Boson peak in ultrathin alumina layers investigated with neutron spectroscopy

Bulk glasses exhibit extra vibrational modes at low energies, collectively known as the boson peak. The vibrational dynamics in nanoscale alumina glasses have an impact on the performance of qubits and other superconducting devices; however, the frequency of the boson peak has not been previously measured. Here we report neutron spectroscopy experiments on Al/Al2O3 nanoparticles consisting of spherical metallic cores with a radii from 20 to 1000 nm surrounded by a 3.5-nm-thick alumina glass. A low-energy peak is observed at ωBP = 2.8 ± 0.6 meV for highly oxidized particles, indicating 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 redshifted by 3 meV with respect to the van Hove singularity of γ-phase Al2O3 nanocrystals. Molecular-dynamics simulations of α-Al2O3, γ-Al2O3 and α-Al2O3 show that the observed boson peak is a signature of the ultrathin glass surface and the characteristic frequency is reduced compared to the peak in the bulk glass.