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

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