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1.
Far infrared reflection spectra of ZnxHg1−xSe with x = 0 to 0.4 showed characteristic of two TO-phonon modes behavior and the plasmon-LO phonon coupling in the region from 20 to 500 cm−1 at 5 and 300 K. The composition dependence of TO modes on the frequency is linear for the HgSe-like mode TO1 and constant for the ZnSe-like mode TO2. The additional TO mode due to clustering effect is observed at 113 cm−1. The plasmon-LO phonon coupled modes are explained well by our model taken into account the two-mode behavior and the contribution of interband transitions in dynamic dielectric function.  相似文献   

2.
We consider a valence force field model for the phonon spectrum of (CH)x and find that phonon coupling to extended π electron states must be included to describe the observed Raman active modes of the polymer. The calculated phonon spectra are shown to possess the zone center dispersion anomalies characteristic of a condensed Peierls ground state.  相似文献   

3.
A model pseudopotential depending on an effective core radius treated as a parameter is used for alkaline earth metals in bcc and fcc phases to study the Binding energy, Interatomic interactions, phonon dispersion curves, Phonon density of states, Debye-Waller factor, mean square displacement, Debye-Waller temperature parameters, dynamical elastic constants (C11, C12 and C44), bulk modulus (B), shear modulus (C′), deviation from Cauchy relation (C12C44), Poisson's ratio (σ), Young's modulus (Y), behavior of phonon frequencies in the elastic limit independent of the direction (Y1), limiting value in the [1 1 0] direction (Y2), degree of elastic anisotropy (A) and propagation velocities of the elastic waves. The contribution of s-like electrons is incorporated through the second-order perturbation theory due to model potential. The theoretical results are compared with the existing experimental data. A good agreement between theoretical investigations and experimental findings has confirmed the ability of our potential to yield large numbers of lattice mechanical properties of certain alkaline earth metals.  相似文献   

4.
We present results for the absorption spectrum due to a localized Γ1+ → Γ4? transition that take into account the properties of the linear electron- phonon (e-p) interactions transforming as Γ1+, Γ3+ and Γ5+ in the excited electronic state, in the cases of strong and weak e-p interaction coupling. We show that in the strong e-p interaction coupling limit the asymmetric shape of the structured band is due to the commutation relations of the e-p interaction matrices. Moreover, in the weak coupling limit we present an expression for the spectrum line shape obtained by taking into account the time ordering of the e-p interaction matrices and the phonon propagators at all times. It is shown in the latter case that the densities of phonon states corresponding to the electronic excited state are different from those corresponding to the ground state, and the e-p coupling constants are redefined due to the Jahn-Teller interactions.  相似文献   

5.
The thermal shifts of R1 and R2 lines of Cr3+-doped garnets Y3Ga5O12 (YGG), Y3Sc2Al3O12 (YSAG) and Gd3Sc2Al3O12 (GSAG) are studied by considering both the static contribution (which is frequently neglected in the previous papers) due to lattice thermal expansion and the vibrational contribution due to electron–phonon interaction. In the studies, the static contribution is calculated with the thermal expansion coefficient of the corresponding cluster in the host garnet crystals. The results indicate that the static contributions in sign are opposite to and in magnitude are about 10% of the corresponding vibrational contributions. The true electron–phonon coupling parameters α′ obtaining by taking both contributions into account increase more than 10% in comparison with the corresponding apparent electron–phonon coupling parameters α determined by considering only the vibrational contribution in the previous paper. So, to obtain the complete understanding of thermal shift of a spectral line and the true rather than apparent electron–phonon coupling parameters, one should take account of both the static and vibrational contributions.  相似文献   

6.
The Eliashberg theory generalized for electron—phonon systems with a nonconstant density of electron states and with allowance made for the frequency behavior of the electron mass and chemical potential renormalizations is used to study T c in the SH3 phase of hydrogen sulfide under pressure. The phonon contribution to the anomalous electron Green’s function is considered. The pairing within the total width of the electron band and not only in a narrow layer near the Fermi surface is taken into account. The frequency and temperature dependences of the complex mass renormalization ReZ(ω), the density of states N(ε) renormalized by the electron—phonon interactions, and the electron—phonon spectral function obtained computationally are used to calculate the anomalous electron Green’s function. A generalized Eliashberg equation with a variable density of electron states has been solved. The frequency dependence of the real and imaginary parts of the order parameter in the SH3 phase has been obtained. The value of T c ≈ 177 K in the SH3 phase of hydrogen sulfide at pressure P = 225 GPa has been determined by solving the system of Eliashberg equations.  相似文献   

