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.     

Optical phonons in CuAlS2

The optical phonons at k=0 of CuAlS₂ have been investigated by Raman scattering, infrared reflectivity and absorption measurements from 50 to 1000 cm^-1 at T=300 K. Eleven of the thirteen expected optically active phonons have been observed and identified with respect to their symmetry types. The phonon frequencies appear in a range from 498 to 76 cm^-1 with predominant polar modes at 445 and 266 cm^-1. The dielectric dispersion for E ⊥ c and E ∥ c has been determined by Kramers-Kronig integrations.     

Raman and infrared spectra of ZnSiAs2

The optical phonons at k = 0 of ZnSiAs₂ have been investigated by Raman scattering and infrared reflectivity measurements at 300 K. Eleven of thirteen expected optically active phonons have been observed and identified with respect to their symmetry types. The phonon frequencies appear in the range from 415 cm^-1 to 75 cm^-1 with predominant polar modes at 400 cm^-1 (gG₅), 389 cm^-1 (Γ₄) and 242 cm^-1 (Γ₄). The dielectric dispersion for E ⊥ c and E 6 c has been determined by Kramers-Kronig integrations.     

Surface enhanced Raman scattering of light by ZnO nanostructures

Raman scattering (including nonresonant, resonant, and surface enhanced scattering) of light by optical and surface phonons of ZnO nanocrystals and nanorods has been investigated. It has been found that the nonresonant and resonant Raman scattering spectra of the nanostructures exhibit typical vibrational modes, E ₂(high) and A ₁(LO), respectively, which are allowed by the selection rules. The deposition of silver nanoclusters on the surface of nanostructures leads either to an abrupt increase in the intensity (by a factor of 10³) of Raman scattering of light by surface optical phonons or to the appearance of new surface modes, which indicates the observation of the phenomenon of surface enhanced Raman light scattering. It has been demonstrated that the frequencies of surface optical phonon modes of the studied nanostructures are in good agreement with the theoretical values obtained from calculations performed within the effective dielectric function model.     

High-pressure Raman spectroscopy of antimony: As-type, incommensurate host-guest, and bcc phases

The vibrational dynamics of elemental solids that form incommensurate host-guest structures are of fundamental interest. High-pressure Raman scattering has been used to examine the vibrational spectrum of the group-V element Sb up to 33 GPa. A_1g and E_g phonons of the ambient pressure rhombohedral A7 phase display a marked decrease with pressure, i.e., prior to the transition to the tetragonal host-guest Sb-II phase at 8.6 GPa, via the monoclinic host-guest Sb-IV phase. The Raman spectrum of the incommensurate host-guest Sb-II phase, has five bands between 80 cm⁻¹ and 200 cm⁻¹ that increase with pressure. For the bcc structure stable above 28 GPa, we observe one weak disorder-induced band that increases with pressure.     

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.     

Resonant Raman scattering in nanostructures with InGaAs/AlAs quantum dots

Raman scattering by optical phonons in In_xGa_{1 ? x} As/AlAs nanostructures with quantum dots has been studied experimentally for compositions corresponding to x = 0.3?1 under out-resonance conditions. Features due to scattering by GaAs-and InAs-like optical phonons in quantum dots have been detected, and the phonon frequencies have been determined as a function of the dot composition. With increasing excitation energy, a red shift is observed in the frequency of the GaAs-like phonon in quantum dots, which testifies to Raman scattering selective by the size of quantum dots. Under resonant conditions, multiphonon light scattering by optical and interface phonons is observed up to the third order, including overtones of the first-order phonons of InGaAs and AlAs materials and their combinations.     

Raman scattering from magnons in CoCl2 and FeCl2

One-magnon Raman scattering has been observed in the metamagnets CoCl₂ and FeCl₂. The k = 0 magnon energies are 16 ± 1 cm^-1 at 21 K and 16.4 ± 0.4 cm^-1 at 12 K, respectively and these values are in good agreement with previous AFMR and neutron scattering results. A search for two-magnon scattering in both compounds was unsuccessful, largely because of masking from nearby first-order phonons and a weak temperature dependent broad band at 140 cm^-1 in CoCl₂, which is assigned to two-phonon scattering from acoustic phonons.     

Electron localization in 1D conductors due to the phonon scattering

Electron localization in 1D conductors due to scattering by dispersionless optical phonons at low temperatures has been studied by applying the Berezinsky diagram technique. The large localization length l_loc, much longer than the mean free path l, has been found. The low frequency optical absorption is described by the law: Re σ(ω) ～ ω² |ln³ω|.     

Field dependence of the thermal conductivity of K2CuCl4 · 2H2O near its ferromagnetic Curie temperature

The thermal conductivity of the ferromagnetic insulator K₂CuCl₄ · 2H₂O has been measured near its Curie temperature T_c. The measurements were made as a function of temperature in constant external magnetic field and as a function of field along isotherms. The results indicate a relaxation rate for magnetic critical scattering of phonons varies as H^?1/2.     

Disorder-induced Raman scattering of In1−xGaxP in the transverse acoustic phonon region

Raman scattering measurements have been performed on In_1?xGa_xP (0.62?x?1) over the entire frequency range of first and second order scattering. Besides the already known disorder activated band between the LO(Γ) and TO(Γ) modes a new disorder activated band is found in the region of transverse acoustic phonons around 87 cm^-1. The position of the new band shifts only slightly with composition while its strength and line-shape change.     

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.     

