Dispersion Of Optical Phonons Research Articles

Lattice dynamics of CuAu-orderedCuInSe2

The lattice vibrational properties of ${\mathrm{CuInSe}}_{2}$ with CuAu-type ordering of the cation sublattice are investigated theoretically by a first-principles calculations of the structure and the lattice dynamical characteristics including zone-center optical mode frequencies, phonon dispersion and density of states, and elastic constants. The results obtained for CuAu-ordered ${\mathrm{CuInSe}}_{2}$ are compared with related experimental data and comparative theoretical calculations for the chalcopyrite phase of the compound. A critical analysis is given of the partly contradictory experimental lattice vibration data reported for chalcopyrite ${\mathrm{CuInSe}}_{2}.$

Anomalous dispersion of longitudinal optical phonons in Nd(1.86)Ce(0.14)CuO(4+delta) determined by inelastic x-ray scattering.

The phonon dispersions of Nd(1.86)Ce(0.14)CuO(4+delta) along the [xi,0,0] direction have been determined by inelastic x-ray scattering. Compared to the undoped parent compound, the two highest longitudinal phonon branches, associated with the Cu-O bond stretching and out-of-plane oxygen vibration, are shifted to lower energies. Moreover, an anomalous softening of the bond-stretching band is observed at about q = (0.2,0,0). These signatures provide evidence for strong electron-phonon coupling in this electron-doped high-temperature superconductor.

Raman scattering in GaN pillar arrays

We present a detailed study of GaN pillar arrays by atomic force microscopy (AFM), Raman spectroscopy, and photoluminescence (PL) spectroscopy. AFM is used to characterize the shape of the GaN pillars and revealed a large roughness of etched pillar surfaces. Raman scattering spectra of the pillars are well described by angular dispersion of polar optical phonons induced by the three-dimensional shape of the pillar. Additional Raman scattering has been tentatively assigned to the activation of the high frequency B1 silent mode by defects introduced during the ion etching. This result is well correlated with the appearance of donor and acceptor-related PL of the GaN pillars. N vacancy or/and Ga interstitials would be likely candidates for donors in the nonstoichiometric GaN near the surface of the etched pillars.

Analysis of dispersion of long-wavelength optical phonons in zinc with the help of light scattering

The temperature dependences (5–300 K) of the Raman spectra of E2g phonons and optical constants in zinc single crystals are measured in the excitation energy range 1.4–2.54 eV. It is found that phonon damping decreases upon an increase in the wavelength of exciting radiation. The obtained results are compared with the dependence of the phonon width on the excitation energy (the probed wave vector of the excitations under investigation), which are presented for the first time for the transition metal osmium, as well as with the calculated electron-phonon renormalization of damping, taking into account the actual distribution of wave vectors.

Nonlinear dielectric function associated with dispersive optic vibrations

In the present work, a nonlinear dielectric function is derived using the Born–Huang equation and taking into account dispersion of optical phonons and nonlinear terms in the polarization vector P( u, E). The theoretical results show an obvious shift of the profile of the dielectric function. The same expressions for the linear dielectric functions as previous ones are also obtained when nonlinear factors are set to be zero in the present formulae.

Exciton–LO-phonon interaction in zinc-compound quantum wells

The properties of optical phonons and excitons in four zinc-compound quantum wells, in which the well layers are made of ternary mixed crystals, are investigated theoretically. The optical phonon frequencies and the electron--LO-phonon coupling constants of the well materials are calculated by using a mixed crystal theory. The interface optical phonon energies and dispersion relations, the exciton binding energies and the exciton--LO-phonon interaction energies are calculated as functions of the well width and the concentration x. The effects of the mixed crystal on the exciton--optical-phonon interaction in the quantum wells are discussed.

Forward Raman scattering in GaAs/AlAs superlattices: Study of optical phonon anisotropy

We present the forward Raman scattering study of zone-centre optical phonon anisotropy in short-period GaAs/AlAs superlattices. Experiments were performed on specially prepared superlattice structures having anti-reflection dielectric coatings and removed substrates. The experimental data are compared with the angular dispersion of superlattice optical phonons calculated within the dielectric susceptibility model. We have found a good agreement between the experimental data and the calculations taking into account interface disorder.

