1,721,063 research outputs found
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
Surface Lattice Dynamics in Semiinfinite Transition Metals
No full textWe present calculations of the surface phonon spectra in semiinfinite crystals of Iron and Thungsten,bound by an ideal (001) face,based on a force-constant parametrization of the bulk dynamics. The method allows to treat equally well stongly localized and deeply penetrating atomic vibrations. Local densities of phonon states are shown for the surface and for the first layer underneath,both for the localized modes and for the travelling waves,which are considerably modified by the presence of the surface
Role of three-body interactions in the calculator of the total energy and the shear modulus of hexagonal metals
No full textBy calculating the total energy and the shear modulus of hexagonal metals we show that the three-body interactions are not very important in the determination of the stable structure, but must be taken into account when studying the dynamical properties
Acoustic Phonons at the (111) Surface of Chromium
No full textRecent Brillouin scattering data for the (111) surface of Cr (Sandercock 1978) are compared with the theoretical cross section computed for a semi-infinite anisotropic elastic medium undergoing free-surface boundary conditions. The surface ripple mechanism is found to be responsible for the continuum of phonon states localised between the transverse and longitudinal thresholds
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 Lattice Dynamics of Trigonal Se and Te
No full textVery recently Brillouin scattering from thermally excited phonons has proved to be the most succesfull technique to directly measure surface vibrations, both in the discrete and in the continuos part of the spectrum
Brillouin Scattering at Interfaces and Longwavelength Acoustic Phonons
No full textWe present the theory of Brillouin scattering for an interface composed by a semi infinite medium and a finite slab. According to the thickness of the slab and the nature of the two systems various interesting features are present in the scattering cross section. In backward scattering experiments, to which we refer, are present structures corresponding to the Rayleigh, Sezawa and Lamb modes polarized in the sagittal plane. By considering transparent thick films of sufficiently high thickness it is possible to obtain information on the interface mode, the Stonely wave. This mode gives rise to a maximum in the Brillouin cross section. In the limit in which the thickness of the slab goes to infinity, the Lamb modes give rise to the density of states of the supported semi infinite medium
Acoustic and Optical Surface Phonons
No full textVery recently surface Brillouin scattering from opaque materials and electron energy loss spectroscopy from metallic surfaces with adsorbate were succesfully emplyed to detect surface phonons. We present calculations of surface acoustic phonons in the longwavelength limit for a semi-infinite medium in order to explain the Brillouin cross section. The case of GaAs and Al coated Si are considered. The surface optical phonons that are rapidly decaying in the medium are evaluated with the layer method and the results are compared with the electron energy loss spectra
Surface Brillouin Scattering from Acoustic Phonons: II. Application to Semiconductors
No full textWe have applied to semiconductors the thory of Brillouin scattering from corrugated surfaces developed previously. We have fitted the available spectra measured with 5145 A light in order to evaluate the elastooptic constants of the medium. For GaAs we found k_11=42.5 k_12=51, k_44=25.5. This is the first determination of these constants in GaAs at this wavelenght where other techniques are not applicable. For silicon we found k_11=53 k_12=25, k_44=23. It is also shown that in opaque semiconductors the elastooptic coupling depends strongly on the surface orentation
Valence Charge Density in Aluminum
No full textA comparison is made between the valence charge density obtained using a local on Fermi sphere potential and that derived from a suitable average potential which takes into account the effects due to the nonlocality of the pseudopotential. The two electron distributions present significant differences, showing that the calculation of the valence charge density, which does not take account of the nonlocality of the pseudopotential, can lead to uncertain results
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