Inelastic neutron scattering study of lattice dynamics in α-ZnCl<Subscript>2</Subscript>

Inelastic neutron scattering experiments have been carried out to measure the phonon density of states in polycrystalline α-ZnCl₂ at Dhruva, Trombay. Lattice dynamical calculations, based on an interatomic potential model, are accomplished to study phonons associated with this otherwise extremely hygroscopic compound. Our calculated data are found to be well-compatible with the available measured ones.     

Lattice dynamics of strontium tungstate

We report here measurements of the phonon density of states and the lattice dynamics calculations of strontium tungstate (SrWO₄). At ambient conditions this compound crystallizes to a body-centred tetragonal unit cell (space group I4₁/a) called scheelite structure. We have developed transferable interatomic potentials to study the lattice dynamics of this class of compounds. The model parameters have been fitted with respect to the experimentally available Raman and infra-red frequencies and the equilibrium unit cell parameters. Inelastic neutron scattering measurements have been carried out in the triple-axis spectrometer at Dhruva reactor. The measured phonon density of states is in good agreement with the theoretical calculations, thus validating the interatomic potential developed.      

Phonon density of states of tetracyanoethylene from coherent inelastic neutron scattering at Dhruva reactor

Inelastic neutron scattering experiments to determine phonon density of states of coherent scattering samples of polycrystalline complex solids are generally intensity-limited and therefore are feasible only at high flux facilities. Phonon density of states of the monoclinic phase of tetracyanoethylene at 300 K, obtained using the medium resolution triple axis spectrometer at the new Indian medium flux reactor Dhruva are reported here. The raw data is converted to the “neutron weighted” phonon density of states by applying suitable corrections. Comparison made with results from a theoretical calculation based on a semirigid molecule model of lattice dynamics is fair. Results from Dhruva are also consistent with that obtained (to be published) at the high flux pulsed neutron source (ISIS) of the Rutherford Appleton Laboratory in United Kingdom.     

Lattice dynamics of beryllium

Phonon frequencies in beryllium along the principal symmetry directions have been determined by means of the slow neutron inelastic scattering technique. The data are analysed in terms of a six-neighbour force constant model and the force constants are evaluated. It is concluded that strong tensor forces are present in beryllium and the importance of this finding to basic theories of lattice dynamics is pointed out.     

Negative thermal expansion in framework compounds

We have studied negative thermal expansion (NTE) compounds with chemical compositions of NX₂O₈ and NX₂O₇ (N=Zr, Hf and X=W, Mo, V) and M₂O (M=Cu, Ag) using the techniques of inelastic neutron scattering and lattice dynamics. There is a large variation in the negative thermal expansion coefficients of these compounds. The inelastic neutron scattering experiments have been carried out using polycrystalline and single crystal samples at ambient pressure as well as at high pressures. Experimental data are useful to confirm the predictions made from our lattice dynamical calculations as well as to check the quality of the interatomic potentials developed by us. We have been able to successfully model the NTE behaviour of these compounds. Our studies show that unusual phonon softening of low energy modes is able to account for NTE in these compounds.      

Inelastic neutron scattering and lattice dynamics of GaPO<Subscript>4</Subscript>

We report here measurements of phonon spectrum and lattice dynamical calculations for GaPO₄. The measurements in low-cristobalite phase of GaPO₄ are carried out using high-resolution medium-energy chopper spectrometer at ANL, USA in the energy transfer range 0–160 meV. Semiempirical interatomic potential in GaPO₄, previously determined using ab-initio calculations have been widely used in studying the phase transitions among various polymorphs. The calculated phonon spectrum using the available potential show fair agreement with the experimental data. However, the agreement between the two is improved by including the polarisability of the oxygen atoms in the framework of the shell model. The lattice dynamical models are also exploited for calculations of various thermodynamic properties of GaPO₄.     

Nonlinear radiation response of n-doped indium antimonide and indium arsenide in intense terahertz field

The nonlinear radiation responses of two different n-doped bulk semiconductors: indium antimonide(In Sb) and indium arsenide(In As) in an intense terahertz(THz) field are studied by using the method of ensemble Monte Carlo(EMC)at room temperature. The results show that the radiations of two materials generate about 2-THz periodic regular spectrum distributions under a high field of 100 k V/cm at 1-THz center frequency. The center frequencies are enhanced to about 7 THz in In Sb, and only 5 THz in In As, respectively. The electron valley occupancy and the percentage of new electrons excited by impact ionization are also calculated. We find that the band nonparabolicity and impact ionization promote the generation of nonlinear high frequency radiation, while intervalley scattering has the opposite effect. Moreover, the impact ionization dominates in In Sb, while impact ionization and intervalley scattering work together in In As. These characteristics have potential applications in up-convension of THz wave and THz nonlinear frequency multiplication field.     

