Melting of the cooperative Jahn-Teller distortion in LaMnO 3 single crystal studied by Raman spectroscopy

We have studied the behavior of the Raman phonons of a stoichiometric LaMnO₃ single crystal as a function of temperature in the range between 77 K and 900 K. We focus on the three main phonon peaks of the Pbnm structure, related to the tilt, antisymmetric stretching (Jahn-Teller mode) and stretching modes of Mn-O octahedra. The phonon frequencies show a strong softening that can be fit taking into account their renormalization because of three phonon anharmonic effects in the pseudoharmonic approximation. Thermal expansion effects, in particular the variation of Mn-O bond lengths with temperature, are not relevant above 300 K. On the contrary, phonon width behavior deviates from the three phonon scattering processes well bellow T _c . The correlation between the magnitude of the cooperative Jahn-Teller distortion, that disappears at 800 K, and the amplitude of the Raman phonons in the orthorhombic phase is shown. Nevertheless, Pbnm phonons are still observable above this temperature. Phonon width and intensity behavior around T _c can be explained by local melting of the orbital order that begins quite below T _c and by fluctuations of the regular Mn-O octahedra that correspond to dynamic Jahn-Teller distortions. Received 25 January 2001 and Received in final form 14 March 2001     

Raman scattering from GaAs-Al0.5Ga0.5As-AlAs polytype superlattices

We report the first Raman study of GaAs-Al_0.5Ga_0.5As-AlAs polytype superlattices grown by molecular-beam epitaxy. Folded longitudinal acoustic phonon modes were observed for all the samples. The frequencies agree well with those calculated using an elastic continuum model. The intensities of the folded modes have novel properties, compared with double-constituent superlattices like GaAs-(AlGa)As.     

Chemical composition of matrix-embedded ternary II–VI nanocrystals derived from first- and second-order Raman spectra

A one- and multiphonon Raman scattering study is performed for an extensive set of CdS_1–xSe_x, Cd_1–yZn_yS, Cd_1–yZn_ySe, and CdSe_1–xTe_x nanocrystals to investigate the applicability of first- and second-order Raman spectra for the determination of the matrix-embedded ternary nanocrystal composition. For one-mode ternary systems both the LO and 2LO phonon frequencies in the Raman spectra are shown to be a good measure of the nanocrystal composition. For two-mode systems, the approaches based on the difference of the LO phonon frequencies (first-order Raman spectra) or double LO overtone and combination tone frequencies (second-order Raman spectra) as well as on the LO phonon band intensity ratios are analysed. The weak electron–phonon coupling in the II–VI nanocrystals and the polaron constant values for the nanocrystal sublattices are discussed.     

Residual strain and alloying effects on the vibrational properties of step-graded InxAl1−xAs layers grown on GaAs

Lattice-relaxed In_xAl_1−xAs-graded buffer layers grown by MBE on GaAs substrate have been studied by micro-Raman scattering and photoluminescence (PL). In these heterostructures, the indium composition was gradually increased in six or four intermediate layers each of 100 nm thickness. The alloying effect in the In_xAl_1−xAs layers has been interpreted using the modified random element isodisplacement (MREI) formalism. The dependence in the MREI model of the longitudinal optical (LO) phonon energy and the In composition in the InAlAs alloy with PL measurements and Raman analysis allow the evaluation of disorder degree and give the In composition in the InAlAs active layers. The obtained InAs- and AlAs-like phonon frequencies from the fitting of Raman spectra are in reasonable agreement with those calculated according to the MREI model. Raman spectra show that InAs-like phonon frequencies are not strongly dependent either on the buffer structure or on the residual strain in the active layers. Using the AlAs-like LO phonon frequency shifts, we have calculated the residual strain in the In_xAl_1−xAs active layer. Raman results show that the slope of the grade is an important parameter that allows the growth of samples with good quality. The lower residual strain value was obtained by thick buffer with a smaller grading rate. PL measurements show that In compositions of active layers of the different studied samples are slightly higher than those measured during growth.     

