A Raman scattering study of long-wavelength optical phonons in hexagonal ammonium fluoride single crystals

All the predicted Raman active translatory vibrations of wurtzite ammonium fluoride (space group C⁴_6v are observed and assigned on the basis of their polarization selections obtained with single crystals. The fundamental ammonium ion librations could not be detected, but a frequency of about 545 cm^?1 is deduced from the analysis of the observed second order Raman spectrum.The TO-LO splitting for the A₁ and E₁ polar phonons is a lot more important than the A₁ — E₁ splitting (125 cm^?1 compared to 1–3 cm^?1); thus, the effect of long-range electric forces strongly dominates over that of the anisotropy of the shortrange interatomic forces. Only very weak directional dispersion could be detected on the quasi-mode spectra. From the observed TO and LO frequencies, one evaluates ?₀ ? 3.95 in good agreement with dielectric experimental data.     

Raman scattering of light by optical phonons in Si-Ge-Si structures with quantum dots

Raman scattering of light by optical phonons in Si-Ge-Si structures with pseudomorphic germanium quantum dots has been investigated. Resonance amplification of the scattering intensity on E ₀ (Γ₇−Γ₈) transitions has been observed. It is shown that as a result of the formation of the layer of germanium quantum dots, the resonance energy is ∼0.3 eV higher than in the two-dimensional case. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 203–207 (10 August 1996)     

Anisotropy of long-wavelength optical phonons in GaAs/AlAs superlattices

The angular anisotropy of optical phonons in GaAs/AlAs (001) superlattices is investigated by Raman scattering spectroscopy. Scattering configurations allowed for phonons with wave vectors oriented along the superlattice layers and normally to them are used. For phonons localized in GaAs layers, the theoretically predicted mixing of the LO1 longitudinal modes with TO1 transverse modes in which atomic displacements occur along the normal to the superlattice is observed experimentally. These modes possess noticeable angular anisotropy. For transverse modes in which atoms move in the plane of the superlattice, the angular anisotropy is small.     

Angular dependence of Raman scattering selection rules for long-wavelength optical phonons in short-period GaAs/AlAs superlattices

The angular dependence of Raman scattering selection rules for optical phonons in short-period (001) GaAs/AlAs superlattices is calculated and experimentally studied. Experiments are performed using a micro-Raman setup, in the scattering geometry with the wavevectors of the incident and scattered light lying in the plane of superlattices (so-called in-plane geometry). Phonon frequencies are calculated using the Born model taking the Coulomb interaction into account in the rigid-ion approximation. Raman scattering spectra are calculated in the framework of the deformation potential and electro-optical mechanisms. Calculations show an angular dependence of the selection rules for optical phonons with different directions of the wavevectors. Drastic differences in the selection rules are found for experimental and calculated spectra. Presumably, these differences are due to the Fröhlich mechanism in Raman scattering for short-period superlattices.     

Intrasubband light absorption in a parabolic quantum well with consideration of scattering on the three-dimensional optical phonons

Within the frameworks of the second order perturbation theory the light absorption by free carriers in a parabolic quantum well (QW) is investigated taking into account the scattering by the three-dimensional optical phonons. An analytical expression for the absorption coefficient is obtained taking into the consideration two processes, with initial absorption of a photon and further scattering by optical phonons, and with the initial scattering by the optical phonons and the subsequent absorption of the photon. Frequency characteristics and dependences on the temperature and the width of QW are examined.     

Anomalous dispersion of optical phonons at the neutral-ionic transition: evidence from diffuse x-ray scattering

Diffuse x-ray data for mixed-stack organic charge-transfer crystals approaching the neutral-ionic phase transition can be quantitatively explained as due to the softening of the optical phonon branch. The interpretation is fully consistent with vibrational spectra, and underlines the importance of electron-phonon coupling in low-dimensional systems with delocalized electrons.     

Absolute efficiency and dispersion of Raman scattering by phonons in silicon

We have measured the absolute Raman efficiency for first order scattering of the optical zone center phonon with two different incident photon energies (1.16 and 0.94 eV) by comparison with the Raman scattering from diamond. A value significantly lower than the experimental data published previously in this region has been found. For the first order scattering no dispersion of the scattering efficiency has been observed at the indirect gap, in contrast to the second order scattering which shows a pronounced resonance behaviour in this energy range.     

Raman scattering by optical phonons and polaritons in CuCl

We present data on forward and backward scattering in CuCl at liquid N₂ temperature. Peaks at 146 and 172 cm^-1 which appear in both forward and backward scattering spectra are attributed to other than first order scattering by optical phonons. The data on polarition scattering indicate that the atomic displacement contribution to the scattering by TO phonons is rather small. From the experimentally determined polarition dispersion curve we obtain a value of 174 ±2.5 cm^-1 for ω_T and a value of 5.3 for ?_S.     

Resonant Raman scattering of optical phonons in self-assembled quantum dots

We have investigated the carrier relaxation mechanism in InGaAs/GaAs quantum dots by photoluminescence excitation (PLE) spectroscopy. Near-field scanning optical microscope successfully shows that a PLE resonance at a relaxation energy of 36 meV can be seen in all single-dot luminescence spectra, and thus can be attributed to resonant Raman scattering by a GaAs LO phonon to the excitonic ground state. In addition, a number of sharp resonances observed in single-dot PLE spectra can be identified as resonant Raman features due to localized phonons, which are observed in the conventional Raman spectrum. The results reveal the mechanism for the efficient relaxation of carriers observed in self-assembled quantum dots: the carriers can relax within the continuum states, and make transitions to the excitonic ground state by phonon emission.     

