Metal-Insulator Transition in a Generalized Hubbard Model with Correlated Hopping at Half-Filling

In the present paper metal-insulator transition is studied in a generalized Hubbard model with correlated hopping at half-filling and zero temperature. Single-particle Green function and energy spectrum of electron system are calculated. The expressions for energy gap width and the concentration of polar states (holes or doublons) are obtained. The conditions for metallic and insulating states are found.Comment: 11 pages, 2 eps figures, Latex 2.09, submitted to Phys. Stat. Sol. (B

Recent developments and diffuse scattering studies at beamline F1 (Hasylab/DESY)

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Computational study of LnGaO3 (Ln=La+Gd) perovskites

Atomistic simulation techniques have been used to study the thermal properties of perovskite-type LnGaO3 (Ln = La-Gd). A set of interatomic potentials describing interatomic interactions in these compounds was developed and tested over a wide temperature range through utilizing free energyminimization.The predicted dielectric constants, thermal expansion coefficients, phonon density of states and its projections, heat capacity and entropy, elastic moduli, Gruneisen parameters, surface energies for main crystallographic directionsand Debye temperatures are in good agreement with the limited available experimental data. Perovskite-type LnGaO3 (Ln = La-Gd) compounds have been examined under conditions to which substrate materials are typically subjected. Only a narrow region in the phase diagram of LnGaO3 (Ln = La-Gd) and their solid solutions is recommended for use in substrate applications

Ausbau und Betrieb des Kappa-Diffraktometers am Strahl F 1 Schlussbericht

Available from TIB Hannover: F02B1535 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Bildung und Forschung, Berlin (Germany)DEGerman

Renormalization of the phase transition in lead phosphate, Pb3(PO4)2Pb_3(PO_4)_2, by high pressure: Lattice parameters and spontaneous strain

The unit-cell parameters of lead phosphate, Pb3(PO4)2, have been measured by single-crystal X-ray diffraction in a diamond-anvil cell under hydrostatic conditions to a maximum pressure of 6 GPa. The compound undergoes an improper ferroelastic phase transition at 1.81 ± 0.01 GPa at 298 ± 1 K from a low-pressure monoclinic phase (space group C2/c) to a high-pressure trigonal phase. The space group of the high-pressure phase is R3m or R3m and the phase boundary is the same as that crossed at ca 450 K at room pressure. The evolution of the spontaneous strain components arising from the phase transition is linear with pressure, indicating an effective critical exponent for the phase transition of [beta] = 0.50 ± 0.03. Thus, the application of pressure strongly renormalizes the effective critical exponent of the order parameter from the value of 0.236 found in high-temperature experiments

Hard mode spectroscopy: The concept and applications

Hard modes are, in the context of this review, optically active phonons which show systematic changes of their Raman and/or IR spectra when the structural properties of a material are changed (e.g. by heating, application of pressure or chemical reactions).As the characteristic length of high-frequency phonons is very short (the Ornstein–Zernike correlation length) the structural variations are measured on an atomic scale. This feature is a great advantage for the analysis of heterogeneous materials, e.g. exsolution pattern, disordered systems.The interpretation of frequency shifts, variations of the intensities and line width of optical spectra is largely based on symmetry arguments which show that the renormalization of phonon spectra is, in most cases, proportional to AQ 2 + BQ 4, where Q is a structural order parameter and A, B are numerical constants. Recipes for the analysis of phonon spectra including the use of reference spectra, profile analysis and the application of spectral autocorrelation functions are discussed.In the case of powder IR spectra the effect of the embedding matrix in a powder pellet has to be analysed. A simple approach to the “effective medium theory” is reviewed.The effect of short-range structural order on the phonon spectra is discussed using the phase transitions in lead phosphate (Pb3(PO4)2), titanite CaTiSiO5 and lead scandium tantalate PbSc0.5.Ta0.5O3