Splitting of transverse optical phonon modes localized in GaAs quantum wires on a faceted (311)A surface

The energy splitting of fundamental localized transverse optical (TO1) phonon modes in GaAs/AlAs superlattices and quantum wires grown by molecular-beam epitaxy on a faceted (311)A GaAs surface is observed by Raman spectroscopy. The form of the Raman scattering tensor makes it possible to observe the TO_x and TO_y modes separately, using different scattering geometries the y and x axes are the directions of displacement of the atoms and are directed parallel and transverse to the facets on the (311)A surface). Enhancement of the splitting of the TO1_x and TO1_y modes is observed as the average thickness of the GaAs layers is decreased from 21 to 8.5 Å. The splitting is probably due to the effect of the corrugation of the GaAs/AlAs (311)A hetero-interface on the properties of localized phonon modes. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 1, 45–48 (10 July 1997)     

Effects of Ca substitution on the phonon anomalies in the Raman scattering of YBa<Subscript>2</Subscript>Cu<Subscript>4</Subscript>O<Subscript>8</Subscript>

We report on the temperature dependence of the frequencies and linewidths in the phonon Raman scattering for Y _1-y Ca y Ba ₂ Cu ₄ O ₈ system ( y = 0 - 0.15). The phonon anomalies above T _c of the frequencies are observed for the out-of-phase O(2)-O(3) A _g and O(4) A _g modes, and these onset temperatures decrease with increasing Ca content. These features are consistent with the spin-gap behaviors associated with Ca doping reported previously. Furthermore, we find that the smaller gap exists near or just above the Ba phonon frequency at in the undoped samples and its energy increases with Ca doping. Received 9 June 1999     

Surface and interface optical phonon modes and electron-phonon interaction in a multi-shell spherical system

Within the framework of the dielectric continuum model, interface optical(IO) and surface optical(SO) phonon modes and the Fr?hlich electron-IO (SO) phonon interaction Hamiltonian in a multi-shell spherical system were derived and studied. Numerical calculation on CdS/HgS/H₂O and CdS/HgS/CdS/H₂O spherical systems have been performed. Results reveal that there are two IO modes and one SO mode for the CdS/HgS/H₂O system, one SO mode and four IO modes whose frequencies approach the IO phonon frequencies of the single CdS/HgS heterostructure with the increasing of the quantum number l for CdS/HgS/CdS/H₂O. It also showed that smaller l and SO phonon compared with IO phonon, have more significant contribution to the electron-IO (SO) phonon interaction. Received 16 October 2001 and Received in final form 23 January 2002 Published online 25 June 2002     

Surface morphology and Raman spectroscopy of thin layers of antimony and bismuth chalcogenides

The phonon spectra in thin layers of bismuth telluride and solid solutions of Bi_2–xSb_xTe_3–ySe_y of different composition, belonging to three-dimensional topological insulators, have been investigated by micro-Raman spectroscopy, and the morphology of an interlayer van der Waals (0001) surface in them has been studied by semicontact atomic force microscopy at room temperature. The analysis of the Raman spectra and the intensity ratio of active and inactive longitudinal optical modes depending on the composition, morphology of the interlayer surface, and thickness of the layers enabled the estimation of the effect of topological surface states of Dirac fermions, associated with the strengthening of the electron–phonon interaction as a result of resonance Raman scattering, and the identification of the compositions, in which the contribution of topological surface states becomes dominant.     

Probing electron-phonon coupling in high-Tc superconductors by site-selective isotope substitution

We consider changes in the electron-phonon coupling in high-T _c cuprates caused by site-selective oxygen isotope substitution. Contrary to the total substitution, the site-selective replacement influences the coupling constant for each phonon mode due to the induced changes in the phonon eigenvectors. The relative changes for some modes can be larger than 100%. The measured properties sensitive to these changes are discussed. Received 9 August 2001 and Received in final form 11 January 2002     

Soft modes and phonon interactions in studied by neutron scattering

The low frequency lattice dynamics and its relationship to the second order paraelectric-to-ferroelectric transition in Sn₂P₂S₆ is studied. The dispersion branches of the acoustic and lowest lying optical phonons in the a^*-c^* plane have been obtained in the ferroelectric phase, for x-polarized phonons. Close to the phase transition a considerable softening is found for the lowest optical mode (P_x), comparable to the behaviour observed in previous Raman investigations. As found previously in Sn₂P₂Se₆, a strong coupling between the TO(P_x) and TA(u_xz) phonons is observed, although, apparently, not strong enough to lead to an incommensurate phase. The soft TO(P_x) mode at the zone center is observed. The temperature dependence of its frequency and damping shows that the transition is not entirely displacive. At low temperatures an unusual apparent negative LO-TO splitting is observed which is shown to arise from the coupling of the x-polarized soft mode to the nearby z-polarized optical phonon. For comparison, the soft TO(P_x) dispersion in the a^*-b^* plane is measured in both the paraelectric and ferroelectric phases. Consistent frequency changes and LO-TO splitting are observed, revealing a significant interaction between the TA(u_yx) and LA(u_xx) acoustics branches and the TO and LO soft optic branches, respectively. In contrast, the nearby y-polarized optic branch shows almost no temperature dependence. Finally, the influence of piezoelectric effects on the limiting acoustic slopes in the ferroelectric phase is discussed. Received: 11 May 1998 / Revised and Accepted: 15 June 1998     