Phonons in disordered solids

The effects of crystalline disorder on the frequency distribution and lifetimes of phonons are investigated. As the disorder parameter g characterizing the scattering of normal modes is increased to g>g_c, the normal mode spectrum becomes unstable against arbitrarily small nonlinearities while the ground state (if any) becomes increasingly degenerate. The bandwidth of the acoustic phonon spectrum increases with g, from a value ω_Dat g = 0 to 1.24ω_D at g_c. The speed of sound at long wavelengths decreases with increasing g; at threshold (g_c) it has decreased by some 29.3% from its value at g = 0. For g < g_c the scattering lifetime of long-wavelength normal modes satisfies Rayleigh's law (1/τ = A(_g)ω⁴). At g_c the damping satisfies a different law: 1/τ = Bω². Similar effects are obtained for the optic mode phonons. For these reasons, it appears that g_c marks the threshold of amorphous behavior, although the regime g >g_c is beyond the scope of our model.     

Effect of dislocations on the lattice thermal conductivity of superconducting niobium

Thermal conductivity measurements on single crystal Nb samples in the superconducting state have demonstrated a resonant scattering of thermal phonons at roughly 5 × 10¹⁰Hz. The assumption of a mechanical resonance associated with the dislocation structure accounts for the present data and is consistent with other data found in the literature. The thermalization of phonons at an abraded sample surface, and the attendant failure of the relation l^?1 = ∑_jl_j^?1 for phonon mean free paths, was also observed.     

Inelastic neutron scattering from the quasi-one-dimensional conductor K1.75[Pt(CN)4]·1.5H2O

Neutron inelastic scattering has been used to study the longitudinal and one of the transverse acoustic phonons, propagating along the [001]¹ direction in triclinic K_1.75[Pt(CN)₄]·1.5H₂O. This material appears to be a quasi-one-dimensional conductor, with a commensurate distortion. We observe a reasonably well defined Kohn anomaly, which shows little temperature dependence between 80 and 300 K.     

Specific features of thermal phonon scattering and ferromagnetic properties of ZnO varistor ceramics

The scattering of thermal phonons (T = 2.3–3.8 K) in ZnO single crystals and related 97.5ZnO + 0.5Bi₂O₃ + 0.5Co₃O₄ + 0.5MnO₂ + 1Sb₂O₃ (mol %) ceramics has been studied. It is established that the transport characteristics of phonons with thermal frequencies in ceramics at liquid-helium temperatures are determined by the presence of Co²⁺ ions in ZnO grains. A model of phonon transport in ZnO-based ceramics is proposed that takes into account the electron states of cobalt. The energy of the corresponding level is estimated at Δ ≈ 5 K. Manifestations of the ferromagnetic effect are revealed.     

Raman E 1, E 1+Δ1 resonance in unstrained germanium quantum dots

Raman scattering by optical phonons in unstrained Ge quantum dots obtained in GaAs/ZnSe/Ge/ZnSe structures was studied using molecular beam epitaxy. A shift in the E ₁, E ₁+Δ1 resonance energy due to the quantization of the spectrum of electron and hole states in quantum dots was observed. The properties observed were explained with the use of a simplest model of localization with allowance for the spectrum of Ge electron states.     

Phonons in the organic conductor TEA(TCNQ)2 studied by neutron inelastic scattering

The lattice dynamics of the one-dimensional organic conductor TEA(TCNQ)₂ is studied by inelastic neutron scattering at temperatures 77, 175 and 295 K. Special attention is paid to the phonons propagating along c¹ which is approximately along the conducting axis. These phonons show a well defined Kohn anomaly at 2k_F as well as gaps which we attribute to both the tetrametic structure of the crystals and to the electron- phonon coupling. The temperature variation of the phonons along c¹ is found to be typical for a system where electron-phonon coupling dominates.     

Interpretation of thermoelectric power behaviour of Zinc nanowire composites: Phonon-scattering mechanism

We develop a theoretical model for quantitative analysis of temperature-dependent thermoelectric power (S) of Zn nanowires. In doing so, we first use the Mott expression to compute the electron diffusive thermoelectric power (S_c^diff.) using Fermi energy as electron-free parameter, S_c^diff. shows linear temperature dependence. Further, the S_c^diff. contribution is subtracted from the experimental data and the difference (S_experimental-S_c^dif) is characterized as phonon drag thermoelectric power (S_ph^drag) which is obtained within the relaxation time approximation where the thermoelectric power is limited by the scattering of phonons with impurities, grain boundaries, charge careers and phonons in the nanowires. The S_ph^drag shows anomalous temperature-dependent behaviour, which is an artifact of various operating scattering mechanisms. The observed anomalies are well accounted in terms of interaction among the phonons-impurity, phonon-grain boundaries, phonon-electron and the umklapp scattering. It is also shown that for phonons the scattering and transport cross-sections are proportional to ω⁴ in the Rayleigh regime where ω is the frequency of the phonons. Numerical analysis of thermoelectric power from the present model shows similar results as those revealed from experiments.     

Electron mobility and electron scattering in CdxHg1−xSe mixed crystals

We report a detailed study, both experimental and theoretical, of electron mobility and Hall coefficient in small-gap Cd_xHg_1?xSe mixed crystals. The electron mobility is calculated by the variational technique. The results obtained with no adjustable parameter are within 15% of the experimental values in the range of temperature 4.2–300 K, electron concentrations 7 × 10¹⁶?5 × 10¹⁸cm^?3 and composition 0 < x ? 0.25.The scattering of electrons by charged centres, optical phonons (polar and nonpolar interaction), acoustic phonons as well as disorder (alloy) scattering is taken into account. It is shown that the composition-dependent dielectric function and the band edge symmetry play an important role in the explanation of the experimental results.