Inelastic He-atom scattering from metal surfaces

In this paper we review the recent advances in the theory of He-atom inelastic scattering used to study the lattice dynamics of metal surfaces. We discuss the importance of the He-surface interaction potential and the various techniques employed up to now to determine the Surface phonon dispersions. The anharmonic contributions to the one-phonon linewidth are also discussed

The phonon frequencies of the (111) surface of Platinum: existence of three localized modes

The He surface reflection coefficients have been evaluated for the Pt(111) surface by using a long range force constants model for the bulk phonons. The lowering of the lateral surface force constants allows one to explain in a quantitative manner the existence of all the peaks observed in the experimental spectra

Surface and gap intrinsic localized modes in one-dimensional III-V semiconductors

A theoretical investigation has been made of intrinsic localized vibrational modes in an anharmonic one-dimensional diatomic lattice with alternating force constants coupling successive neighbours. This system simulates a row of atoms in the (1 1 1) direction of a III-V semiconductor. Specific calculations have been carried out for GaN, because it has a large gap between acoustic and optical branches. We study small-amplitude atom vibrations (up to 0.4 Angstrom), accessible to experimental detection, in order to legitimize the expansion of the full potential to include cubic and quartic terms. We consider then nearest-neighbour interactions through harmonic as well as cubic and quartic anharmonic interactions to study the interplay between cubic and quartic terms in the frequencies of the localized modes. The force constants were determined empirically by fitting the longitudinal branches in the Gamma-L direction of GaN. We have studied both gap and surface intrinsic localized modes. Zinc-blende-structure chains are of particular interest, because the lack of inversion symmetry prevents the classification of the modes as even or odd parity. Nevertheless, modes were found that closely resemble the even- or odd-parity modes of an NaCl-structure chain. Their frequencies lie inside the gap for GaN. The absence of inversion symmetry permits a variety of surface modes to exist, depending on whether the bond at the surface is strong or weak and the atom at the surface is light or heavy. All surface mode frequencies for GaN lie inside the gap as found with the use of the full potential

Localized modes in monoatomic and diatomic chains due to the anharmonicity

Intrinsic localized modes due to the anharmonicty are discussed in detail in the case of monoatomic chains considering a finite (even or odd) number of particles interacting with nearest neighbours harmonic and quartic anharmonic force constants. We find odd parity and even parity localized modes above the maximum harmonic frequency in the monoatomic case. These modes persist modifying the anharmonic force constant at the center of the chain, but their envelope shape becomes double peaked. Preliminary resylts for diatomic chains are also presented. In tis case we find odd localized modes above the maximum harmonic frequency and in the gap between the acoustic and optic continuum

Intrinsic localized modes in the bulk and at the surface of anharmonic diatomic chains

Intrinsic localized vibrational modes in a diatomic anharmonic chain are discussed in detail. We consider a finite (even or odd) number of particles interacting with nearest-neighbor harmonic and cubic and quartic anharmonic potentials. For suitable values of the potential parameters we find two odd parity modes localized about light and heavy atoms, respectively, with frequencies above the top of the optical branch. In the gap there are localized modes arising from the top of the acoustic branch with odd parity if they are centered on a heavy atom and even parity if centered on a light atom. There are also two localized modes originating from the bottom of the optical branch. The odd parity mode is centered on a light atom, while the even parity mode is centered on a heavy atom. In addition, we have found two types of surface modes. One is the anharmonic version of the surface mode occurring in a finite diatomic harmonic chain with frequency in the gap, and the other is entirely due to the anharmonicity. The latter mode has its frequency above the top of the optical branch. A comparison with other work is given

Anharmonic effects in a finite Frenkel-Kontorova type chain

The features of the ground state and the phonon frequency spectrum of a finite Frenkel-Kontorova type model are investigated in presence of quartic anharmonic nearest-neighbor interactions. The different computational techniques used allow us to determine with great accuracy the critical parameter for the ground-state symmetry breaking transition, showing its dependence on the strength of the interatomic anharmonicity introduced. The phonon gap. the linearized vibrational modes and their parity are also studied in the vicinity of the transition point. (C) 2002 Elsevier Science B.V. All rights reserved

Interaction of an external impurity with the surface intrinsic mode in a Heisenberg chain

In this paper we have demonstrated the existence of surface intrinsic localized modes in one-dimensional classical spin systems. These modes have frequencies above the top of the magnon band and are more localized than the corresponding modes that were found in the lattice dynamics. By numerical methods we have studied the forward evolution in time. These entities are stable in time for many time steps. Also, we consider the interaction of these surface excitations with an external dipolar field. We have first considered the case in which the external dipole is parallel to the spins of the bulk. According to the distance of the impurity from the surface we find that at a particular distance a new surface mode appears, that we have called a mixed mode because it is formed by a localized state at the surface plus a plane wave. At large distances the dipole traps the surface mode, while at small distances the external impurity potential is so strong that all the spins are forced to be aligned in the z direction. For the case in which the external magnetic dipole is antiparallel to the bulk spins there are no mixed modes and the external impurity is so strong that the surface modes are trapped at large distances, and the bulk modes are trapped at very short distances

Anticorrugation in inelastic He-atom scattering from Rh(111) surfaces

Recent measurements supported by nb initio calculations have shown that the interaction of He atoms with transition metal surfaces is anticorrugating, i.e., the electron density seen by He is higher in the bridge positions than in the on-top positions. By shifting the electron charge density away from the atomic positions and using the Esbjerg-Norskov approach to determine the interaction potential, we evaluate the inelastic reflection coefficients of He with Rh(lll). The structure factor introduced by the anticorrugation explains, in a simple way, the high intensity observed for the resonant mode

Theory of intrinsic localized modes in diatomic chains: beyond the rotating wave approximation

Intrinsic localized modes in the gap of a diatomic chain with free ends are discussed in detail by going beyond the rotating wave approximation. We include in the time dependence of the displacements terms up to cos(2omegat). We consider a finite chain of particles interacting with nearest-neighbour interactions. We study amplitudes of the intrinsic localized modes smaller than 0.25 Angstrom. In this range of amplitudes the full potential can be well represented by an expansion in powers of the displacements up to fourth-order terms. The use of a force constant model allows us to simplify the problem. As a test case we consider a chain of LiI atoms. We found intrinsic localized modes in the gap. The amplitudes of the first harmonic term (cos(omegat)) are of even or odd parity, whereas we prove that the amplitudes of the static part and those of the second harmonic can have only even symmetry. The main result of the paper is that the amplitudes associated with the second harmonic are two or three orders of magnitude less than those of the first harmonic. Furthermore, the frequency of the localized modes are modified by less than 1% by the inclusion of the second harmonic