NEUTRON SCATTERING AND LATTICE DYNAMICAL STUDIES OF THE HIGH-PRESSURE PHASE ICE (I)

Lattice dynamical calculations of ice VIII have been carried out by using a slightly modified set of force constants obtained recently for ice Ih (Li J C and Ross D K 1993 Nature 365 327). A weak interaction was introduced between the two interpenetrated sublattices in the ice VIII structure. The calculated results for H₂O and D₂O ice VIII are in reasonable agreement with the measured inelastic neutron scattering spectra. The eigenvectors of phonon modes in the range of translational and librational bands have been studied in order to understand the properties of the vibrational modes. It is found that the third peak at 26.7meV in the translation results from weak hydrogen bond interactions, and the first peak (14.7meV) is much higher than it is in ice Ih (～7.1meV), which is partially due to the interactions between the two sublattices.     

Anisotropic Hubbard model on a triangular lattice — spin dynamics in HoMnO3

The recent neutron scattering data for spin-wave dispersion in HoMnO₃ are well-described by an anisotropic Hubbard model on a triangular lattice with a planar (XY) spin anisotropy. Best fit indicates that magnetic excitations in HoMnO₃ correspond to the strong-coupling limit U/t >∼ 15, with planar exchange energy J = 4t ² /U ≃ 2.5 meV and planar anisotropy ΔU ≃ 0.35 meV.      

Phonon Band Filling and Photon Emission by Phonons

Phonon generation by electrons is supplied in n-type Si crystals in electric fields E100 kV/cm at the lattice temperature of 80 K employing the ensemble Monte Carlo technique. Electron transfer between equivalent energy valleys is accounted for the g-type- and f-type phonon absorption and emission. Acoustic phonons are accounted for the quasi-elastic scattering of electrons within the energy valleys. Excess phonon number is determined using numerical data on phonon generation rate and experimental values of phonon lifetimes. The feasibility of stimulated emission of infrared-range photons due to direct optical transitions between the phonon bands is discussed.     

NEUTRON SCATTERING AND LATTICE DYNAMICAL STUDIES OF THE HIGH-PRESSURE PHASE ICE (II)

Lattice dynamical calculations have been carried out for ice II based on the force field constructed for ice Ih. In order to fully understand ice II inelastic neutron scattering spectra, the decomposed phonon density of states was shown mode by mode. Calculated results have shown that the hydrogen bond force constant between the six-molecule rings is significantly weaker, 75eV/nm², compared with the force constant, 220eV/nm², within the rings. Inelastic neutron scattering spectra of clathrate hydrate H₂O+He are almost the same as ice II. This means that the absorption of He atoms cannot affect the bond strengths of the ice II host lattice. Based on the force field model for ice II, the van der Waals interactions between water molecules and helium atoms are considered. The results obtained are consistent with experimental data. Lattice dynamical calculations have been carried out for ice II using seven rigid pairwise potentials. It was found that MCY makes the stretching and bending interactions in ice II too weak and makes the O-O bond length too long (～5%), thus its lattice densities are obviously lower than other potential lattices or experimental values.     

Effect of nanocrystallinity on lattice dynamics in Bi2Te3 based thermoelectrics

The lattice dynamics in as‐cast and nanocrystalline thermoelectric Bi₂Te₃ based p‐type and n‐type material were investigated using inelastic neutron scattering. Generalized densities of phonon states show substantial agreement between the lattice dynamics in as‐cast samples and previous studies. The lattice dynamics in the nanocrystalline materials differ significantly from its as‐cast counterparts in the acoustic phonon regime. In nanocrystalline p‐type and n‐type compounds, the average acoustic phonon group velocity was found to be reduced to 80(5)% and 95(2)% of the value in as‐cast material. It is argued that point‐defect and strain contrast scattering may play an important role for the understanding of lattice thermal conductivity in (nanocrystalline) Bi₂Te₃ based thermoelectrics beside the observed decrease of sound velocity. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Phonon spectrum and related thermodynamic properties of microcrack in bcc-Fe

The phonon spectrum and the related thermodynamic properties of microcracks in bcc-Fe are studied with the recursion method by using the Finnis--Sinclair (F--S) N-body potential. The initial configuration of the microcracks is established from an anisotropic linear elastic solution and relaxed to an equilibrium by molecular dynamics method. It is shown that the local vibrational density of states of the atoms near a crack tip is considerably different from the bulk phonon spectrum, which is closely associated with the local stress field around the crack tip; meanwhile, the local vibrational energies of atoms near the crack tip are higher than those of atoms in a perfect crystal. These results imply that the crack tip zone is in a complex stress state and closely related to the structure evolution of cracks. It is also found that the phonon excitation is a kind of local effect induced by microcracks. In addition, the microcrack system has a higher vibrational entropy, which reflects the character of phonon spectrum related to the stress field induced by cracks.     