7.
《Current Applied Physics》2014,14(4):590-595
In this study, we reported the effects of the boron (B) substitution into the Ca site in the Ca3Co4O9 system on the electrical, thermal and magnetic properties between 300 K and 5 K. The results indicated that the B-substitution into the system caused an increase of resistivity due to the decrease on carrier concentration. Thermal conductivity decreased for the x = 0.5 B-substituted sample and then increased with increasing the B-content. Analysis on the thermal conductivity of samples showed that the phonon–phonon interaction term is the dominant component in the total thermal conductivity for all the samples. It was found that the point defect contribution to the thermal conductivity increased by increasing the B-content. The temperature dependence of magnetic susceptibility showed a paramagnetic behavior at room temperature and ferrimagnetic behavior below 20 K for unsubstituted sample. But, the magnetization decreased in the B-substituted samples. The substitution of B into the Ca site destroyed the interlayer coupling, which resulted in the decrease of the ferromagnetic behavior. The susceptibility data was fitted using Curie–Weiss law with temperature independent term and the μeff values were calculated to be 1.42 μB and 3.89 μB for unsubstituted sample and the highest B-substitution, respectively.  相似文献   

8.
A complete analysis of the phonon conductivity κ, still lacking in the literature, is presented in the two-mode conduction model for Germanium. First a method is derived from which the correction term κc of the Callaway model is separated into its longitudinal and transverse parts and then the effects of strong phonon dispersion and the role of longitudinal and transverse phonons on κc are studied. For this purpose we have also proposed some new empirical expressions for the three phonon relaxation rates τ3ph−1's which are valid in the entire temperature range. This improvised model, when applied simultaneously to the phonon conductivity data of both normal and enriched Ge, yields some new results. These are (i) κc neglected by the earlier workers in the two-mode phonon conduction model, gives a substantial contribution beyond the conductivity maximum and (ii) the longitudinal phonons are the major carriers of heat at high temperature.  相似文献   

9.
In this paper, the microscopic theory of the relative change in velocity of sound with temperature of La0.5Ca0.5MnO3 is reported. The phonon Green function is calculated using the Green function technique of Zubarev in the limit of zero wave vector and low temperature. The lattice model electronic Hamiltonian in the presence of the phonon interaction with hybridization between the conduction electrons and the l-electrons is used. The relative change in velocity of sound at various temperatures is studied for different model parameters, namely the position of the l-level, the effective phonon coupling strength and hybridization strength. The phonon anomalies observed experimentally at different temperatures are explained theoretically. An abrupt change in velocity at Neel temperature (TN) is observed clearly. It is observed that different parameters influence the velocity of sound.  相似文献   

10.
The contribution of a carrier-soft optical phonon interaction to the electrical resistivity of IV–VI compounds is discussed. The calculated resistivity based on a soft mode theory shows a large increase near the transition temperature Tc. It is foound that the observed increase of resistivity of p-SnTe is reproduced by the carrier-soft TO phonon interaction.  相似文献   

11.
We report spatially resolved Raman scattering from Si-doped epitaxial laterally overgrown GaN structures to investigate spatial variations in stress and free electron concentration. The doping-induced increase in the free electron concentration is relatively higher in the laterally overgrown regions than in the coherently grown regions due to the increased contribution of the high-energy A1 longitudinal optical phonon–plasmon coupled mode. In addition, the E2(high) [E2(low)] phonon energy shifts downward (upward) more significantly in the laterally overgrown regions than in the coherently grown regions. The doping-induced Raman shifts of the E2(high) and E2(low) phonons in the laterally overgrown regions are approximately ?0.6 and 0.11 cm?1, respectively, corresponding to the in-plane stress of ~0.22 GPa.  相似文献   

12.
In this Letter the microscopic theory of the relative change in velocity of sound with temperature of La0.5Ca0.5MnO3 is reported. The phonon Green function is calculated using the Green function technique of Zubarev in the limit of zero wave vector and low temperature. The lattice model electronic Hamiltonian in the presence of the phonon interaction with hybridization between the conduction electrons and the l-electrons is used. The relative change in velocity of sound at various temperatures is studied for different model parameters namely the position of the l-level, the effective phonon coupling strength and hybridization strength. The phonon anomalies observed experimentally at different temperatures are explained theoretically. An abrupt change in velocity at Neel temperature (TN) is observed clearly. It is observed that different parameters influence the velocity of sound.  相似文献   