Infrared spectra and lattice vibrations of the spin-chain compound LiCuVO 4

The infrared spectra of the one-dimensional antiferromagnet LiCuVO₄ are measured in the frequency range from 10 cm^-1 to 10 000 cm^-1 and at temperatures from 2 K to 300 K, for the electric field vector E of the radiation polarized either along the a- or along the b-crystallographic directions. For each polarization six infrared active phonon modes are observed in accordance with factor group analysis of the crystal structure of LiCuVO₄. The theoretical group analysis of the possible spinel low-symmetry phases is performed within the framework of Landau's theory of phase transitions. The parameters of several phonon lines show noticeable anomalies around 150 K where the magnetic correlations appear in the copper chains, which may indicate a finite interaction between the phonon and the magnon subsystems in LiCuVO₄. Received 19 February 2001 and Received in final form 26 June 2001     

Phonon freedom and confinement in GaAsAlxGa1−xAs superlattices

Phonon modes in GaAsAl_xGa_1?xAs superlattices simplify when the phonon wavevector q is perpendicular to the plane of the layers. We have studied such modes using a Raman back-scattering technique on SL's grown by MBE. The results are consistent with simple ideas of LA phonon freedom and LO phonon confinement suggested by one-dimensional lattice dynamical calculations. The longitudinal acoustic (LA) modes show zone folding due to mini-zone formation. Their frequencies occur in doublets linearly dependent on q and show little mini-gap formation. This is consistent with a picture of approximately free plane wave propagating through the interfaces with Raman coupling due to SL layering of the photoelastic coefficient. By contrast, Raman data on LO modes in small period GaAsAlAs SL's suggest that these modes are standing waves strongly confined in either GaAs or AlAs.     

Raman study of V/III flux ratio effect in InP/InAlAs/InP heterostructures grown by MOCVD

Micro‐Raman measurements have been carried out in order to study the V/III flux ratio effect in InP/InAlAs/InP heterostructures grown by metal‐organic chemical vapor deposition (MOCVD). Photoluminescence (PL) studies in InP/InAlAs/InP heterostructures [1] , [2] show a strong dependence of the PL band linewidth on V/III molar ratio. In addition to the observation of the two‐mode behavior and the disorder activated modes in InAlAs alloy, an analysis of Raman spectra shows a line shape broadening and wavenumber shift of Raman peaks for various V/III molar ratios, with minimum linewidth and lattice mismatch occurring at V/III = 50. Also, a strong dependence on the composition modulation of the AlAs‐like longitudinal optic (LO_AlAs−like) phonon was observed due to clustering. Calculation of the in‐plane strain shows that the lattice mismatch between the epilayer and the substrate is relatively insensitive to flux ratio variation within the range investigated. Therefore, the high arsenic overpressures used have an insignificant adverse effect on the quality of the hetero‐interfaces. Copyright © 2009 John Wiley & Sons, Ltd.     

The origin of disorder in CH 3 HgX (X = Cl, Br and I) crystals investigated by temperature dependent Raman spectroscopy

Methyl-mercury(II) halides CH₃HgX (X = Cl, Br and I) were studied by means of temperature dependent Raman spectroscopy from 10 K to 410 K. In addition to the previously reported soft phonons, new changes in the low frequency spectra were observed at T ≈ 70 K in CH₃HgBr and at T ≈ 100 K in CH₃HgI. The bandwidths of the two internal modes in CH₃HgBr, the CH₃ symmetric stretching band and the C-Hg-Br bending band, rise towards a local maximum at T ≈ 50 K as the temperature is raised from 10 K to 300 K. On the other hand the bandwidths of the two corresponding modes in CH₃HgI crystals monotonously increase with temperature, obeying an Arrhenius law. Besides the three phonon modes present in the Raman spectra of CH₃HgCl at room temperature, the fourth phonon band that has been observed at temperatures below 245 K might correspond to the freezing of methyl librations. The huge bandwidth of the C-Hg-Br bending mode could suggest the presence of additional weak bonding of a mercury atom with bromine atoms from other molecules, thus inducing positional disorder. Received 19 November 1999 and Received in final form 10 November 2000     

