Search results for "Phonon"
showing 10 items of 466 documents
Resonant Raman characterization of InAlGaN/GaN heterostructures
2006
InAlGaN/GaN heterostructures and thin films with In composition ranging from 0.03 to 0.1 are characterized by means of Raman scattering excited at various energies in the ultra violet range, tuning the laser excitation energy through the band gap of In x Al y Ga 1-x-y N. It is shown that the addition of In to the Al y Ga 1-y N alloy diminishes considerably the vibration energy of the A 1 (LO) phonon mode. The phonon line is asymmetric on the low energy side, and the asymmetry increases with In content, while the main peak shifts to lower energies. A shift of the phonon energy has also been observed when the excitation energy is close to the absorption edge of the In x Al y Ga 1-x-y N layer.…
Photonic band gaps in highly ionic medium: CuCl, CuBr, CuI
2003
Abstract Using the transfer-matrix-method, we have studied the propagation of electromagnetic waves through two-dimensional (2D) and three-dimensional (3D) dispersive photonic band gap (PBG) structures constructed from copper halides materials, especially from CuCl compounds. A special attention has been paid to the effect of the polariton gap on the PBG properties. This study reveals that “Twin gaps” and “Twin brothers” concepts and the flattened bands phenomena in both polarizations and for both structures (i.e. 2D and 3D) are all consequences of the strong photon–phonon coupling, particularly near the long wave length transverse optical phonon frequency. Furthermore, results for comparis…
Interaction of Lamb modes with two-level systems in amorphous nanoscopic membranes
2007
Using a generalized model of interaction between a two-level system (TLS) and an arbitrary deformation of the material, we calculate the interaction of Lamb modes with TLSs in amorphous nanoscopic membranes. We compare the mean free paths of the Lamb modes with different symmetries and calculate the heat conductivity $\kappa$. In the limit of an infinitely wide membrane, the heat conductivity is divergent. Nevertheless, the finite size of the membrane imposes a lower cut-off for the phonons frequencies, which leads to the temperature dependence $\kappa\propto T(a+b\ln T)$. This temperature dependence is a hallmark of the TLS-limited heat conductance at low temperature.
Phonon Dispersion of Wurtzite CdSe: The Bond Charge Model
2000
The phonon dispersion of wurtzite CdSe is presented along the main directions of the Brillouin zone. The study has been performed by using a bond charge model for wurtzite-type semiconductors with only six adjustable parameters. The results are compared against neutron scattering data and ab initio calculations. The phonon eigenvectors corresponding to the vibrational modes at the Γ-point are in very good agreement with the ab initio calculations.
Phase separation in the quaternary Heusler compound CoTi(1−x)MnxSb – A reduction in the thermal conductivity for thermoelectric applications
2010
We investigate the phase separation of the solid solution CoTi(1−x)MnxSb into the two Heusler compounds CoTiSb and CoMnSb. Energy-dispersive X-ray spectroscopy measurements on the two-phase material reveal the presence of size- and shape-tunable CoTiSb regions in a CoMnSb matrix. We demonstrate that the formed phase and grain boundaries have a considerable influence on the phonon scattering processes, which leads to a reduction in the thermal conductivity by a factor of three compared to single-phase CoTiSb.
High-pressure structural, lattice dynamics, and electronic properties of beryllium aluminate studied from first-principles theory
2021
Abstract The present work reports the complete study of structural, vibrational, mechanical, and electronic properties of BeAl2O4 (known as Chrysoberyl) using first-principles computing methods. The calculated ground-state properties agree quite well with previous experiments. The computed phonon dispersion curves do not show imaginary frequencies confirming the dynamical stability. In addition, the calculated elastic constants also ensure the mechanical stability through fulfillment of mechanical stability criteria. Apart from that, the theoretically determined phonon frequencies agree quite well with previous Raman and infrared experiments at ambient conditions. Various thermodynamic prop…
Elastic and piezoelectric properties of BaTiO3 at room temperature
1999
Abstract We have calculated the phonon dispersion curves of barium titanate (BaTiO 3 ) at room temperature. A lattice dynamical formalism using the shell model is used. This microscopic model includes the short-range interactions of axially symmetric type and the long-range Coulomb interactions and taking into account the electronic polarizability of constituent ions. Zone center phonon and a few phonons in the first Brillouin zone are used for fitting. The values of relevant parameters are critically analyzed. A calculation of the elastic and piezoelectric properties is presented and compared with the available experimental data.
Lattice phonon modes of the high-pressure phase CuCl-IV
1999
Raman spectra of isotopically pure CuCl samples ${(}^{63}\mathrm{CuCl}$ and ${}^{65}\mathrm{CuCl})$ were measured under hydrostatic pressure up to 14 GPa at low temperature $(5 \mathrm{K}).$ Up to six Raman modes were resolved for the binary BC8-analog phase CuCl-IV in the range from 4.1 to 14 GPa. The phonon dispersion relations of this phase were calculated within a rigid ion model. Based on this calculation an assignment for the observed Raman-active modes is proposed. The calculated pressure dependences of mode frequencies reproduce the experimental results. Raman spectra support the existence of the phase CuCl-IIa in a narrow pressure range from 3.3 to 4.1 GPa.
Enhanced Debye level in nano Zn1+xSb, FeSb2, and NiSb: Nuclear inelastic spectroscopy on121Sb
2014
The121 Sb partial density of phonon states (DPS) in nanopowder antimonides were obtained with nuclear inelastic scattering on , , and NiSb prepared by a wet chemistry route. The DPS is compared with the bulk counterpart. An increase of the Debye level indicative of a decrease of the isothermal speed of sound is systematically observed. This observation reveals that the decrease in speed of sound observed in nanostructured thermoelectric materials is not restricted to sintered nanocomposites.
Polar phonons in relaxor ferroelectric 0.2PSN-0.4PMN-0.4PZN
2008
Relaxor ferroelectrics 0-2PbSc(1/2)Nb(1/2)O(3) - 0.4PbMg(1/3)Nb(2/3)O(3)-0.4PbZn(1/3)Nb(2/3)O(3) ceramics were studied by means of the Fourier transform infrared reflection and THz transmission spectroscopy in the temperature range of 20-500 K. On heating from low temperatures, the A(1) component of the strongly split TO1 mode softens towards the Burns temperature, but the softening ceases near 400 K, which could be a signature of polar cluster percolation temperature.