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 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. MD simulations of the nonlinear dynamics of Ni and Nb confirmed that high frequency ILMs may exist in these metals.
Abstract. 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 them. 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 to large distances along crystallographic directions. If the recoil energy exceeds tens of eVs, vacancies and interstitials can be formed, being 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.
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.Comment: 17 pages, 5 figure
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, MD calculations of vibrations under strong local excitation are carried through with high precision. The results fully confirm the prediction.
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.
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