Search results for "PHONON"
showing 10 items of 466 documents
Modeling the magnetic properties and Mössbauer spectra of multifunctional magnetic materials obtained by insertion of a spin-crossover Fe(III) comple…
2013
In this article, we present a theoretical microscopic approach to describe the magnetic and spectroscopic behavior of multifunctional hybrid materials which demonstrate spin crossover and ferromagnetic ordering. The low-spin to high-spin transition is considered as a cooperative phenomenon that is driven by the interaction of the electronic shells of the Fe ions with the full symmetric deformation of the local surrounding that is extended over the crystal lattice via the acoustic phonon field. The proposed model is applied to the analysis of the series [Fe(III)(sal2-trien)] [Mn(II)Cr(III)(ox)3]·solv, in short 1·solv, where solv = CH2Cl2, CH2Br2, and CHBr3.
An interface effect in c-oriented (Y/Pr)Ba2Cu3O7 Superlattices: Raman scattering by ‘Forbidden’ phonons
1997
Abstract Raman results of ultrathin layer high Tc superlattices ( Y m Pr 1 )Ba2Cu3O7 (m = 2, 3, 4) exhibit a common feature: new peaks, absent in the bulk compounds. These peaks are attributed to oxygen B1u symmetry modes confined in the YBa2Cu3O7 layers. To explain their Raman activity a mechanism based on the asymmetry of crystal field at Y-Pr layer interfaces is proposed.
Phonon Scattering through a Local Anisotropic Structural Disorder in the Thermoelectric Solid Solution Cu_2Zn_(1−x)Fe_xGeSe_4
2013
Inspired by the promising thermoelectric properties of chalcopyrite-like quaternary chalcogenides, here we describe the synthesis and characterization of the solid solution Cu(2)Zn(1-x)Fe(x)GeSe(4). Upon substitution of Zn with the isoelectronic Fe, no charge carriers are introduced in these intrinsic semiconductors. However, a change in lattice parameters, expressed in an elongation of the c/a lattice parameter ratio with minimal change in unit cell volume, reveals the existence of a three-stage cation restructuring process of Cu, Zn, and Fe. The resulting local anisotropic structural disorder leads to phonon scattering not normally observed, resulting in an effective approach to reduce th…
Raman scattering study of the anharmonic effects in CeO2−ynanocrystals
2007
We have studied the temperature dependence of the F2g Raman mode phonon frequency and broadening in CeO2−y nanocrystals. The phonon softening and phonon linewidth are calculated using a model which takes into account the three-and four-phonon anharmonic processes. A detailed comparison of the experimental data with theoretical calculations revealed the predominance of four-phonon anharmonic processes in the temperature dependence of the phonon energy and broadening of the nanocrystals. On the other hand, three-phonon processes dominate the temperature behavior of phonons in polycrystalline samples. The anti-Stokes/Stokes peak intensity ratio was also investigated and found to be smaller for…
Vibrational properties of ZnTe at high pressures
2002
Raman spectra of ZnTe were measured under hydrostatic pressures up to 15 GPa at T = 300 K. Results for the frequencies of first- and second-order Raman features of the zincblende phase (0-9.5 GPa) are used to set up a rigid-ion model of the phonon dispersion relations under pressure. Calculated phonon densities of states, mode Gruneisen parameters and the thermal expansion coefficient as a function of pressure are discussed. The effect of pressure on the widths and intensities of Raman spectral features is considered. Raman spectra of high-pressure phases of ZnTe are reported. These spectra indicate the possible existence of a new phase near 13 GPa, intermediate between the cinnabar and ort…
First-principles phonon calculations of Fe4+impurity in SrTiO3
2012
The results of hybrid density functional theory calculations on phonons in Sr(Fe(x)Ti(1-x))O(3) solid solution within the formalism of a linear combination of atomic orbitals are presented. The phonon density of states (DOS) calculated for 6.25% Fe(4+) impurities is reported and defect-induced phonon modes are identified. Based on our calculations and group-theoretical analysis, we suggest for the first time an interpretation of experimentally observed Raman- and IR-active modes.
Anomalous thermoelastic behavior of (KI)1-x(NH4I)x
1990
Abstract An anomalous softening of the shear mode c 44 is observed in (KI) 1- x (NH 4 I) x with Brillouin scattering (10 K ⩽ T ⩽ 300 K ). It is caused by the weak quadrupolar moment of the NH 4 + ion, induced by the C 3 v symmetry of the potential. A parallel decrease of the lifetime of this transverse phonon at lower temperatures is observed. Static random strain fields due to admixing KI with NH 4 I are small. The results have been treated in terms of a mean field theory. The c 11 elastic constant does not show any anomalous behavior. Also are given the elastic constants c 11 and c 44 for x =0.14, 0.43 and 1 and 1 at T = 300 K and 10 K .
Time resolved CARS measurements of I2 in solid Kr
2006
Dephasing is a central concept in condensed phase spectroscopy. It determines how long a system will maintain its coherence. The dephasing time of a system is determined by dynamic intermolecular interactions, and therefore measurements of dephasing time can provide information on interactions and couplings between a molecule and its environment. This chapter illustrates the application of the femtosecond coherent anti-Stokes Raman scattering (CARS) method to investigate the vibrational dephasing of I 2 in solid krypton. Dephasing of vibrational states between v = 2 and v = 16 is studied in the temperature range T = 2.6–32 K. The low vibrational states show dephasing times on the order of a…
Electron–phonon effects on the direct band gap in semiconductors: LCAO calculations
2002
Abstract Using a perturbative treatment of the electron–phonon interaction, we have studied the effect of phonons on the direct band gap of conventional semiconductors. Our calculations are performed in the framework of the tight-binding linear combination of atomic orbitals (LCAO) approach. Within this scheme we have calculated the temperature and isotopic mass dependence of the lowest direct band gap of several semiconductors with diamond and zincblende structure. Our results reproduce the overall trend of available experimental data for the band gap as a function of temperature, as well as give correctly the mass dependence of the band gap on isotopic. A calculation of conduction band in…
Two-phonon magneto-Raman scattering in quantum wells: Fröhlich interaction
1996
We have developed a theoretical model of two-phonon resonant magneto-Raman scattering in a semiconductor quantum well (QW). Frohlich electron-phonon interaction has been considered and the corresponding selection rules are derived for Faraday geometry and backscattering configuration. The resonant profiles are analyzed as a function of magnetic field and laser energy. To simplify the discussion a three-band model with parabolic masses has been used as a first approach, studying later the role of heavy-hole light-hole admixture in the scattering process. It is shown that, due to mixing effects, Frohlich interaction contributes to the two-phonon Raman spectra in the parallel (z(σ ± , σ ± ) z)…