Surface Vibrational Modes Research Articles

Surface-related phonon mode in porous GaP

Porous GaP layers prepared by electrochemical anodization of (1 0 0) and (1 1 1) A-oriented n-GaP crystalline substrates in HF solution have been studied by Raman spectroscopy. A surface vibrational mode at 397 cm −1 was observed in porous GaP. The process of anodization results in downward shifts of the TO and LO phonon frequencies, which are attributed to phonon confinement in crystalline GaP particles of nanometer size.

Hydrogen interaction with Sb-terminated GaAs and InP (110) surfaces.

The interaction of atomic hydrogen with InP(110) and GaAs(110) surfaces terminated with a well-ordered antimony monolayer was investigated using spectroscopic ellipsometry and Raman spectroscopy. The characteristic features of the surface dielectric function associated with the Sb monolayer and the Raman lines from the excitation of surface vibrational modes disappear with increasing hydrogen exposure, indicating the etching of the Sb monolayer. No additional structure in the surface dielectric function and no shift in the phonon frequencies were detected during the etching process. By comparing the experimental results with ab initio calculations we conclude that the etching of the Sb monolayer starts at defect sites of the Sb monolayers and then propagates through the Sb surface chains.

Optical characterization of surface electronic and vibrational properties of epitaxial antimony monolayers on III–V (110) surfaces

The electronic and vibrational properties of InP(110), GaAs(110), GaP(110), and InAs(110) surfaces terminated with ordered Sb monolayers are studied by Reflectance Anisotropy Spectroscopy (RAS) and Surface Resonant Raman Spectroscopy (SRRS). In the RAS spectra of the monolayer covered surfaces optical anisotropies attributed to transitions of the electronic surface band structure of the monolayer can be identified. On all four substrates, prominent surface transitions are found in the photon energy range from 1.8 to 2.8 eV. The SRRS, on the other hand, is utilized to monitor surface vibrational modes of the monolayer terminated surfaces. The scattering cross section reveals a clear correlation to the surface transitions detected by RAS; for resonance condition to the surface electronic transitions maximum scattering efficiency occurs. This can be understood in terms of deformation potential scattering. Since the Sb monolayer vibrational modes modulate mainly the bonds of the first few atomic layers, the corresponding modulation of the polarizability is dominated by the surface electronic band structure.

Three-dimensional phonon confinement in CdSe microcrystallites in glass

Results are presented which demonstrate the existence of unambiguous three-dimensional quantum confinement of phonons within crystallites of the II-VI semiconductor CdSe, which were prepared by the method of diffusive growth within a doped glass matrix. The confinement was investigated by means of Raman scattering from zone centre longitudinal optical (LO) phonons. These Raman spectra illustrate LO phonon peaks showing a low-energy shift and broadening with decreasing crystallite size. The experimental measurements are compared with a theoretical model of phonon confinement which includes a size distribution parameter. The experimental results are in good agreement with the predictions of this model. Surface vibrational modes and the effects of hydrostatic pressure from the glass matrix do not appear to be of significance in the samples investigated.

Collective modes in a strangelet.

Possible existence of various collective modes, such as the compressional, the polarizational, and the surface vibrational modes in a strangelet, are discussed in the framework of hydrodynamic model. The eigenenergies of these collective modes are estimated using an equation of state for the interacting quark matter based on the quark bag model and are compared with the corresponding modes in the nuclei. It is found that, within the uncertainties associated with the bag model parameters, the collective eigenenergies in a strangelet are 1.5\char21{}4 times higher than those in the nuclei of same baryon number. The collective mode behavior may distinguish a strangelet from a large nucleus and give signature of the formation of quark-gluon plasma in ultrarelativistic heavy-ion collision experiments.

First-principles molecular dynamics study of surface vibrations and vibrational mode coupling on the H/Si(111)1×1 surface

The vibrational dynamics of hydrogen on the H/Si(111)1×1 surface are studied using the first principles Car–Parrinello molecular dynamics method. The calculated frequencies of the surface vibrations are found to be in good agreement with experimental results. The combination band spectra indicate there are anharmonic couplings between the Si–H stretching mode and other lower frequency silicon surface phonon modes.

Low frequency Raman scattering from surface vibrational modes and size of microheterogenities in an amorphous matrix

Polarized room-temperature Raman scattering spectra measured on heat-treated GeO 2 -based glasses show features resembling particle peaks. They are related to the discrete particle-like structure of thermally treated samples. We have also found that our light scattering spectra have characteristic polarization properties

Anharmonic surface vibrations in photoemission from alkali metals.

The phonon widths of outermost core-electron photoemission spectra from (110)-oriented overlayers of Na, K, and Rb metals show the expected Debye behavior for the bulk atoms, but significant deviations for the surface atoms. The data indicate a softening of the surface vibrational mode above 200 K. This effect, which is weak in Na but strong in K and Rb, demonstrates that the vibrational mode normal to the surface is anharmonic.

