S. A. Kiselev

Generation of intrinsic vibrational gap modes in three-dimensional ionic crystals

The existence of anharmonic localization of lattice vibrations in a perfect three-dimensional diatomic ionic crystal is established for the rigid-ion model by molecular dynamics simulations. For a realistic set of NaI potential parameters, an intrinsic localized gap mode vibrating in the [111] direction is observed for fcc and zinc-blende lattices. An axial elastic distortion is an integral feature of this mode which forms more readily for the zinc blende than for the fcc structure. Molecular dynamics simulations verify that in each structure this localized mode may be stable for at least 200 cycles. {copyright} {ital 1997} {ital The American Physical Society}

Anharmonic impurity modes in a 1-D lattice with two-body potentials

Abstract The dynamics of a light impurity in a 1-D monatomic lattice with standard two-body potentials is studied. For sufficiently large amplitude the frequency crosses into the plane wave spectrum and the localized impurity mode becomes a resonant mode. A localized dc expansion of the lattice around the impurity accompanies the vibrational excitation in either case.

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.

Intrinsic Localized Modes in Anharmonic Lattices

The success of the harmonic approximation in describing vibrations of condensed matter systems comes about because the plane wave amplitude at any particular site is extremely small hence it does not matter that the intermolecular potentials themselves are considered to be anharmonic in nature. Anharmonicity is generally introduced perturbatively to explain experimentally observed phenomena such as thermal expansion and phonon combination bands. Here we examine the dynamical properties of a locally excited anharmonic lattice such as might be expected to occur during an optical excitation of a coupled electron-lattice system so that the anharmonicity can play a more significant role. One development presented here is a straightforward demonstration of intrinsic localized modes in perfect one-dimensional monatomic lattices with quartic anharmonicity. These modes are similar to those associated with previously studied force constant defects, but they may be located anywhere in a perfect lattice and can actually move from site to site given the appropriate initial conditions. Associated with each localized vibrational mode in monatomic lattices with cubic and quartic anharmonicity is a localized dc distortion which forms an integral part of this new dynamical configuration. The softening of the potential with large displacements from equilibrium also produces a red-shift in the local mode frequency, but stable modes are observed in molecular dynamics simulations for a large range of anharmonicity parameters. The building blocks presented here for the development of localized modes in the one dimensional anharmonic chain can be used directly in the discussion of more complex nonlinear systems.

Eigenvectors of strongly anharmonic intrinsic gap modes in three-dimensional ionic crystals

Abstract An artificial dynamical simulated annealing technique of the Car-Parrinello-type is used to demonstrate that anharmonic localization of lattice vibrations in a perfect 3D diatomic ionic crystal can occur. Molecular dynamics simulations with the rigid-ion two-body potential model make possible a study of the properties of intrinsic gap mode eigenvectors for different crystal structures. These eigenvectors consist of two very different parts with axial symmetries: an AC vibrational component and a DC distortion of the lattice. For the same crystal potential model intrinsic gap model form more readily for the lower symmetry zinc-blende structure than for the higher symmetry fcc one.

Localized vibrational modes in perfect crystals

The possibility that photo-physical persistence in solids may stem not only from the way atoms are rearranged but also from the way vibrational energy is distributed locally has led us to reexamine the anharmonic properties of simple lattice systems. We find that lattice anharmonicity may make localized vibrational excitations in perfect crystals allowed. In addition, they may be trapped at defects at low temperatures only to be released into the lattice at a higher temperature. Two defect systems KI: Ag + and Pb:Sn, which exhibit anomalous low-temperature properties, are reexamined from this perspective

Localized anharmonic defect modes in a one-dimensional lattice with standard two-body nearest-neighbor potentials

Abstract The frequency of an anharmonic local impurity mode is shown to decrease with increasing amplitude. When the frequency crosses into the phonon band, a resonant mode appears. A localized DC expansion of the lattice around the impurity increases as its vibrational frequency decreases.