  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.
