1,721,064 research outputs found
Inelastic He-atom scattering from metal surfaces
No full textIn 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
Brillouin Scattering and Surface Acoustic Phonons
No full textIn this work we present the theory of the scattering of light from an anisotropic semiinfinite medium. We show that the Brillouin scatterin can occour with two different mechanisms. The first one is related to the surface corrucation induced by the thermal phonons. The ripple effect is dominant in the scattering from metalli surfaces where the penetration depth of the light is very small. The second scattering mechanism is due to the modulation of the dielectric function of the whole medium induced by the phonons. It results particularly important for opaque semiconductors (elastooptic effect). In order to investigate the effects of the anisotropy of the medium we prent results for various faces of clean semiconductors
Anharmonic effects in a finite Frenkel-Kontorova type chain
No full textThe 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
Intrinsic localized modes in the bulk and at the surface of anharmonic diatomic chains
No full textIntrinsic 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
Surface Density of Acoustic Phonons in GaAs
No full textWe calculate the surface density of acoustic phonons for GaAs(110) and relate it to the Brillouin elasto-optic cross section. A continuum of surfacelike excitations is found in good agreement with the position and the shape of the surface structure observed in the measured Brillouin spectrum. The role played by the optical absorption coefficient in determining different scattering mechanisms is also discussed
Theory of intrinsic localized modes in diatomic chains: beyond the rotating wave approximation
No full textIntrinsic 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
Brillouin Scattering from Surface Ripples in GaAs
No full textWe compute the power spectrum of the surface displacements normal to the boundary for the faces (001), (110) and (111) of GaAs in the long wavelength limit. Recent experimental results of Brillouin scattering of reflected light are interpreted
Surface localized modes in anharmonic systems at low dimensionality
No full textIn this paper we discuss in detail the localized vibrational modes in monoatomic and diatomic one-dimensional chains induced by anharmonicity. Analysis of the effects of the quartic anharmonic interactions in the Vibrational spectrum shows, in the monoatomic case, the appearance of surface modes, not allowed in the harmonic field, and of high-frequency modes. In the diatomic case we found two types of surface mode, the first reminiscent of the harmonic one and the second entirely due to the anharmonicity, and two localized gap modes arising from the top of the acoustic branch with odd parity centered on a heavy atom and even parity centered on a light atom. The introduction of cubic anharmonicity produces odd and even gap localized modes arising from the bottom of the optical branch and affects the surface modes
Role of substrate geometry in sliding friction
No full textVia a Frenkel-Kontorova (FK) model approach, we study, in the context of solid friction phenomena, the role of the substrate potential geometry on the underdamped dynamics of a chain of interacting atoms. The choice of an on-site potential defined by the sum of two sinusoidal functions with different periodicity allows us to simulate microscopic sliding over simply periodic, multiple-well periodic and quasiperiodic substrates. We analyse the mobility-force characteristics and the related static friction, commenting on the nature of the particle dynamics in the vicinity of the pinning-depinning transition point and considering the role played by the coverage variable on the depinning mechanism. For the quasiperiodic cases, we also examine the hysteretic behaviour of the chain mobility as a function of the adiabatically varying force at zero temperature. For multiple-well periodic potentials, we observe the possible formation of commensurate dynamical structures during sliding
The phonon frequencies of the (111) surface of Platinum: existence of three localized modes
No full textThe 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
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