Dispersion of optical and acoustical phonons in the zinc-blende group III-nitride superlattices

A comprehensive theoretical study of phonons in cubic GaN, AlN, Ga1−xAlxN, and (GaN)m/(AlN)n superlattices (SLs) is reported using both simple and microscopic models. The new periodicity introduced by the SL structure causes zone folding of the acoustical modes allowing phonons with large q vectors, normally inactive in light scattering, to become Raman active. We predicted the frequencies of such modes in GaN/AlN SLs using elastic continuum, diatomic linear-chain, and rigid-ion models. In the rigid-ion model (RIM), the short range forces are optimized in terms of elastic constants and phonon energies at critical points while the long-range Coulomb forces are evaluated exactly via Ewald summation. The advantage of using RIM is that it permits the construction of a dynamical matrix entering into the secular equation for the SL vibrations of a given wavevector directly from the dynamical matrices of the bulk materials. For short period (GaN)m/(AlN)n SLs, the dependence of phonons on wavevectors both parallel and perpendicular to the growth direction [001] is investigated. Theoretical results for the GaN/AlN superlattices are compared and discussed with the GaAs/AlAs system as well as with the existing model calculation of Grille and Bechstedt. Unlike GaAs/AlAs, the larger optical phonon values and partially changed confinement characteristics in GaN/AlN superlattices are believed to have significant affects on the relaxation properties of electrons in group III-nitride material systems.

Anisotropy of optical phonons in semiconductor superlattices: Raman scattering experiments

Experiments on Raman scattering in the “forward” geometry, permitting observation of anisotropy of the optical phonons, are performed on specially prepared short-period GaAs/AlAs superlattice structures with the substrates removed and the surfaces covered with an antireflective layer. The experimental data agree well with the computational results obtained for the angular dispersion of optical phonons in superlattices on the basis of a modified continuum model.

Localized optical phonons in GaAs/AlAs superlattices grown on (311)A and (311)B surfaces

A study is reported of optical vibrational modes in [311]-grown GaAs/AlAs superlattices. An analysis of the TO and LO localized modes observed in IR reflectance spectra showed that the difference between the TO and LO mode frequencies in superlattices grown on (311)A and (311)B surfaces is due to the different localization lengths of these modes. The dispersion of transverse optical phonons in GaAs derived from IR reflectance spectra is in a good agreement with Raman scattering data.

Raman scattering of the optical vibrational modes in (GaAs)n(AlAs)n superlattices grown on (311)A and (311)B surfaces.

Raman spectra of confined optical phonons in (GaAs${)}_{\mathit{n}}$(AlAs${)}_{\mathit{n}}$ superlattices grown by molecular-beam epitaxy on (311)A, (311)B, and (100) surfaces have been measured in order to study the corrugation of (311) interfaces. Careful analysis of the experimental data reveals no evidence of such a corrugation. The dispersion of optical phonons in both GaAs and AlAs layers has been obtained and was found to be in good agreement with recent published data.

Dispersion of superlattice optical phonons

Abstract Microscopic models of longwavelength zone-centre optical modes in superlattices predict an anisotropic behaviour of the dipole active modes. Recent micro-Raman data has unambiguously demonstrated this anisotropy for LO1 and it is shown here that a continuum model that simply mixes confined LO and interface modes gives very good agreement with the data.

Reduced differential current: A conserved quantity in short-scale transport.

An electronic transport invariant, the reduced differential current (RDC), is introduced. The RDC is approximately conserved over short distances typical for modern semiconductor devices. In a steady-state transport, the divergence of the RDC vanishes if the only source of inelastic collisions is due to the interaction with monochromatic optical phonons. Rates of the violation of the RDC invariance due to the inelastic acoustic-phonon scattering and to the dispersion of optical phonons are calculated for exemplary nonequilibrium electron distributions.

