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     

The thermal conductivity of disordered and amorphous metal films

The thermal conductivity of quenched condensed polycrystalline and amorphous Pb and Pb_0.9Cu_0.1 films has been measured between 0.5 and 11 K, i.e. in the superconducting (T7 K) and in the normal state (T7 K). Whereas, in agreement with previous results, phonon heat transport is very small for crystalline films, a considerable portion of heat is carried by phonons in amorphous films, owing to the absence of extended lattice defects. Phonon scattering in these latter films is analyzed in terms of scattering from conduction electrons aboveT _c, whereas well belowT _c it is very likely due to low energy excitations inherent in the amorphous structure.Work performed within the research program of the Sonderforschungsbereich 125 — Aachen/Jülich/Köln     

Electron Raman scattering in metals with strong electron-phonon coupling

Inelastic light scattering by the carriers interacting with phonons in the anisotropic metals with large penetration depth is theoretically studied. It is shown that the strong temperature dependence of the Raman scattering intensity in the region of phonon frequencies is the main characteristic feature of these processes. The effects of anisotropy, impurities and the strength of electron-phonon interaction on the frequency and temperature dependences of the polarization operator are analysed. Taking into account the anisotropy vertex corrections which obey a system of the Boltzman-type integral equations should leads to the considerable changes of the frequency behavior of scattering cross section for low frequencies. However, the changes of the temperature dependence are not so drastic. Increasing the electron-phonon coupling constant affects the particle-hole polarization operator in two possible ways to weaken temperature dependence and to make flatter frequency curves. The same effects are also from impurities. Some theoretical consequences which concern the role of electron-phonon interaction for electron Raman scattering in high-T _c superconductors aboveT _c are proposed.     

Dynamical properties of the Peierls-Fröhlich state on the many-phonon-coupling model

The amplitude and phase phonons and the frequency dependent conductivity below the mean-field Peierls-Fröhlich transition temperature T_c, and the Kohn anomaly and fluctuation induced charge-density-wave conductivity above T_c, are discussed on the basis of the many-phonon-coupling model recently introduced by Rice, Duke and Lipari. For dominant intramolecular phonon coupling an isotope effect in T_c is related to the isotopic shift in the small polaron binding energy.     

Interpretation of temperature-dependent thermoelectric power behaviour of La0.67Ba0.33MnO3 manganites

The temperature dependence of the thermoelectric power S(T) in polycrystalline La_0.67Ba_0.33MnO₃ has been investigated. In the ferromagnetic regime, the phonon thermoelectric power is evaluated by incorporating the scattering of phonons with impurities, grain boundaries, charge carriers and phonon. The Mott expression is used to compute the electron diffusive thermoelectric power (S_c^diff.) using Fermi energy as electron-free parameter. The S_c^diff infers linear temperature dependence and S_ph^drag increases exponentially with temperature, which is an artefact of various operating scattering mechanisms. The behaviour of the S(T) is determined by competition among the several operating scattering mechanisms for the heat carriers and a balance between carrier diffusion and phonon drag contributions in the La_0.67Ba_0.33MnO₃. Numerical analysis of thermoelectric power of the present model shows similar results as those revealed from experiments.     

Far-infrared reflectivity of sintered YBa2Cu3O7 in the normal and superconducting state

We report on a study of the far-infrared reflection for a sintered YBa₂Cu₃O₇ sample that contained a large portion of preferentially oriented crystallites with thea–b plane parallel to the surface and that showed extraordinary high far-infrared reflectivity. From experimental reflection data we determined, by Kramers-Kronig analysis, the dynamical conductivity and extracted the contributions due to free charge carriers and phonons, respectively. We find evidence for an anomalous behavior of the dynamical conductivity at temperatures aboveT _c ; the dynamical conductivity increases strongly for temperatures approachingT _c and is strongly frequency dependent. By use of the Mattis-Bardeen theory we obtain an estimate for the superconducting energy gap of 2/kT _c 4.6 (forTT _c ). We find that the lowest frequency infrared-active phonon mode is less damped in the superconducting state than in the normal state.     

Propagation of nonequilibrium phonons in ferroelectric ceramics and single crystals

Phonon transport in ferroelectric ceramics and single crystals has been experimentally investigated. Our measurements indicate that, in the temperature range 1.7–3.8 K studied, the effective phonon diffusion coefficient behaves as D _eff∞T ⁻⁵ in ferroelectrics with a broadened phase transition. This experimental dependence is in accord with the presence of a plateau in the thermal conductivity of such materials. The scattering by domain walls in BaTiO₃ single crystals has been identified, and our results are in quantitative agreement with calculations. Zh. éksp. Teor. Fiz. 115, 624–631 (February 1999)     

Low-temperature thermal conductivity of terbium-gallium garnet

Thermal conductivity of paramagnetic Tb₃Ga₅O₁₂ (TbGG) terbium-gallium garnet single crystals is investigated at temperatures from 0.4 to 300 K in magnetic fields up to 3.25 T. A minimum is observed in the temperature dependence κ(T) of thermal conductivity at T _min = 0.52 K. This and other singularities on the κ(T) dependence are associated with scattering of phonons from terbium ions. The thermal conductivity at T = 5.1 K strongly depends on the magnetic field direction relative to the crystallographic axes of the crystal. Experimental data are considered using the Debye theory of thermal conductivity taking into account resonance scattering of phonons from Tb³⁺ ions. Analysis of the temperature and field dependences of the thermal conductivity indicates the existence of a strong spin-phonon interaction in TbGG. The low-temperature behavior of the thermal conductivity (field and angular dependences) is mainly determined by resonance scattering of phonons at the first quasi-doublet of the electron spectrum of Tb³⁺ ion.     

