Raman scattering in InN films and nanostructures

Studies of lattice dynamics devoted to wurtzite InN are presented. Raman scattering experiments on both InN thin films and nanometric islands grown by Metal–Organic Vapor Phase Epitaxy (MOVPE) were performed at room temperature. From the Raman spectra recorded from InN films under hydrostatic pressure up to 13 GPa, linear pressure coefficients and the corresponding Grüneisen parameters for both E₂ and A₁(LO) phonons were extracted for the wurtzite structure up to 11 GPa, close to the starting pressure of the hexagonal to rock-salt phase transition of InN. Spectra at higher pressure suggest that InN undergoes a gradual phase transition, and the reverse transition exhibits a strong hysteresis effect during the downstroke. Then, we discuss recent results on large single InN islands grown on GaN buffer layers, obtained by spatially resolved micro-Raman measurements. The magnitude of the residual strain is estimated, using a recent determination of phonon deformation potentials. It is found to vary linearly as a function of island height.     

The Tersoff potential for phonons in GaAs

We report on the use of the Tersoff empirical potential applied to GaAs. Cohesive and elastic properties, as well as phonon frequencies and Grüneisen parameters at the Γ and X points, are calculated and compared to experimental data. The limitations of the potential and its existing parametrisations are discussed.     

Phase transition, elastic, thermodynamic properties of zinc-blend BeSe from first-principles

The structural phase transition, elastic, thermodynamics properties of BeSe in zinc-blende were investigated by performing first-principles calculations within generalized gradient approximation. The phase transition pressure P_t between the B3 phase and the B8 phase of BeSe was determined. The pressure dependencies of elastic constants, shear modulus, Young's modulus, and Poisson's ratio of BeSe are calculated. The thermodynamic properties of the zinc-blende structure BeSe are calculated by using the quasi-harmonic Debye model. The pressure and temperature dependencies of the heat capacity and the thermal expansion coefficient, as well as the Grüneisen parameter are investigated systematically in the ranges of 0–50 GPa and 0–1200 K.     

The influence of thes-f interaction on the conduction band behaviour in ferromagnetic semiconductors. A theory of the red shift effect

The mechanism of the thermomechanical instability of an elastically slowly strained solid is explored, by a normal mode analysis of temperature and strain fluctuations. The strain dependence of the Grüneisen parameter is found to play a major role, since it controls the rates of damping of the lowest wavevector acoustic excitations pumped by the imposed strain. For -titanium the instability is found to occur at a critical strain comparable with that of the elastic limit. The instability can be envisaged in terms of a fast growing deformation, presumably associated with an acoustic emission.Gruppo Nazionale di Struttura della Materia del Consiglio Nazionale delle Ricerche, UR7     

Analysis of volume dependence of Grüneisen ratio

Various models on volume dependence of the Grüneisen ratio have been analyzed in the present study. The Sharma model [Mod. Phys. Lett. B 22/31 (2008) 3113] is found to be similar to that used by Nie [Phys. Stat. Sol. (b) 219 (2004) 241] on the basis of approximation made by Jeanloz [J. Geophys. Res. 94 (1989) 5929]. The Nie expression is amended in a manner so that the resulted expression follows the constraint of high pressure thermodynamics in the limit of infinite pressure. The newly developed relationship is applied successfully on materials for which experimental data are accessible such as epsilon-iron, NaCl, Li, Na, and K.     

Temperature dependence of elastic constants for ionic solids

In this paper the thermal variation of volume for NaCl, KCl, MgO and CaO has been investigated on the basis of Anderson model. We have evaluated the values of elastic constants C₁₁, C₄₄ and K_T at different temperature on the basis of Murnaghan and Tallon models. Tallon's model with second approximation presents slightly better agreement with experimental results which shows that the Anderson–Grüneisen parameter is directly proportional to the volume ratio. Tallon's model can be used to evaluate the values of elastic constants for solids at different temperatures.     

Mode grüneisen parameters for long-wave phonons under pressure of Si and Ge

The elastic stiffness constants under pressure of Si and Ge are studied theoretically using the higher-order perturbational formalism and the local Heine-Abarenkov model pseudopotential, and the mode Grüneisen parameters for long-wave phonons are estimated as a function of the compressed volume. The results obtained for atmospheric pressure are in good agreement with the experimental data. The long-wave LA mode Grüneisen parameters decrease slightly as the volume is reduced by compression. The TA mode Grüneisen parameters show a large decrease as function of the volume strain, and become negative near the covalent-metallic phase transition. The volume-dependences of the long-wave mode Grüneisen parameters are useful in the study of the anharmonic properties of Si and Ge.     

