M. Klopov

Prediction of high-frequency intrinsic localized modes in Ni and Nb

It is found that in some metals, an intrinsic localized mode may exist with frequency above the top of the phonon spectrum. The necessary condition, requiring a sufficiently high ratio of quartic to cubic anharmonicity, may be fulfilled because of screening of the interaction between ions by free electrons. Starting from the known literature values of the pair potentials, we have found that in Ni and Nb the derived localized mode condition is fulfilled. Molecular-dynamics simulations of the nonlinear dynamics of Ni and Nb confirmed that high-frequency intrinsic localized modes may exist in these metals.

Theory and molecular dynamics simulations of intrinsic localized modes and defect formation in solids

Molecular dynamics (MD) simulations of recoil processes following the scattering of x-rays or neutrons have been performed in ionic crystals and metals. At small energies (< 10 eV) the recoil can induce intrinsic localized modes (ILMs) and linear local modes associated with these modes. As a rule, the frequencies of such modes are located in the gaps of the phonon spectrum. However, in metallic Ni, Nb and Fe, due to the renormalization of atomic interactions by free electrons, the frequencies mentioned are found to be positioned above the phonon spectrum. It has been shown that these ILMs are highly mobile and can efficiently transfer a concentrated vibrational energy over large distances along crystallographic directions. If the recoil energy exceeds tens of eVs, vacancies and interstitials can be formed, which are strongly dependent on the direction of the recoil momentum. In NaCl-type lattices the recoil in (110) direction can produce a vacancy and a crowdion, while in the case of a recoil in (100) and in (111) directions a bi-vacancy and a crowdion can be formed.

Transverse intrinsic localized modes in monatomic chain and in graphene

Abstract In this paper an analytical and numerical study of anharmonic vibrations of monatomic chain and graphene in transverse (perpendicular) with respect to the chain/plane direction is presented. Due to the lack of odd anharmonicities and presence of hard quartic anharmonicity for displacements in this direction, there may exist localized anharmonic transverse modes with the frequencies above the spectrum of the corresponding phonons. Although these frequencies are in resonance with longitudinal (chain) or in-plane (graphene) phonons, the modes can decay only due to a weak anharmonic process. Therefore the lifetime of these vibrations may be very long. E.g. in the chain, according to our theoretical and numerical calculations it may exceed 10 10 periods. We call these vibrations as transverse intrinsic localized modes.

Linear local modes induced by intrinsic localized modes in a monatomic chain

A theory is developed to describe the effect of an intrinsic localized mode (ILM) on small vibrations in a monatomic chain with hard quartic anharmonicity. One prediction is the appearance in the chain of linear local modes nearby the ILM. To check this result, molecular dynamics calculations of vibrations under strong local excitation are carried through with high precision. The results fully confirm the prediction.

Standing and Moving Discrete Breathers with Frequencies Above the Phonon Spectrum

It is found that discrete breathers with frequencies above the top of the phonon spectrum may exist in some metals (Ni, Fe, Cu) and semiconductors (Ge, diamond). It is shown that these excitations in metals may propagate in crystals along crystallographic directions transferring energy of \(\gtrsim \)1 eV over large distances.

Effects of long-range forces in nonlinear dynamics of crystals: creation of defects and self-localized vibrations

A method of performing MD simulations of the local dynamics of 3D nonlinear crystal lattices has been presented, based on the nonlinear integral equations of the atomic motions in the site representation. The method takes into account long-range forces and has been verified on a perfect NaI crystal, where the radiation-produced recoil processes have been investigated. The energy thresholds of the defect formation have been determined to vary from 1.3 eV to 12 eV in dependence of the recoil direction. At lower recoil energies the properties of the created intrinsic localized vibrational modes (ILM) have been studied. A comparison with the traditional methods based on the clusters of nonpolarizable ions has been provided.

Electronic and optical properties of magnesium and calcium hydroxides: the role of covalency and many-body effects

Magnesium and calcium hydroxides X(OH)2 (X = Mg, Ca) are multifunctional materials that have many important applications in industry, technology, and research. In solid-state electronics, the emerging applications of these compounds are related to photovoltaic devices. In the present paper we review electronic properties of X(OH)2, band gaps, work function, and features of chemical bonding and discuss theoretically predicted exciton effects.

Self-consistent theory of intrinsic localized modes: Application to monatomic chain

A theory of intrinsic localized modes (ILMs) in anharmonic lattices is developed, which allows one to reduce the original nonlinear problem to a linear problem of small variations of the mode. This enables us to apply the Lifshitz method of the perturbed phonon dynamics for the calculations of ILMs. In order to check the theory, the ILMs in monatomic chain are considered. A comparison of the results with the corresponding molecular dynamics calculations shows an excellent agreement.

Intrinsic localized modes and trapped phonons in crystal lattices

An analytical theory of intrinsic local modes (ILMs) in infinite perfect lattices is proposed. One prediction of the theory is that for some conditions linear local modes appear near the ILM. High-precision MD calculations of the vibrations of diatomic chains confirm the prediction.