Lattice dynamics of cubic PbTiO3 by inelastic neutron scattering

High-temperature dispersion relations of the phonon modes in a cubic PbTiO₃ single crystal have been investigated along the [ξ 0 0] and [ξ ξ 0] directions by inelastic neutron scattering. Above T _c, the phonon dispersions are only temperature-dependent close to the Brillouin zone centre where the mode softening comes through. The measurements indicate large cubic anisotropy of the elastic tensor and relatively low anisotropy of the soft mode dispersion. The differences from an earlier inelastic neutron scattering study are discussed.     

Electronic thermal conductivity in suspended and supported bilayer graphene

Electronic thermal conductivity κ_e is investigated, using Boltzmann transport equation approach, in a suspended and supported bilayer graphene (BLG) as a function of temperature and electron concentration. The electron scattering due to screened charged impurity, short-range disorder and acoustic phonon via deformation potential are considered for both suspended and supported BLG. Additionally, scattering due to surface polar phonons, is considered in supported BLG. In suspended BLG, calculated κ_e is compared with the experimental data leaving the phonon thermal conductivity. It is emphasized that κ_e is important in samples with very high electron concentration and reduced phonon thermal conductivity. κ_e is found to be about two times smaller in supported BLG compared to that in suspended BLG. With the reduced extrinsic disorders, in principle, the intrinsic scattering by acoustic phonons can set a fundamental limit on possible intrinsic κ_e.     

Phonon linewidths in YNi<Subscript>2</Subscript>B<Subscript>2</Subscript>C

Phonons in a metal interact with conduction electrons which give rise to a finite linewidth. In the normal state, this leads to a Lorentzian shape of the phonon line. Density functional theory is able to predict the phonon linewidths as a function of wave vector for each branch of the phonon dispersion. An experimental verification of such predictions is feasible only for compounds with very strong electron-phonon coupling. YN₂B₂C was chosen as a test example because it is a conventional superconductor with a fairly high T _c (15.2 K). Inelastic neutron scattering experiments did largely confirm the theoretical predictions. Moreover, they revealed a strong temperature dependence of the linewidths of some phonons with particularly strong electron-phonon coupling which can as yet only qualitatively be accounted for by theory. For such phonons, marked changes of the phonon frequencies and linewidths were observed from room temperature down to 15 K. Further changes were observed on entering into the superconducting state. These changes can, however, not be described simply by a change of the phonon linewidth.      

Free carrier absorption in quantizing magnetic fields: Nonpolar optical phonon scattering

A quantum theory of free carrier absorption in nondegenerate semiconductors and in strong magnetic fields which was previously developed to treat the case when acoustic phonon scattering dominates the free carrier absorption process [1] is extended to treat the case when nonpolar optical scattering is important. When the electromagnetic radiation field is polarized parallel to the direction of the applied magnetic field, results are obtained which are similar to those when acoustic phonon scattering is dominant. The free carrier absorption is an oscillatory function of the magnetic field which on the average increases in magnitude with the magnetic field. However, more structure in the free carrier absorption occurs when nonpolar optical phonon scattering dominates. This is due to the fact that there are two periods in the oscillatory magnetic field dependence associated with the emission or the absorption of optical phonons during the intraband transitions. When the cyclotron frequency exceeds the sum of the photon and optical phonon frequencies, i.e. ω_c > θ + ω_o, the free carrier absorption is predicted to increase linearly with magnetic field when ?ω_c? k_BT. The magnetic field dependence of the free carrier absorption can be explained in terms of phonon-assisted transitions between the various Landau levels in a band involving the emission and absorption of optical phonons.     

Raman scattering study of the dynamics of the pressure-induced phase transition in thallium azide (TlN3)

A Raman scattering investigation of the pressure-induced phase transition in tetragonal thallium azide (TlN₃) is reported. The most interesting features of the Raman spectrum of TlN₃ are the anti-resonant line-shapes of two E_g symmetry phonons at 35 and 50 cm^?1 superimposed on a quasi-elastic wing. The scattering data is shown to be consistent with a model in which the two phonons interact via an imaginary coupling term. The phonon at 35 cm^?1 (assigned to a translational shear mode of the Tl⁺ sublattice) softens with increasing pressure and increases in linewidth as P approaches P₀ (=8 kbar) from below. At the same time, the quasi-elastic scattering component (associated with large amplitude N₃^? librational fluctuations) becomes less damped. A displacive structural transition from tetragonal to monoclinic is indicated by the eigenvector of the soft phonon.     