Electron phonon interactions and superconductivity in α ′ -TTF[Pd(dmit)2]2

A simple model is developed to understand superconductivity in α ^′ -TTF[Pd(dmit)₂]₂. We include electron-intra molecular and intermolecular phonon interactions as the mechanism of superconductivity. Intramolecular vibrations included are the eight symmetric A_g modes of the Pd(dmit)₂ molecule. Intermolecular vibrations included are the longitudinal acoustic and transverse acoustic (LA and TA) modes of the Pd(dmit)₂ column. All the electron-phonon coupling constants are calculated from first principles. We find that largest el-intramolecular vibration coupling is to the A_g mode with the highest frequency (1449 cm^-1). The el-intermolecular coupling to the LA mode is found to be larger than the total el-intramolecular couplings. We also find el-(TA)phonon coupling to be at least an order of magnitude smaller than el-(LA)phonon coupling. Estimate of superconducting transition temperature is comparable to experimental result. We also provide a detailed discussion, employing the results of recent numerical calculations on two-chain Hubbard model and the specific material parameters, on the relative importance of el-ph and Coulomb-origin mechanisms of superconductivity in α ^′ -TTF[Pd(dmit)₂]₂ and TTF[Ni(dmit) ₂ ] ₂ . Received 29 March 2001 and Received in final form 7 August 2001     

Isotope effect in resonant Raman scattering and induced IR spectra of trans-polyacetylene

We present a full explanation of the isotope effect in resonant Raman scattering, doping induced, and photo-induced IR-active vibrations of previously reported data in trans-(CH)_x, (¹³CH)_x and (CD)_x. The interpretation is given in terms of Raman scattering from amplitude modes of the dimerized chains and absorption by charged induced IR-active modes. It is shown that the bare phonon frequencies are properly modified by the isotope mass whereas the observed frequencies are derived by the dressed phonon propagator, and therefore do not follow the usual mass rule.     

Hole doping by Li substitution and antiferromagnetism in YBa Cu O studied by neutron powder diffraction measurements

The magnetic structure of tetragonal insulating YBa₂Cu_3-xLi_xO_y has been studied as a function of x and y. The Néel temperature and the mean ordered magnetic moment on the Cu2 sites were determined by neutron powder diffraction measurements. The decrease of these two parameters as compared to YBa₂Cu₃O₆ is much stronger for lithium than for zinc substitution. The difference is quantitatively explained by the presence of holes created in the CuO₂ planes. These holes arise from the substitution of plane Cu²⁺ by Li⁺. We suggest an explanation why such holes are not seen for the same substitution of plane Cu²⁺ by Li⁺ in orthorhombic superconducting YBa₂Cu_3-xLi_xO _{7 - δ} . Received 31 October 2001 and Received in final form 6 March 2002 Published online 25 June 2002     

Infrared and Raman spectroscopies of InP/II–VI core-shell nanoparticles

We use infrared (IR) and Raman spectroscopies to investigate the optically active phonon modes in InP nanoparticles and InP/II–VI core-shell nanoparticles fabricated by similar colloidal chemistry methods. The IR transmission spectra of several InP nanoparticle samples exhibit a common absorption feature, which we assign to the Fröhlich mode. The Raman results for the same samples show transverse and longitudinal optical phonon peaks, and scattering strength in between due to surface optical (SO) modes. Infrared spectra of the InP/ZnSe core-shell nanoparticles () exhibit three absorption features, one due to the InP core, and the others associated with the ZnSe shell layer. Raman measurements (12–292 K) also show three phonon-related peaks, whose intensities vary sharply with temperature. The frequencies of the IR and Raman lines are in approximate accord with dielectric continuum theory.     

MBE生长的In_(1-x-y)Ga_xAl_yAs/InP四元混晶的拉曼散射

ＭＢＥ生长的Ｉｎ１－ｘ－ｙＧａｘＡｌｙＡｓ／ＩｎＰ四元混晶的拉曼散射韩和相汪兆平李国华徐士杰刘南竹朱作明（中国科学院半导体研究所半导体超晶格国家重点实验室北京１０００８３）ＯｐｔｉｃａｌＰｈｏｎｏｎＲａｍａｎＳｃａｔｅｒｉｎｇｆｒｏｍＩｎ１－ｘ－ｙＧ...     

Composition and lattice mismatch dependent dielectric constants and optical phonon modes of InAs1−x−ySbxPy quaternary alloys

Based on the pseudopotential formalism under the virtual crystal approximation, the dielectric and lattice vibration properties of zinc-blende InAs_1−x−ySb_xP_y quaternary system under conditions of lattice matching and lattice mismatching to InAs substrates have been investigated. Generally, a good agreement is noticed between our results and the available experimental and theoretical data reported in the literature. The variation of all features of interest versus either the composition parameter x or the lattice mismatch percentage is found to be monotonic and almost linear. The present study provides more opportunities to get diverse high-frequency and static dielectric constants, longitudinal and transversal optical phonon modes and phonon frequency splitting by a proper choice of the composition parameters x and y (0 ⩽ x ⩽ 0.30, 0 ⩽ y ⩽ 0.69) and/or the lattice mismatch percentage.