Surface vibrations at clean and hydrogenated GaAs(110) from ab-initio molecular dynamics

We present results of finite-temperature Car-Parrinello molecular dynamics simulations of the atomic geometry and the vibrations at clean and hydrogenated (1 and 0.25 ML coverages) GaAs(110) surfaces. Through an accurate analysis of the atomic motion based on signal processing techniques the phonon modes have been investigated. Some surface vibrational modes at high-symmetry k points, with layer-by-layer resolution of the eigenvectors, have been found. The results are compared with available experimental data and previous calculations.

Shear horizontal vibrations at the (0001) surface of beryllium

We report the results of a high-resolution Electron-Energy Loss Spectroscopy (EELS) investigation of the Be(0001) surface. Two dispersive surface vibrational modes are observed in the ▪ to M̄ direction: a sagittal-plane mode (the Rayleigh wave) and a shear horizontal mode. We have compared our experimental results to a 300-layer slab calculation and to a semi-infinite Green's function calculation of Sameth and Mele. Both calculations predict a higher energy for the Rayleigh wave and lower energy for the shear horizontal mode than is experimentally measured. These results are consistent with stronger in-plane bonding and weaker interplanar bonding at the surface, in accord with other theoretical predictions.

Elastic and inelastic interactions of He and Ne atoms with metal surfaces

The experimental information accumulated in recent years on elastic and inelastic scattering of He and Ne atoms from metal surfaces is analysed by describing the atom-surface interaction as a superposition of pseudo-pairwise potentials. These are chosen according to the quite accurate potentials presently available for van der Waals pairs and take a phenomenological account for many body effects. The emerging picture is quite satisfactory and allows the physical interaction of a given atom-surface system to be predicted with excellent accuracy. In particular the coupling of He atoms to the surface vibrational modes is considered with attention focused on He inelastic scattering from Cu(001).

Wavepackets and inelastic tunnelling

Using real-time numerical propagation techniques, the authors consider the tunneling of electron wavepackets through potential barriers that possess internal dynamics. The models considered allow the study of the influence of localized surface vibrational modes on the tunnel current observed in scanning tunnelling microscopy. Non-resonant and resonant processes are considered. In the former case, the authors focus on the behaviour of the total tunnel current when the energy sweeps through an inelastic threshold. In the resonant case, they make contact with analytic theory and consider in detail, using a two-channel Breit-Wigner model, the physics underlying the resonant process, the physical origin of sum-rules obeyed by the tunnelling characteristic, and the conditions under which simple analytic theoretical results can be applied in general.

Low-energy vibrational modes of the monolayer adsorbate CO2/NaCl(001)

Dispersion curves of surface vibrational modes of monolayer CO2/NaCl(001) have been measured by inelastic He atom scattering along the ΓX azimuth. Eight phonon modes could be followed across almost the entire Brillouin zone. The identification of the modes is discussed based on previously calculated normal mode energies at the zone origin.

Semiconductor surface and interface dynamics from tight-binding molecular dynamics simulations

Tight-binding molecular dynamics simulations have been performed to compute the bulk, (110) surface, and (110)-p(1×1)-Sb(1 ML) interfacial atomic vibrational spectra for GaAs and InP. The same tight-binding total energy model that successfully described the static surface and interfacial atomic and electronic structure for these systems is utilized in the molecular dynamics simulations. The results for the bulk vibrational energies are in semiquantitative agreement with experimental results, displaying approximately the same level of variance as other model computations. Moreover, these simulations are used to examine the effects of anharmonicity in the system by investigating the temperature dependence of the vibrational spectra. The (110) surface vibrational energies are in quantitative agreement with the scattering data, and a comparison of the results for GaAs(110) and InP(110) supports the existance of a surface vibrational mode that is characteristic of the relaxed (110) surface, and whose energy is similar for each zinc blende (110) surface. Last, the computed vibrational energies for the III–V(110)-p(1×1)-Sb(1 ML) interface are in semiquantitative agreement with Raman scattering data and illustrate the effects of the overlayer binding on the surface vibrational spectrum.

The phonons of the Al surfaces: comparison between semi-empirical and ab initio calculations

We report inelastic He-atom surface scattering data for the low-index surfaces of aluminum. The experimental data are analyzed by using a semi-empirical Born-von Karman model in which both two-body and three-body interactions are taken into account and by using an ab initio pseudopotential approach. Our analysis shows that the semi-empirical model containing long-range forces agrees with great accuracy with the pseudopotential calculations for what concerns both the range of the interactions and the dispersion of the surface vibrational modes. The calculations of the reflection coefficients allow us to study the nature of the weak resonances observed in the time-of-flight spectra. These structures are related to a picking up of modes with polarization vector normal to the surface due to the modification introduced by the surface on the entire force field. Our analysis of Al proves that for simple metals an accurate semi-empirical model can reproduce with great accuracy both the surface lattice dynamics, without introducing new free parameters in the surface interatomic potential, as well as an ab initio pseudopotential calculation.