Influence of magnetoelastic effects on lattice vibrations in the ferromagnetic invar alloy Fe0.65Ni0.35

Neutron scattering measurements have been made of phonons in a Fe₆₅Ni₃₅ crystal at several temperatures. Marked softening of the [110] acoustic shear modes and a dip in the dispersion relation are found at low temperatures. Above the magnetic ordering temperature T_c, the frequency shift is removed completely, which suggests phonon perturbation by magnetoelastic coupling.     

Heat transfer in the plastic phases I and II of cyclopentane

Thermal conductivity of solid cyclopentane C₅H₁₀ has been measured at isochoric conditions in the plastic phases I and II for samples of different densities. Isochoric thermal conductivity is nearly constant in phase II and increases with temperature in phase I. Such behaviour is attributed to weakening of the translational orientational coupling which, in turn, leads to a decrease of phonon scattering on rotational excitations. The experimental data are described in terms of a modified Debye model of thermal conductivity with allowance for heat transfer by both low-frequency phonons and diffusive modes.     

Ab initio study on the high superconducting transition temperature in calcium under high pressure

For calcium in the phases IV and V, we estimated the superconducting transition temperature T _c by the use of the Allen–Dynes formula. Setting the effective screened Coulomb repulsion constant μ* at 0.1 in the formula, we obtained T _c =23.42 K at 100 GPa for Ca-IV and T _c =15.87 K at 120 GPa for Ca-V. In order to clarify the origin of such high values of T _c , first, we investigated the band character of electrons and found that the high T _c is not necessarily related to the so called s–d transfer. Then we analyzed the electron–phonon coupling at each phonon mode in Ca-V where the highest T _c in elements has been experimentally observed. As a result, we discovered that an optical mode at the Γ point has the strongest electron–phonon coupling. Such phonon mode can exist only in the complex crystal structure of Ca-V, and the result shows that the high T _c seems to be closely linked with the complex crystal structures like Ca-IV and Ca-V.     

Optical investigation of coherent and thermal phonons in high-T c superconductors

Low-energy phonons in NdBa₂Cu₃O_7?x and Bi₂Sr₂CaCu₂O_8+x superconducting single crystals have been studied by Raman scattering and reflectivity measurements with femtosecond-scale resolution. Raman scattering provides information on equilibrium thermal phonons, whereas information about a coherent state of the phonon system is obtained by measuring in the time domain when the phonon system is pumped by an ultrashort pulse and subsequently probed by a properly gated second pulse. It is shown that both methods yield similar results for the phonon mode energies, while the energy relaxation and dephasing rates exhibit a tendency to disagreement.     

Observation of a central peak in lead germanite by light scattering

Brillouin-Raman scattering measurements on single domain lead germanate over the temperature interval 300–455 K reveal an unresolved (less than 80 MHz) central peak. At about T_c?20 K this peak begins to increase in intensity, reaching a maximum at T_c, and then falls away rapidly with increasing temperature above the transition. The central peak anomalous intensity, which is visible to the naked eye, is much larger than the intensity contributions due to phonons, including the ferroelectric soft mode, and appears to be a static effect.     

Thermal conductivity of silver doped lanthanum manganites between 10 and 300 K

Thermal conductivity (λ) of nanocrystalline La_1−xAg_xMnO₃ (x=0.05, 0.15, 0.25, 0.3) pellets prepared by pyrophoric method is reported between 10 and 300 K. Magnitude of thermal conductivity has been found to be strongly influenced by monovalent (Ag) substitution at the La site. Silver doping in LaMnO₃ enhances T_C of the system to ∼299 K. Qualitative nature of the temperature variation of thermal conductivity of the silver substituted lanthanum manganites remains closely similar to that for divalent doped systems. Our analysis demonstrates that in La_1−xAg_xMnO₃ also, the mechanism of heat conduction is predominantly by phonons. The contribution of the electronic part is only ∼1% of the total λ. The spin wave contribution is also estimated close to T_C, which for all the samples lies within ∼2%. At temperatures below ∼100 K, the measured data have been analyzed using phonon relaxation time method and the strengths of the various phonon scattering processes have been estimated. Our analysis further suggests strong influence of phonon scattering by 2D like defects in the thermal conductivity of monovalent doped lanthanum manganites at low temperatures (<70 K) in the ferromagnetic region.     

Anharmonic oxygen vibration and mode softening in Tl2Ba2Ca2Cu3O10 superconductor

The softening of the IR active 300 cm⁻¹ phonon mode in Tl-2223 superconductor, aroundT _c has been explained by considering a sixth order polarization potential at the off-center oxygen ion site in the Tl-O plane, and by using a nonlinear lattice dynamical theory. The present theory explains, more or less satisfactorily, the unusual temperature dependence of oxygen ion vibration and frequency shifts at higher temperature. The existence of strong nonlinear electron-phonon interaction atT _c has been emphasized.