Structural origins of the boson peak in metals: From high-entropy alloys to metallic glasses

TL;DR

This study investigates the origins of the boson peak in metallic glasses and high-entropy alloys, revealing that structural disorder, rather than chemical disorder or density reduction, induces the excess vibrational modes associated with the boson peak.

Contribution

The paper demonstrates through computer modeling that structural disorder alone can produce the boson peak in metallic systems, challenging previous theories attributing it to defects or density effects.

Findings

Structural disorder induces the boson peak in metallic alloys.

Chemical disorder does not produce sufficient local softening for the boson peak.

Density reduction is not responsible for the boson peak in the studied metallic systems.

Abstract

The boson peak appears in all amorphous solids and is an excess of vibrational states at low frequencies compared to the phonon spectrum of the corresponding crystal. Until recently, the consensus was that it originated from "defects" in the glass. The nature of these defects is still under discussion, but the picture of regions with locally disturbed short-range order and/or decreased elastic constants has gained some traction. Recently, a different theory was proposed: The boson peak was attributed to the first van Hove singularity of crystal lattices which is only smeared out by the disorder. This new viewpoint assumes that the van Hove singularity is simply shifted by the decreased density of the amorphous state and is therefore not a glass-specific anomaly. In order to resolve this issue, we use computer models of a four-component alloy, alternatively with chemical disorder (high-entropy alloy), structural disorder, and reduced density. Comparison to a reference glass of the same composition reveals that the boson peak consists of additional vibrational modes which can be induced solely by structural disorder. While chemical disorder introduces fluctuations of the elastic constants, we find that those do not lead to sufficient local softening to induce these modes. A boson peak due to a reduction of density could be excluded for the present metallic system.