Surface vibrational modes of silanol groups on silica

Infrared spectroscopy has been used to study the vibrational modes associated with free (isolated) silanol groups on an aerosil silica and a precipitated silica which have been activated in vacuum in the temperature range from 450 to 800 o C. Both silicas exhibit two Si-O-H angle deformation modes at 760 and 840 cm -1 , indicative of two types of isolated silanol species, called type I and II, respectively

The Physics of Phonons

Elements of crystal symmetry: Direct lattice Reciprocal lattice Brillouin zone Crystal structure Point groups Space groups Symmetry of the Brillouin zone Jones zone Surface Brillouin zone Matrix representations of point groups. Lattice dynamics in harmonic approximation - semiclassical treatment: Introduction Lattice dynamics of a linear chain Lattice dynamics of three-dimensional crystals - phenomenological models Density of normal modes Numerical calculation of g(w) Lattice heat capacity. Lattice dynamics in the harmonic approximation - ab initio treatment: Introduction The frozen-phonon approach The linear response approach The planar force constant method. Anharmonicity: Introduction Hamiltonian of a general three-dimensional crystal Effect of anharmonicity on phonon states Effects of the selection rules on three-phonon processes Hamiltonian of an anharmonic elastic continuum Evaluation of three-phonon scattering strengths The quasi-harmonic approximation and Grueneisen's constant. Theory of lattice thermal conductivity: Introduction Relaxation-time methods Gree-Kubo linear response theory Second sound and Poiseuille flow of phonons. Phonon scattering in solids: Boundary scattering Scattering by static imperfections Phonon scattering in alloys Anharmonic scattering Phonon-electron scattering in doped semiconductors Phonon scattering due to magnetic impurities in semiconductors Phonon scattering from tunnelling states of impurities Phonon-photon interaction. Analysis of phonon relaxation and thermal conductivity results: Anharmonic decay of phonons Lattice thermal conductivity of undoped semiconductors and insulators Non-metallic crystals with high thermal conductivity Thermal conductivity of complex crystals Low-temperature thermal conductivity of doped semiconductors. Phonons in low dimensional solids: Introduction Surface vibrational modes Attenuation of surface phonons Phonons in superlattices Thermal conductivity of superlattices. Phonons in impure and mixed crystals: Introduction Localised vibrational modes in semiconductors Experimental studies of long-wavelength optical phonons in mixed crystals Theoretical models for long-wavelength optical phonons in mixed crystals Phonon conductivity of mixed crystals. Phonons in quasi-crystalline and amorphous solids: Introduction Phonons in quasi-crystals Structure and vibrational excitations of amorphous solids Vibrational properties of amorphous solids Low-temperature properties of amorphous solids. Phonon spectroscopy: Introduction Heat pulse technique Superconducting tunnel junction technique Optical techniques Phonons from Landau levels in 2DEG Phonon focusing and imaging Frequency crossing phonon spectroscopy Phonon echoes. Phonons in liquid helium: Introduction Dispersion curve and elementary excitations Specific heat Interactions between the excitations Kapitza resistance Quantum evaporation. Appendices: Density functional formalism The pseudopotential method Evaluation of integrals in sections 6.4.1.4 Negative-definitenss of the phonon off-diagonal operator ^D*L. References. Index.

Lattice dynamics on surfaces by atom scattering

Recently helium atom scattering from surfaces has been refined such that it allows studies of surface vibrational modes by investigating the inelastic component of the backscattered flux. Even though surface lattice dynamics has been accessible to theory for a long time, these experiments have provided a wealth of interesting and sometimes unexpected anomalies in the surface phonon dispersion. A most peculiar anomaly is that of a temperature dependent surface phonon mode. It has been found that such a ‘soft’ mode drives the well-known two-dimensional structural phase transitions on W(001) and Mo(001), the reconstruction of these surfaces. In addition, elastic He scattering has yielded surprising new information on the low-temperature phase structure of these systems. Most interesting are the striking differences between LEED patterns and those obtained by He atom diffraction.

Sub-barrier fusion of 37Cl with 58,60,62,64Ni.

Near-barrier and sub-barrier fusion excitation functions have been measured for the $^{37}\mathrm{Cl}$${+}^{58,60,62,64}$Ni systems using a recoil velocity spectrometer to identify evaporation residues. A considerable amount of enhancement was observed for all systems studied. The measured excitation functions for $^{37}\mathrm{Cl}$${+}^{58,62,64}$Ni are not in agreement with published results for these systems. The data were analyzed in the context of a coupled-channels model which took into account either the dependence of the fusion cross section on the coupling to surface vibrational modes, or, alternatively, on the orientation of deformed colliding nuclei. Very good agreement was obtained with the experimental data under the assumption that $^{37}\mathrm{Cl}$ exhibits a moderate oblate static deformation, while the light Ni isotopes are spherical vibrators. On the other hand, the data for $^{37}\mathrm{Cl}$${+}^{64}$Ni favor a moderate static oblate deformation for $^{64}\mathrm{Ni}$.

Dispersion relations of low-energy branches in the vibrational spectrum of cleaved GaAs(110).

A realistic calculation of the low-frequency phonon spectrum of cleaved GaAs(110) is presented. Our calculation is based on Chadi's tight-binding theory for structural energies and the structural energies are simply determined by the bond-orbital approximation. A variety of interesting surface vibrational modes are found which agree well with the observation, in He-atom scattering experiments, of some acoustic branches of surface phonons.