U. Schröder

Surface relaxation, surface reconstruction and surface dynamics close to the antiferrodistortive phase transition of SrTiO3(001) slabs with free SrO and TiO2 surfaces

The authors discuss the relaxation, reconstruction and dynamics of both (001) surfaces of SrTiO3 (SrO and TiO2 surfaces) in the framework of shell models. The treatment of the SrO surface (surface I) on the basis of a bulk shell model leads to relaxation- and surface-dynamical results, the former of which are in good agreement with recent LEED measurements. However, for the TiO2 surface (surface II), surface-related changes in the Ti-O short-range interactions have to be introduced in the model in order to obtain agreement with the LEED results, as well as to assure dynamical stability of this surface. Both surfaces exhibit soft-mode behaviour which, upon lowering the temperature, leads to surface reconstruction that takes place prior to the antiferrodistortive transition of the bulk.

Lattice dynamics of the high-Tc superconductor La2−xMxCuO4

Abstract The phonon dispersion curves, the one-phonon density of states and the specific heat of the high-T c superconductor La 2− x M x CuO 4 have been calculated in the framework of a shell model. The zone center frequencies are classified according to their vibrational character and compared with the available IR and Raman data. A slight adjustment of the parameters obtained from the known interaction potentials of oxides with perovskite and rock salt structure yields reasonable agreement with the IR, Raman, specific heat, and inelastic neutron scattering data of the polycrystalline material.

Phonons in Nd2−xCexCuO4

We have measured far infrared reflection and Raman spectra on high quality single crystals and ceramic samples of the Nd2−xCexCuO4 system. The TO and LO frequencies of all infrared-active phonons were determined from a Kramers-Kronig analysis of the infrared reflection spectra. The frequencies of the detected Raman-active phonons are 228 cm−1 (A1g Nd), 328 cm−1 (B1g O2) and 480 cm−1 (Eg 02). A resonantly enhanced peak at 580 cm−1 is observed for Ce-doped samples only with polarization properties like an A1g phonon. We have also performed lattice-dynamical calculations with parameters extracted from those of perovskites and metallic oxides. The frequencies obtained agree well with our experimental findings. We present the eigenvectors of all zone center phonons. Finally, we also detected a feature at 165 cm−1 in the ab-polarized infrared spectra in Nd2CuO4 which might be of magnetic origin.

Relaxation and dynamics of the (111) surfaces of the fluorides CaF2 and SrF2

The authors have treated the relaxation and the dynamics of the (111) surfaces of the fluorides CaF2 and SrF2 in the framework of shell models, in which the short-range interactions are represented by Born-Mayer potentials. The relaxation in the outer layer of the crystal, consisting of three closely spaced F-Ca/SrF layers, is significant. The calculated normal vibrations of a 19-layer slab are presented in 3-dimensional diagrams of the normalized surface squared amplitude, which provide a very direct and pictorial display of the surface localization of vibrational states. For CaF2 the calculations are compared with the HREELS data of Longueville et at. (1990).

Structure and dynamics of hydrogenated GaAs(110) and InP(110) surfaces

We have investigated the structural and vibrational properties of hydrogen-covered GaAs(110) and InP(110) surfaces in the framework of the density-functional theory. The plane-wave pseudopotential method combined with the slab supercell description for the surfaces has been employed to determine the relaxation geometries of H:GaAs(110) and H:InP(110) for the half- and one-monolayer coverages. Our results are in good agreement with all available experimental data. For both surfaces and for all coverages we have determined the surface vibrations by means of an ab initio linear-response formalism. The calculated bond-stretching frequencies compare very well with the results from high-resolution electron-energy-loss spectroscopy. The half-monolayer coverages on InP(110) exhibit vibrational states which are related to the mechanism which leads to the structural decomposition of the surface experimentally observed at high hydrogen exposures.

Effect of pressure on Raman modes in MBa2Cu3O7-type materials

Abstract The effect of pressure on Raman modes of ceramic superconducting samples of MBa 2 Cu 3 O 7−x (M = Y, Eu) has been investigated up to 16 GPa at 300 K. Linear pressure coefficients are reported for the strongest modes at around 145, 300–340, 440, and 500 cm −1 . All these modes exhibit an increase in frequency, with average mode Gruneisen pameters falling in the range from 1.2 to 2.2. The pressure dependence of the mode frequencies is calculated within the framework of a shell model, which has recently been used to aid the mode assignment. Calculated mode Gruneisen parameters are found to be in satisfactory agreement with experiment if the proper volume dependence of the polarizability is taken into account.

Surface phonons of Si(111):H-(1 × 1) and Si(111):As-(1 × 1)

Abstract The full phonon dispersions of the fully relaxed (Si(111):H-(1 × 1) and Si(111):As-(1 × 1) surfaces are studied within a first-order density-functional perturbation scheme. The results are in excellent agreement with the experiment. It is shown that, despite of significant deviations in the surface force constants, most differences in the two phonon spectra are an effect of the different masses of the surface atoms. Furthermore, the Debye-Waller factors are calculated and discussed. For the atoms in the surface region, they exhibit a pronounced anisotropy. Convergence towards the bulk value is not reached until the eighth atomic layer.

Density-functional calculation of phonons in adsorbate-covered semiconductor surfaces

Abstract We present the results from density-functional calculations of the phonon dispersion of adsorbate-covered semiconductor surfaces. The plane-wave method is used together with the slab-supercell description for the surfaces. We focus on vibrational states that characterize the chemisorption site of the adsorbed atoms and phonon modes of the interface. By comparing the vibrational states in the presence of the adsorbates with those of the clean surfaces, adsorption-induced changes of the surface geometry and force constants can be identified. We study the chemisorption of hydrogen, antimony, and group-III elements on the (110) surfaces of III–V compounds, as well as on the (001) and (111) surfaces of silicon.

Lattice dynamics of high-Tc superconductors: Pb2Sr2YCu3O8

Abstract Raman and infrared active modes of Pb2Sr2YCu3O8 have been calculated in the framework of a shell model which uses short range overlap and long range Coulomb potentials. Eigenfrequencies and eigenvectors for the zone center modes are presented and compared with available Raman scattering data. The agreement is excellent.

Relaxation and dynamics of the (001) surfaces of KZnF3and KMnF3

Abstract We report calculations of the static and dynamical properties of the K-F and Zn-F 2 (Mn-F 2 ) (001) surfaces of the fluoridic perovskites KZnF 3 and KMnF 3 . Several surfacelocalized modes have been identified in both relaxed and unrelaxed geometries. In particular a soft mode at M was obtained for the K-F surface of KMnF 3 . The surface reconstruction associated with this soft mode is discussed: it can be considered as a precursor of the bulk transition.