Resonant interaction in excited states of a bound Frohlich polaron

Resonant interaction is investigated between the 2p (2s) state and the continuum of the 1s plus one-photon states of a bound Frohlich polaron in the case of non-zero optical phonon dispersion. The author first presents general theory for expansion of the resolvent of this system, which is not necessarily diagonal with respect to electron quantum numbers. On the basis of this theory he calculates the spectral functions for the 2s and 2p states with various strengths of Coulomb binding and electron-phonon coupling. It is found that the resonant interaction causes the level splitting and broadening for the 2p (2s) state and a very small third peak appears as a result of the mixing of the 1s one-phonon continuum.

Second-order energy shifts of the three types of bound polarons: the Frohlich, deformation potential and piezoelectric polarons

The second-order energy shifts of the 1s, 2s and 2p states of a bound polaron are calculated for the three types of electron-phonon interactions; the Frohlich, deformation potential and piezoelectric interactions. For the bound Frohlich polaron, a non-zero optical phonon dispersion is taken into account. The second-order perturbation theory of excited states interacting with a continuum of energies is used in the case of the resonance between the 2s and 2p states and the states with a 1s state plus one phonon, and the natures of the resonances for the three types of the bound polarons are discussed.

Raman Scattering in CdI2Polytypes and Dispersion of Optical Phonons

Raman spectra of various CdI 2 polytypes have been measured at liquid He temperature. Several folded modes of the E symmetry optical branch have been observed in higher polytypes. The dispersion curve of the E symmetry branch in the [00ξ] direction has been obtained from the experimental frequencies of the folded modes. The pressure dependence of the Raman active modes has been also measured. These results are analyzed by using a linear chain model and a polarizable ion model. The analysis indicates that the long range forces contribute little to the interlayer forces and that the difference in long range forces as well as the short range forces among the polytypes is very small.

Rotation–translation coupling and the lattice dynamics of RbCN

The dispersion of acoustic and optical phonons of rubidium cyanide has been measured at 300 K and near the phase transition from the disordered cubic high-temperature phase to the ordered monoclinic phase. At both temperatures the acoustic branches are affected by rotation–translation coupling in a way similar to the findings in KCN. The data are explained by a model which combines a shell model and rotation–translation coupling. The rotation–translation coupling is described on the basis of the theory of Michel and Naudts. Details of the model are discussed.

Lattice dynamics in RbCN

The dispersion of acoustic and optical phonons of rubidium cyanide has been measured at 300 K. The acoustic branches are affected by rotation-translation coupling in a way similar to the findings in KCN. The data are explained by a model which combines a shell model and rotation-translation coupling.

Inelastic neutron scattering in SbSi near the ferroelectric phase transformation

AbstractInelastic neutron studies are performed on SbSI. The (013) Bragg peak intensity is strongly temperature dependent and gives evidence for the first order ferroelectric transition. Experimental reasons (poor statistics, small samples) do not permit conclusions concerning the overdamped and rather broad coupled soft mode‐central peak entity which is observed in light scattering. An underdamped soft mode can be excluded. Acoustic and optical phonon dispersion surfaces are measured around the [0ζ0] direction and polarized in [001], which exhibit deep valleys along [0ζ0]. This correlates well with previous diffuse X‐ray results. The depth of the valleys increases strongly in changing the phase from paraelectric to ferroelectric.

Inelastic neutron scattering in SbSI near the ferroelectric phase transformation

Inelastic neutron studies have been performed on SbSI. The (013) Bragg peak intensity is strongly temperature dependent and gives evidence for the first order ferroelectric transition. No underdamped soft mode has been observed. Experimental reasons (poor statistics, small sample) did not permit any conclusions concerning the rather broad coupled soft mode-central peak entity observed in light scattering. Acoustic and optical phonon dispersion surfaces have been measured around the [0ξ0] direction and polarized in [001], which exhibit deep valleys along [0ξ0], the depth of which changes strongly in going through the transition.