On the determination of the volume dependence of Grüneisen parameters in cubic and non-cubic solids

For a material of orthorhombic symmetry under hydrostatic stress, three generalized Grüneisen parameters relative respectively to the three principal directions of the deformation can be defined. Finite strain expressions for these parameters are derived in terms of the Lagrangian strain tensor and the frame-indifferent analogue of the Eulerian strain tensor. The expressions require for their evaluation the thermal expansion coefficients, the elastic moduli and their pressure and temperature derivatives, and the specific heat of the material, so that there are no arbitrary constants or “curve fitting.” In the case of cubic materials, it is possible to determine the unique Grüneisen parameter as a function of volume directly. For non-cubic materials, the volume dependence of the Grüneisen parameters is calculated using a quasi-harmonic finite strain model of equation of state. Numerical applications are given for some cubic and hexagonal metals and the results are discussed.     

二维Mo2C的晶格热导率的第一性原理研究

Mo2C是构建Mxene基器件的重要材料之一,对Mo2C二维材料声子输运的理解非常必要。文章结合第一性原理方法和声子玻尔兹曼输运方程,研究了二维Mo2C材料的晶格热导率。研究表明,室温下二维Mo2C导热系数非常低,其锯齿方向和扶手椅方向的晶格热导率分别为7.20 和 5.04 W/mK。计算了声学振动和光学振动模式对晶格热导率的贡献,揭示总热导率主要由面内声学横波的振动模式所贡献。还进一步计算了声子群速度、声子弛豫时间、三声子散射空间和模式格林艾森参数,发现二维Mo2C中的声子群速度和声子弛豫时间对晶格传输有重要的影响。     

Phonon dispersion of zinc chalcogenides under extreme pressure and the metallic transformation

The effect of high pressure on the one- and two-phonon Raman spectra of ZnTe, ZnSe, and ZnS has been measured up to their metallic phase transitions, at 95, 137, and 150 kbar respectively. Zone boundary transverse acoustic phonons “soften” with pressure in each case. For ZnTe, ZnSe, ZnS, Si, and GaP a linear correlation is found between the mode Grüneisen parameters for TA(X) and the transition pressures.     

Thermal expansion behavior of silicon at low temperatures

Lattice parameters, thermal expansion coefficients and Grüneisen parameters of silicon are determined by an X-Ray diffraction method in the temperature range of 180–40 K without the use of liquid gases. Thermal expansion of silicon becomes negative below 120 K which is discussed in terms of its lattice vibrations and crystal structure.     

Analysis of thermodynamic identities for materials under extreme compression

Some basic relationships for materials under extreme compression are analyzed with the help of the calculus of indeterminates. The analysis presented here provides an understanding of the origin of identities and constraints at infinite pressure which are satisfied by all physically acceptable equations of state. These identities involve the bulk modulus and its pressure derivatives, the Grüneisen parameter and its volume derivatives, the thermal expansivity, and the Anderson-Grüneisen parameter. The identity for the third-order Grüneisen parameter in terms of the pressure derivatives of the bulk modulus at extreme compression is valid even if the free-volume parameter changes with pressure.     

Energy losses of 2D electron gas due to near-surface acoustic phonon scattering

We have shown that for quantum wells placed close to the stress-free surface of the semiconductor heterostructure, the energy relaxation rate of two-dimensional electrons interacting with acoustic phonons at low temperatures (Bloch–Grüneisen regime) is changed considerably in comparison with that of a two-dimensional electron gas placed in a bulk of semiconductor. The relaxation rate is enhanced in the case of a semiconductor–vacuum system and is suppressed in the case of the surface covered by a thin metal film. The enhanced energy loss is caused by additional scattering at localized and reflected acoustic waves, and the decrease appears due to suppression of piezoelectric scattering in the vicinity of the metal.     