Manifestation of directional dispersion of overdamped optical phonons in Raman spectra

This study is devoted to the analysis of the features of the manifestation of the directional dispersion of strongly damped (overdamped) optical phonons in Raman spectra. The results of the experimental study of the directional dispersion of the overdamped soft mode of symmetry B ₂(z) of the KDP crystal (KH₂PO₄) are presented.     

Resonant Raman scattering from screened longitudinal optical phonons in EuTe

Forbidden resonant Raman scattering from screened longitudinal optical, LO, phonons has been observed in the back-reflection geometry from n-type EuTe at 2°K. The Raman shift increased with increasing excitation frequency but was always between the LO and TO phonon frequencies. This effect is explained in terms of a varying ‘effective’ carrier concentration as a function of laser penetration depth through the surface depletion layer in the situation of large phonon wave vector. Conduction band and lattice parameters have been calculated from infrared Reststrahlen and plasma edge measurements.     

Anisotropic scattering of holes by acoustic and optical phonons in germanium

The anisotropic scattering of holes by acoustic and optical phonons in germanium is investigated with allowance for the complex structure of the energy spectrum and the specifics of the scattering mechanisms. The temperature dependence of the hole mobility is analyzed in the temperature interval 10°K T 300°K. The results are in good agreement with the experimental data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 3–7, May, 1982.     

Scattering of long-wavelength acoustic phonons by isotopic impurities

We consider the problem of the relaxation of an arbitrary initial distribution function of a gas of long-wave acoustic phonons scattered by isotopic impurities embedded in a crystalline medium with cubic symmetry. The spectral decomposition of the collision integral of the suitable Boltzmann-Peierls equation is obtained. The spectrum of the collision operator is purely discrete and in addition to the eigenvalue 0 consists of three other eigenvalues. Explicit analytic expressions for these eigenvalues are obtained. Within the Chapman-Enskog approximation we derive the diffusion equation for the density of phonons and obtain the explicit expression for the diffusion coefficient. The dependency of the eigenvalues of the collision operator and the diffusion coefficient on the elastic constants of the medium is studied.     

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. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 50–55 (10 July 1998)     

Conductivity of a superlattice during electron scattering by optical phonons

The conductivity is calculated for a semiconductor with a superlattice with simultaneous actions of constant (E) and varying (E_o cos t) electric fields, considered quasiclassically. It is shown that in calculating the current-voltage characteristics (CVC) at low temperatures (k_oT h _o, _o being the optical phonon frequency) and e_o = 0 it is necessary to consider electrons as both absorbing and nonabsorbing optical phonons. The frequently used approximation of constant relaxation time is valid only for k_oTh_o, when scattering by optical phonons dominantes.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 81–84, January, 1982.     

Theory of magnetoresistive and thermomagnetic effects with electron scattering by optical phonons in semiconductors

The magnetoresistance and transverse Nernst-Ettingshausen effect in classical magnetic fields are calculated for the case of low-temperature scattering of electrons by optical phonons. It is shown that even though the electron relaxation time does not depend on the energy, not only are the indicated effects still possible, they can have a magnitude much greater than for other scattering mechanisms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No.2, pp. 116–120, February, 1976.The author is indebted to V. L. Bonch-Bruevich and A. G. Mironov for a useful discussion.     

Calculation of long-wavelength optical phonons and lattice reflection spectra of Al1−xGaxP mixed crystals

The two-mode behaviour of the infrared lattice vibrations of Al_1–x Ga _x P mixed crystals is described by the random-element-isodisplacement model with an effective electric field and phenomenological damping (EREI). The calculated long-wavelength transverse and longitudinal optical frequencies in dependence on the compositionx are in agreement with experimental measurements. The infrared lattice reflection spectra are calculated and fitted to the measured one by variation of damping parameters. The final values of the damping parameters are represented in dependence on the compositionx.This work has been done in the Arbeitsgemeinschaft AIII-BV-Halbleiter of the Karl-Marx-Universität Leipzig and was supported by the VEB Halbleiterwerk Frankfurt/Oder.     

Effect of optical phonons scattering on electronic current in metallic carbon nanotubes

The effect of optical phonons scattering on electronic current has been studied in metallic carbon nanotubes. The current has been calculated self-consistently by total voltage equation and the heat transport equation. The total voltage equation consists of three terms, optical phonons collision term, acoustic phonon scattering term, and contact resistance one. Including LO, A₁, and E₁(2) phonons in collision term, we can reproduce the experimental I-V curves displaying negative differential conductance. Furthermore, one conclusion is made that the more optical phonons are scattered by electron, the lower current is in metallic carbon nanotubes. By comparing the current under different conditions, we can make another conclusion that there should be nonequilibrium optical phonons under high bias in spite of whether the metallic nanotube is suspended or not. This result agrees well with the others [M. Lazzeri, F. Mauri, Phys. Rev. B 73 (2006) 165419]. Based on these results, we do not only explain the experiment, but also propose to design a heat-controlling electronic transistor with metallic carbon nanotubes as its channel, in which the electronic current can be controlled by optical phonons.