x}$ nanocrystals. Molecular dynamics simulations of $\alpha$-Al$_2$O$_{3-x}$, $\gamma$-Al$_2$O$_{3-x}$ and a-Al$_2$O$_{3-x}$ show that the observed boson peak is a signature of the ultrathin glass surface, and the frequency is softened compared to that of the hypothetical bulk glass.