Papers by Keyword: Phonons

Abstract: We measured the low temperature wavelength modulated absorption and low temperature photoluminescence spectra of relatively high purity (n~10¹⁴-10¹⁵ cm^-3) 6H SiC boule and epi layers at high wavelength resolution (0.1-0.7 Å) to adequately separate and identify phonon-assisted absorption and recombination processes due to free excitons. As a result we have identified newly resolved or weak features in both spectra which we associate with previously unidentified momentum conserving (MC) phonons. We obtain the energies of 21 of the 36 possible MC phonons in 6H SiC and a more accurate estimate of the exciton bandgap, E_gx = 3.0225±0.0003 eV. In several of the phonon-assisted absorption onsets we also observe fine structure and variations in the measured spin-orbit splitting, which relate to a fourfold splitting of the free exciton energy bands.

Abstract: We investigated the crystal structure, vibrational and elastic properties of crystals with a rare-earth sublattice related to different structural types at ab initio level of modeling: elpasolite Cs₂NaRF₆ −> pyrochlore R₂Ti₂O₇ −> ferroborate RFe₃(BO₃)₄, where R is a rare-earth ion or yttrium. The calculations were performed in the framework of a density functional theory using the hybrid functionals containing local and non-local contribution (i.e. Hartree-Fock exchange term) to the exchange energy. We used CRYSTAL program for simulating periodic structures in the MO LCAO approximation. To describe the internal shell of a rare-earth ion up to 4f, we used the nonrelativistic pseudopotential («4f-in-core») that describes the effect of internal electrons on the outer valence shells. The results of the calculations are in good agreement with the available experimental data of IR and Raman experiments, X-ray diffraction analysis for the rows of elpasolites, pyrochlores and ferroborates.

Abstract: Thermal conductivity κ (T) of LaFeAsO is theoretically investigated below the spin density wave (SDW) anomaly. The lattice contribution to the thermal conductivity (κ_ph) is discussed within the Debye-type relaxation rate approximation in terms of the acoustic phonon frequency and relaxation time below 150 K. The theory is formulated when heat transfer is limited by the scattering of phonons from defects, grain boundaries, charge carriers, and phonons. The lattice thermal conductivity dominates in LaFeAsO and is an artifact of strong phonon-impurity and-phonon scattering mechanism. Our result indicates that the maximum contribution comes from phonon scatters and various thermal scattering mechanisms provide a reasonable explanation for maximum appeared in κ (T).

Abstract: Based on phonon theory the interaction of high frequency sound (ultrasound and hypersonic) with crystal lattices in solids was estimated. The coefficients of absorption in dielectrics and metals, with respect to temperature and sound frequency, were calculated. Analysis of the calculated dependences allows obtaining of nanomaterials with the set sound conductivity and sound absorption in high frequency range.

Abstract: Structural properties of MnS have been studied using plane wave pseudopotential density functional theory as implemented in Quantum Espresso code. Local density approximation (LDA) along with ultrasoft pseudopotential has been used for total energy calculations. The calculated total energies are fitted to Murnaghan equation of state to calculate equilibrium lattice constant, isothermal bulk modulus and pressure derivative of isothermal bulk modulus for NaCl-type structure of MnS and compared with previous experimental and theoretical calculations and good agreement is achieved with those results. Phonon frequencies have also been derived for B1 phase of MnS along high symmetry directions using the density functional perturbation theory at ambient condition.

