Coherent lattice dynamics of highly photo-excited tellurium

Received: 16 May 1997/Accepted: 16 May 1997     

BaVS3晶格动力学研究

采用基于密度泛函理论的平面波赝势方法,在局域密度近似下采用线性响应的密度泛函微扰理论研究了具有六角结构的BaVS3化合物的品格动力学性质,得到了整个声子谱.计算得到的г点拉曼频率和实验数据进行了比较,其中E22g模、A1g模和实验测量值符合得比较好.对于E32g模,采用线性响应计算的结果与实验值差别较大.对该模应用冻结声子方法研究后认为差异主要是由于E32g模的较强的非谐性引起的.此外V原子在平面内的振动模E12g出现了虚频.虚频的出现预示着六角相的BaVS3结构的不稳定性,从而很好地解释了该材料由六角相到正交相的结构相变.     

The lattice dynamics of an anharmonic crystal

The theory of the physical properties of an anharmonic crystal is discussed by using the thermodynamic Green's functions for the phonons. A perturbation procedure is developed to obtain the Green's functions and it is shown that for some purposes a quasi-harmonic approximation is useful, in which the frequencies of the normal modes are those determined by infra-red or neutron spectrometry. The thermodynamic, elastic, dielectric and scattering properties of an anharmonic crystal are discussed in terms of the Green's functions, and detailed expressions are given for the more important contributions. Detailed numerical calculations are presented of the thermal expansion, dielectric properties and shapes of some of the inelastically scattered neutron groups, for sodium iodide and potassium bromide. The calculations, which give reasonable agreement with experiment, show that even at quite low temperatures, the lifetimes of some of the normal modes can be quite short. By using the quasi-harmonic approximation it is shown that the large temperature dependence of the normal modes in a ferroelectric crystal can be treated adequately.     

The rigid molecule approximation in lattice dynamics

The theory of the lattice dynamics of molecular crystals with and without the rigid molecule approximation is developed and the two compared. To make the two compatible, the equilibrium conditions for the internal molecular dimensions are replaced in the rigid molecule approximation by equations representing the constrained constant values of the molecular dimensions expanded in vibrational coordinates.     

Continuum limit of lattice dynamics

A scheme of construction of the time evolution in QFT in algebraic framework is presented and verified for a linear theory in the flat space-time. The tool used is the lattice approximation.     

The lattice dynamics of some type-I and type-II alloys

Summary  The lattice dynamical calculations are performed on some type-I and type-II alloys by using the third-neighbor Clark-Gazis-Wallis (CGW) model. The theory is used to compute the phonon dispersion curves, the frequency spectra and the lattice specific heat of the studied alloys. The obtained results are in good agreement with the experimental findings, and better than those obtained using other models.     

The lattice dynamics of SrCl2

Phonon dispersion curves of Sr Cl₂ have been measured at room temperature in the [001], [110] and [111] directions using inelastic neutron scattering. The results are interpreted by means of a polarizable ion model.     

The lattice dynamics of CsCl,CsBr and CsI

Calculated dispersion curves and frequency spectra are presented for CsCl, CsBr and CsI. These calculations have been made using the deformation dipole model including both first and second neighbor non-coulomb forces. The best fit to the experimental results is provided by a simple five parameter first neighbor model.     

BaVS3晶格动力学研究

采用基于密度泛函理论的平面波赝势方法,在局域密度近似下采用线性响应的密度泛函微扰理论研究了具有六角结构的BaVS₃化合物的晶格动力学性质,得到了整个声子谱.计算得到的Γ点拉曼频率和实验数据进行了比较,其中E²_2g模、A_1g模和实验测量值符合得比较好.对于E³_2g模,采用线性响应计算的结果与实验值差别较大.对该模应用冻结声子方法研究后认为差异主要是由于E³_2g模的较强的非谐性引起的.此外V原子在平面内的振动模E¹_2u出现了虚频.虚频的出现预示着六角相的BaVS₃结构的不稳定性,从而很好地解释了该材料由六角相到正交相的结构相变. 关键词： 晶格动力学 密度泛函 赝势方法 3')" href="#">BaVS₃     

Nonlinear dynamics of a two-dimensional lattice

The dynamics of the nonlinear excitations in a two-dimensional (2D) φ⁴-diatomic lattice, with nonlinear on-site electron-phonon coupling at the polarizable ion site has been presented, without considering the self consistent phonon approximation. One of the major results obtained from our calculations is in the understanding of continuous structural phase transition, where we have obtained the minimum in soft mode frequency at a soft mode temperatureT _s (>T _c), not at critical temperatureT _c. This occurs due to the anisotropy of such 2D systems.     

Surface lattice dynamics of potassium halides

The Green function method for the surface dynamics of ionic crystals, developed in a previous paper, is applied to the (001) surface of potassium halides. An interesting feature of these crystals is the existence of a surface sagittal optical mode, originated by the bulk LO band, which is is always below the Lucas modes. It is also found that the Rayleigh wave frequency at the M? point depends strongly on the ionic polarizabilities. The condition for the occurrence of surface instability induced by a change of ionic polarizabilities is briefly discussed.     

On phenomenological models of lattice dynamics

The metallic interaction is split into (i) closed-shell part and (ii) ion- electron-ion part. The former is evaluated using non-central forces upto the third neighbour and the latter is obtained from the concept of bulk modulus of the electron gas. Phonon dispersion relations of h.c.p. scandium have been calculated and compared with the experimental results.