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

The boson peak is an excess in the vibrational spectrum of amorphous solids, which was recently conjectured to be simply related to features already inherent in the vibrations of the corresponding crystal. In this work, the authors reveal that this is not the case, at least for metallic systems. Using atomic-scale computer simulations of a four-component alloy in different structural states, either as a metallic glass or as a high-entropy alloy, they show that the boson peak does not occur in a defect-free lattice, independent of its density. This disproves, at least for the case of amorphous metals, the recent notion that the vibrational spectra of glasses are not distinct from that of crystals, but simply shifted because of the reduced density that always accompanies the amorphous state. Instead, additional modes arise in softened regions of the material, which are connected to structural defects. These defects can take the form of ''soft spots'' in glasses or even a high concentration of lattice defects in crystals. The boson peak is therefore, in fact, a signature of structural disorder.