Abstract: Phonon dynamics and phonon thermal conductivity of f.c.c. Cu are investigated in detail in the temperature range 200 1300 K within the framework of equilibrium molecular dynamics simulations making use of the Green-Kubo formalism and one of the most reliable embedded-atom method potentials. It is found that the temporal decay of the heat current autocorrelation function of the f.c.c. Cu model at low and intermediate temperatures demonstrates a more complex behaviour than the two-stage decay observed previously for the f.c.c. Ar model. After the first stage of decay, it demonstrates a peak in the temperature range 200 800 K. The intensity of the peak decreases as the temperature increases. At 900 K, it transforms to a shoulder which diminishes almost entirely at 1200 K. It is suggested that the peak may be activated by the influence of the Cauchy pressure in f.c.c. Cu on the phonon dynamics. A decomposition model of the heat current autocorrelation function of a monatomic f.c.c. lattice is introduced. This model can capture all contributions to the function discussed in the literature. It is found that the temperature dependence of the phonon thermal conductivity of the f.c.c. Cu model is in good agreement with previous calculations on the f.c.c. Ar model which follows an exponent close to-1.4, i.e. varies more rapidly than the T-1 law predicted by the theory. The calculated phonon thermal conductivity of the f.c.c. Cu is found to be about one order of magnitude higher than the f.c.c. Ar. This is explained by the inclusion of the electronic contribution to the bulk lattice properties during the fitting of the embedded-atom method potential functions to the experimental or ab initio data. It is demonstrated that the electronic contribution to the total thermal conductivity of f.c.c. Cu dominates over the whole studied temperature range. Nevertheless, the phonon contribution increases as the temperature decreases. The contribution can be estimated to be about 0.5 % at 1300 K and about 5 % at 200 K.

Abstract: Density Functional Theory (DFT) calculations were performed. They were firstly implemented to optimize the structure and refine the stoichiometry of the only ternary compound, CuLi_0.08Mg_1.92 of the Cu-Li-Mg system. Furthermore using DFT, several possible structures of CuMg₂H_x were optimized. Since most of the hydrides are cubic structures or can be considered as distortions of a cubic structure, we have started calculations for CuMg₂H_x (x = 4 - 6) with tetragonal and monoclinic structures, similar to those of the hydrides formed by the nearest neighbors of Cu and Mg in the periodic table: NiMg₂H₄ and CoMg₂H₅ (e.g. monoclinic C2/c and tetragonal P4/nmm, respectively). It can be concluded that the most stable configuration corresponds to CuMg₂H₅ with C2/c structure. We have performed several neutron scattering experiments that are in agreement with the first principles calculations.

Abstract: Optical tools such as infra-red absorption, photoluminescence, or Raman spectroscopy have been used for decades to observe the localized vibrational modes associated with impurities in semiconductors. The frequencies of these modes slightly shift with the isotope of the impurity while host-atom isotopes often show up as shoulders in the spectra. These shifts and shoulders are precious indicators of the nature of the defect. But sometimes, very small isotope-related frequency shifts cause very large changes in vibrational lifetimes. Impurity-isotope effects have now been predicted to impact the thermal conductivity of semiconductors containing a few atomic percent of impurities. Impurity isotope effects can be surprisingly large.

Abstract: Energy transport in nanostructures differs significantly from macrostructures because of classical and quantum size effects on energy carriers. Experimental results show that the thermal conductivity values of nanostructures such as superlattices are significantly lower than that of their bulk constituent materials. The reduction in thermal conductivity led to a large increase in the thermoelectric figure of merit in several superlattice systems. Materials with a large thermoelectric figure of merit can be used to develop efficient solid-state devices that convert waste heat into electricity. Superlattices grown by thin-film deposition techniques, however, are not suitable for large scale applications. Nanocomposites represent one approach that can lead to high thermoelectric figure merit. This paper reviews the current understanding of thermal conductivity reduction mechanisms in superlattices and presents theoretical studies on thermoelectric properties in semiconducting nanocomposites, aiming at developing high efficiency thermoelectric energy conversion materials.

Abstract: We report on the study of the interband pairing interaction in the two-band superconductor MgB2 by tunneling spectroscopy using thin film tunnel junctions. The films were deposited in situ by an approach comprising a conventional planar B sputter gun and a special homemade Mg evaporator providing a high vapor pressure. For the tunneling experiments sandwich-type crossed-strip tunnel junctions with a native MgB2 oxide as the potential barrier and Al, In or Pb counterelectrodes were prepared. Voltage-dependent differential conductance measurements revealed estimates of the barrier thickness and height of 1.5 nm and 1.6 eV, respectively, and allowed us to determine the phonon-induced structures in the tunneling density of states of the phonon-mediated superconductor MgB2. The analysis of the reduced density of states using the standard single-band Eliashberg equations yielded an effective electron-phonon spectral function accounting for the smaller energy gap. A further analysis involving ab-initio LDA calculations and the two-band Eliashberg equations revealed that the dominant feature in the effective spectral function, a strong peak at 58 meV, was mainly due to the interband pairing interaction.