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Delocalized nonlinear vibrational modes (DNVMs) are exact solutions of the equations of motion, and therefore, DNVMs exist at any vibration amplitude and do not depend on interaction potentials. For the first time, modulation instability of four one-component three-dimensional DNVMs is studied in a single crystal of fcc copper with the use of methods of molecular dynamics. DNVMs frequencies, evolution of stresses, kinetic and potential energies, and heat capacity depending on the oscillation amplitudes are analyzed. It is found that all four DNVMs are characterized by a hard-type anharmonicity. Modulation instability of DNVMs results in a formation of chaotic discrete breathers (DBs) with frequency above the upper edge of the phonon spectrum of the crystal. The lifetime of chaotic DBs is found to be in the range of 30–100 ps. At low-oscillation frequencies, longer-lived DBs are formed. The growth of modulation instability leads to an increase in mechanical stresses and a decrease in the heat capacity of the crystal. The results obtained in this work enrich our understanding of the influence of the modulation instability of DNVMs on the properties of metals.

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The discovery of a new two dimensional material is a new trend emerged after the discovery of graphene in 2004. Hundreds of new materials have been predicted, designed and investigated during last decade. Many of them have outstanding properties for potential application in new technologies. One of those materials is so-called borophene, being isolated atomically thin boron. This material can show different electronic properties, including semiconducting, semi-metallic, and metallic states due to its unique structure. Borophene demonstrates unusually high in-plane elasticity and strength. The crucial point for application of borophene is its low structural stability at room temperature. A possible way to solve this problem is the functionalization of its surface by external atoms like hydrogen and fluorine. This work presents a study on the effect of O dopants and the presence of vacancy defects on electronic properties of borophene by using first-principles calculations. It was revealed that unlike other 2D materials, the band structure of borophene is stable by respect to the above-mentioned factors. However, the doping by O atoms is found as an effective way to increase the stability of the borophene sheet.