13.
The quasi-confined (QC) phonon modes, surface optical (SO) phonon modes and corresponding Fröhlich-like Hamiltonian in a wurtzite cylindrical nanowire with ring geometry are investigated in the framework of the dielectric continuum model and Loudon’s uniaxial crystal model. Numerical calculations are focused on the dispersion relations of the SO phonons and the electron–SO phonon coupling strength. Results show that there are only two branches of SO phonon modes. The dispersions of the two branches of SO phonon modes are obvious when the phonon wave-number kz or the azimuthal quantum number m is small. Typical degenerating behavior of the SO modes is evidenced due to the anisotropic effect of wurtzite crystal. Moreover, when kz or m are large enough, the frequencies of the two branches of SO modes converge to a definite limiting frequency in single planar heterostructure. The calculations of the electron–SO phonon coupling strength reveal that the high-frequency SO modes (SO+) play a more important role in the coupling strength than the low-frequency ones (SO). Furthermore, the long-wavelength SO phonons with small m are the main factor contributing to the electron–phonon interaction.  相似文献   

14.
15.
The time-resolved optical response from various crystallographic planes of a YBa2Cu3O7?x monocrystal is investigated. In such measurements of temporal behavior, the phonon system is driven by an ultrashort (subpicosecond) pulse into a coherent state and is probed by a second ultrashort pulse with a given temporal delay. A comparison of the Fourier transforms of the temporal responses with the spontaneous Raman scattering spectra shows that the contribution to a temporal response comes not only from fully symmetric phonons, but also from off-diagonal modes. The mechanism of generation of coherent phonons in high-temperature superconductors is discussed.  相似文献   

16.
The structural, elastic and phonon properties of Mg2Si1?xSnx alloy are investigated by performing density functional theory and density functional perturbation theory calculations. The calculated lattice parameter increases with the increase of Sn content obeying Vegard’s Law that is in good agreement with available experimental data. Shear modulus, Young’s modulus and sound velocities are determined from the obtained elastic constants. Phonon dispersion curves show a pronounced softening with increasing of Sn content. The softening mechanism has been discussed based upon the element mass and bond strength. Besides, phonon contribution to the Helmholtz free energy, the entropy and the constant-volume heat capacity are calculated within the harmonic approximation based on the calculated phonon density of states. Results show Mg2Si1?xSnx is thermodynamically more stable with higher Sn content.  相似文献   

17.
We address here the self-consistent calculation of the spin density wave and the charge density wave gap parameters for high-Tc cuprates on the basis of the Hubbard model. In order to describe the experimental observations for the velocity of sound, we consider the phonon coupling to the conduction band in the harmonic approximation and then the expression for the temperature dependent velocity of sound is calculated from the real part of the phonon Green’s function. The effects of the electron–phonon coupling, the frequency of the sound wave, the hole doping concentration, the CDW coupling and the SDW coupling parameters on the sound velocity are investigated in the pure CDW phase as well as in the co-existence phase of the CDW and SDW states. The results are discussed to explain the experimental observations.  相似文献   

18.
The relaxed configurations of yttria-stabilised zirconia (YSZ) between 3 and 10 mol % of Y2O3 were modeled using the pseudopotential technique. In the displacive limit of a double-well potential model, the vibrational mode corresponding to the soft phonon in pure c-ZrO2 was calculated for each Y2O3 composition. These anharmonic vibrations, associated with the stabilization of YSZ, were investigated within the self-consistent phonon approximation making obtainable the fine structure in spectral density. In studying the phonon dynamics, we use the displacement probability density, which can quantify very accurately the transition temperature necessary for stabilizing the YSZ cubic phase.  相似文献   

19.
We investigate the direct effects of phonon softening on Tc in Nb3Sn. It is inadequate to explain the high Tc, which arises instead from an unusually large electron- phonon coupling to the two Γ'12 optical phonon modes.  相似文献   

20.
The effect of longitudinal optical phonon field on the ground state and low lying-excited state energies of a hydrogenic impurity in a Zn1−xCdxSe/ZnSe strained quantum dot is investigated for various Cd content using the Aldrich-Bajaj effective potential. We consider the strain effect considering the internal electric field induced by the spontaneous and piezoelectric polarizations. Calculations have been performed using Bessel function as an orthonormal basis for different confinement potentials of barrier height. Polaron induced photoionization cross section of the hydrogenic impurity in the quantum dot is investigated. We study the oscillator strengths, the linear and third-order nonlinear optical absorption coefficients as a function of incident photon energy for 1s-1p and 1p-1d transitions with and without the polaronic effect. It is observed that the potential taking into account the effects of phonon makes the binding energies more than the obtained results using a Coulomb potential screened by a static dielectric constant and the optical properties of hydrogenic impurity in a quantum dot are strongly affected by the confining potential and the radii. It is also observed that the magnitude of the absorption coefficients increases for the transitions between higher levels with the inclusion of phonon effect.  相似文献   

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