DECAY PROCESSES OF HIGH-FREQUENCY NONEQUILIBRIUM PHONONS IN PRASEODYMIUM-DOPED LASER CRYSTALS

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Lattice dynamic investigation of the Raman and infrared wavenumbers of orthorhombic R2BaNiO5 (R = Y,Gd) oxides in Immm structure

A short‐range force constant model has been applied to investigate the Raman and the infrared wavenumbers in R₂BaNiO₅ (R = Y, Gd) in their orthorhombic phase of space group Immm. Calculations of zone‐center phonons are made with seven stretching and four bending force constants. The force constants are evaluated by fitting nine Raman and two infrared modes. Two Raman modes are reassigned on the basis of group theoretical calculations. The calculated Raman and infrared modes show good agreement with the observed values. The infrared values are assigned for the first time in these oxides. Copyright © 2006 John Wiley & Sons, Ltd.     

Lattice dynamics of tetragonal R2BaCuO5 (R = La,Nd) oxides in the P4/mbm structure

A short‐range force constant model (SRFCM) has been applied to investigate the Raman and the infrared wavenumbers in R₂BaCuO₅ (R = La, Nd) in their tetragonal phase of space group P4/mbm. Calculations of zone‐center phonons are made with six stretching and five bending force constants. All the Raman and infrared values are then assigned to their corresponding modes. The force constants were evaluated by fitting ten Raman and eight infrared modes. The calculated Raman and infrared modes show good agreement with the observed values. Copyright © 2007 John Wiley & Sons, Ltd.     

Lattice dynamics of tetragonal Nd2BaZnO5

A short‐range force constant model (SRFCM) has been applied to investigate the Raman and the infrared wavenumbers in Nd₂BaZnO₅ in its tetragonal phase of space group I4/mcm. Calculations of zone‐center phonons are made with five stretching and five bending force constants. All the Raman and infrared values are then assigned to their corresponding modes. Two Raman modes are reassigned on the basis of group theoretical calculations. The calculated Raman wavenumbers exhibit good agreement with the observed values. Copyright © 2007 John Wiley & Sons, Ltd.     

Raman investigation of polymorphism in 1,1,4,4‐tetraphenyl‐butadiene

We present a Raman investigation of polymorphism in 1,1,4,4‐tetraphenyl‐butadiene (TPB), a well‐known blue luminescent molecule, which retains its emissive properties in the solid state. The use of low‐wavenumber (10–150 cm⁻¹) Raman microscopy, very sensitive to the crystal packing, allows us to single and pick up four different polymorphs. X‐ray analysis shows that the TPB molecules assume different conformations in the various polymorphs, a fact revealed also by the Raman spectra in the region of intra‐molecular vibrations. Lattice dynamics calculations yield a preliminary assignment of the low‐wavenumber Raman spectra and provide information of the relative stability of the phases. Copyright © 2013 John Wiley & Sons, Ltd.     

Intrinsic bulk vortex dynamics revealed by time resolved small angle neutron scattering

Measurements of intrinsic vortex lattice (VL) dynamics in superconductors as for instance VL melting or Bragg glass transitions are typically performed by e.g. macroscopic transport or surface sensitive measurement techniques. Therefore, usually thin superconducting films are used for microscopic measurements of VL dynamics. A direct consequence of using thin films is the strong influence of surface effects and defects, sample quality and geometry. We succeeded to combine time resolved stroboscopic small angle neutron scattering (SANS) with an advanced, time varying magnetic field setup allowing to extend the time window for slow dynamical processes to the range of 10 ms up to several minutes. The new results demonstrate that it is possible to observe directly the intrinsic dynamics of the VL in a bulk niobium single crystal on a microscopic scale without limitations due to surface effects. Field and temperature dependent relaxation times of the VL from 100 to 500 ms could be observed for the first time, allowing to directly measure the VL of the tilt modulus. This new experimental technique provides the possibility to study also the dynamical magnetic properties of various strongly correlated electron systems.     

Electron scattering with spin flip in indium antimonide and indium arsenide

The longitudinal magnetoresistance has been investigated at temperatures in the range from 2.8 to 200 K in a magnetic field of up to 200 kOe with the aim of determining the temperature range and the magnetic field strength at which charge carrier scattering with spin flip occurs in n-type indium arsenide and n-type indium antimonide. It is established that quantum oscillations of the longitudinal magnetoresistance of indium arsenide exhibit weak zero maxima due to electron scattering with spin flip at temperatures in the range from 4 to 35 K in a magnetic field of 146 kOe. For the longitudinal magnetoresistance of indium antimonide, zero maxima caused by electron scattering with spin flip are revealed in the temperature range from 60 to 80 K in a magnetic field of 132 kOe.