Equation of state and thermodynamics of fcc transition metals: A pseudopotential approach

A simple pseudopotential model is used for the calculation of the phonon spectra at the equilibrium volume and under pressure. The model is based on the secondorder perturbation theory with the local pseudopotential acting on thes electrons while thed electrons contribution is simulated by the repulsive Born-Mayer interatomic potential. Pressure influence on the lattice properties was studied for small compressions (mode Grüneisen parameters) as well as for ultrahigh pressure (equation of state up to 1 TPa). Results of the lattice dynamics calculations were used for determining temperature dependence of the lattice heat capacity and of the macroscopic Grüneisen parameter. The Kohn anomaly at the small wave vectors obtained previously in palladium, platinum and rhodium affects strongly the temperature dependence at low temperature.     

Effect of diffusion layers and defect layer on acoustic phonons transport through the structure consisting of different films

By using the continuum elastic approximation model and the transfer matrix method, we investigate the effect of diffusion layers and defect layer on acoustic phonons transport through the structure consisting of different films. Our work show that most acoustic phonons can easily pass the structure, but some only have much less transmission probabilities and form corresponding dips in the transmission spectrum. With the change of the structure parameters such as the width of diffusion layers and defect layer, the number of unit cell and the density of containing Al in diffusion layers and defect layer, the magnitude of the frequencies of acoustic phonons corresponding to the dips almost remain unchanged, but the transmission coefficients corresponding to the dips change at different degree, and the transmission probabilities of some frequencies are very sensitive to the variation of the above-mentioned structure parameters. These results can provide some references in controlling the transmission coefficients of acoustic phonons, devising parts of acoustic apparatus and theoretical investigation related.     

Vibrational and thermodynamic properties of metals from a model embedded-atom potential

This work provides the first systematic test of validity of the embedded-atom potentials of Mei et al. [Phys. Rev. B 43 (1991) 4653], via a complete study of the vibrational and thermodynamic properties of isoelectronic transition (Ni, Pd, Pt) and noble (Cu, Ag, Au) metals. Phonon dispersion curves and thermal properties are studied within the quasiharmonic approximation. Results for the temperature-dependence of the lattice constants, coefficients of linear thermal expansion, isothermal and adiabatic bulk moduli, heat capacities at constant volume and constant pressure, Debye temperatures and Grüneisen parameters are presented. Electronic contribution to the specific heat is included explicitly via density-functional calculation. The calculated phonon frequencies for Ag and Cu agree well with the results from inelastic neutron scattering experiments. Despite less satisfactory agreement between calculated and measured phonon frequencies for the other four metals, isothermal and adiabatic bulk moduli and the specific heats of all metals are reproduced reasonably well by the model, while the Grüneisen parameter and Debye temperature are underestimated by about 10%. The coefficient of linear thermal expansion is underestimated with respect to measured values in most cases except for Pt and Au. The results are good for Pt up to 1000 K and for Au up to 500 K.     

Second order and third order Grüneisen parameters at extreme compression

On the basis of the free volume theory of Grüneisen parameter (γ) and using the calculus of indeterminates, it is found that the second order Grüneisen parameter (q) and the second pressure derivative of bulk modulus (KK″) change in a similar manner in the limit of extreme compression. The ratio of q and KK″ becomes finite at infinite pressure. This finding has been used further to obtain a relationship for the third order Grüneisen parameter λ in terms of pressure derivatives of bulk modulus up to the third order. The results are found to be consistent with the identities obtained recently by Shanker et al. [14] using the free volume theory.     

Thermodynamic characteristics of the alkali metals (particularly sodium) in the quasiharmonic approximation

The Grüneisen parameters are calculated for several alkali metals. The dependences of the Grüneisen, Slater, and Dugdale-MacDonald versions of this parameter on the relative compression (or extension) of the lattice and on the temperature are derived. A comparison shows that the theoretical isotherms for the sodium equation of state agree satisfactorily with experiment.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 11, pp. 49–52, November, 1969.     

Pressure and volume dependence of Grüneisen parameter of solids

Using the assumption δ_T=δ_T0(V/V₀)^k and the Grüneisen parameter γ macroscopic definition expression, we obtained a relationship for the volume dependence of the Grüneisen parameter γ. We have calculated the Grüneisen parameter γ with this relationship for NaCl and ε-Fe at high pressure under study. The calculated values of γ are found to show fairly in good agreement with available experimental data.