Lattice dynamics,thermodynamic and elastic properties of CdGeAs2

Summary An extended Keating’s model with two-bond-stretching and one-angle-bending force constants within the rigid-ion approximation is applied to the investigation of the lattice dynamics of CdGeAs₂. Five model parameters of the theory are determined by a least-square fitting to the infrared reflectivity measurements. The phonon spectrum along two high-symmetry directions and the one-phonon density of states are calculated. The theoretical values of the elastic constants, the constant-volume specific heat and the Debye temperature show a reasonable agreement with the experimental data reported by other authors.     

Raman active lattice spectrum of single crystal 2,4,6-triphenyltriazine

The Raman spectra of single crystal 2,4,6-triphenyltriazine have been recorded in the lattice region at 300 and 30°K. The interpretation of the results was based on assuming that the molecular structure is D_3h (i.e. complete planarity) instead of the most probable non-planar conformation of C₁ symmetry. Assignment of the lattice vibrations was based on a rigid-body approximation. Considerable temperature effect was observed in the spectrum when the crystal was cooled to 30°K. This seemed to indicate large anisotropic thermal motions in the crystalline solid.     

EPR Study of Sc<Subscript>2</Subscript>SiO<Subscript>5</Subscript>:Nd<Superscript>143</Superscript> Isotopically Pure Impurity Crystals

The Sc₂SiO₅ single crystals doped with 0.001 at.% of the ¹⁴³Nd³⁺ ion were studied by continuous-wave and pulse electron paramagnetic resonance methods. The g-tensors and hyperfine structure tensors for two magnetically non-equivalent Nd ions were obtained. The spin–spin and spin–lattice relaxation times were measured at 9.82 GHz in the temperature range from 4 to 10 K. It was established that three relaxation processes contribute to the spin–lattice relaxation processes. There are one-phonon spin–phonon interaction, two-phonon Raman interaction and two-phonon Orbach–Aminov relaxation processes. It was established that spin–spin relaxation time is of the same magnitude for neodymium ion doped in Sc₂SiO₅ and in Y₂SiO₅.     

Surface reconstruction modes of Cu2O(001) surface: A first principles study

The stabilization mechanism of the polar, copper terminated Cu₂O(001) surface by means of complex surface reconstruction was studied theoretically with a combination of static and molecular dynamics calculations. The experimentally reported “3√2 × 1” surface structure was constructed and characterized for the first time. The combination of simulated annealing with molecular dynamics shows that Cu⁺–Cu⁺ dimers are formed in the first layer along the equivalent [011] and [01?1] directions at elevated temperature. There is a relaxation of the atoms that separates copper cations from nearest neighbor rows. Using the experimentally observed superstructure cell allows decoupling the symmetry equivalent dimers. The structural reconstructions were characterized by the electronic properties calculations. It is observed that the dimers are formed due to the d–d interaction of the copper atoms. Finally, the symmetry driven reconstructed structure was investigated by DFT STM. The simulated STM images show that copper atoms have higher density than oxygen atoms at the surface and produce the positive surface corrugation.     

Mode-coupling theory for the lattice dynamics of anharmonic crystals: self-consistent damping and the 1d Lennard-Jones chain

We present a self-consistent theory for the dynamical one-phonon structure factor in anharmonic crystals. The theory is the phonon analogue of the mode-coupling theory of liquid dynamics of Götze and his coworkers. Starting point is the lattice dynamics treatment based on the Mori- Zwanzig technique as formulated by Götze and Michel. We apply the theory to the one-dimensional (1d) Lennard-Jones chain and show that the nonlinear mode-coupling equations can be readily solved in the time domain. The vertices entering the equations as input are calculated exactly by a Monte Carlo technique. We compare our findings with molecular dynamics (MD) simulations and the results of other theoretical approaches.     

A first-principle study of the structural, elastic, lattice dynamical and thermodynamic properties of PrX (X=P, As)

The structural, phase transition, elastic, lattice dynamic and thermodynamic properties of rare-earth compounds PrP and PrAs with NaCl (B1), CsCl (B2), ZB (B3), WC (B_h) and CuAu (L1₀) structures are investigated using the first principles calculations within the generalized gradient approximation (GGA). For the total-energy calculation, we have used the projected augmented plane-wave (PAW) implementation of the Vienna Ab-initio Simulation Package (VASP). Specifically, some basic physical parameters, e.g. lattice constants, bulk modulus, elastic constants, shear modulus, Young's modulus and Poison's ratio, are predicted. The obtained equilibrium structure parameters are in excellent agreement with the experimental and theoretical data. The temperature and pressure variations of the volume, bulk modulus, thermal expansion coefficient, heat capacity and Debye temperature are calculated in wide pressure and temperature ranges. The phonon dispersion curves and corresponding one-phonon density of states (DOS) for both compounds are also computed in the NaCl (B1) structure.     

Event structure investigation of AuAu interactions from HIJING model using fractal dimensions

This work presents an implementation of fractal geometry methods in the study of event structure for AuAu interactions at collision energies √s _NN = 9.2, 62 and 200 GeV for different interaction dynamics. The events are generated by using the HIJING model. It is shown, that the fractal dimension of events in phase space projections rapidity-transverse momentum (y - p_t) and azimuthal angle-transverse momentum (φ - p_t) are sensitive to the interaction dynamics.     

A theory of magnetism for semi-metallic uranium compounds

A theoretical study is carried out to explain the magnetic properties of uranium compounds of NaCl, anti-Cu₂Sb and Th₃P₄ type. The electronic model used is a modified free electron model along the lines of the band structure for NaCl type uranium compounds previously reported. An exchange interaction via the conduction electrons is assumed for the mechanism of magnetic ordering. The molecular field approximation is used to calculate θ, T_N and the exchange interaction constants. The stability range of various magnetic ordering states and the variation of θ, T_N and exchange interaction constants are obtained as functions of lattice parameter. These results well explain the experimental data.     

Effect of NH3 adsorption on the atomic structure of Si(111) √3 × √3 - Al and Si(111) √3 × √3 - Ag surfaces

The room temperature (RT) adsorption of ammonia (NH₃) on Si(111)√3 × √3-Al and Si(111)√3 × √3-Ag surfaces has been studied using LEED and AES. The transformation from Si(111)√3 × √3-Al surface structure to Si(111)1 × 1-(Al, H) upon NH₃ exposure has been found to be similar to the previously observed structural transformation induced by exposure in the atomic hydrogen. It has been demonstrated that the transformation is caused by hydrogen atoms which are generated by NH₃ dissociation on the Si(111)√3 × √3-Al surface. It has been estimated that about 0.1 ML of ammonia molecules is needed to complete the structural transformation. No interaction of NH₃ with the Si(111)√3 × √3-Ag surface has been found. The dissociation of NH₃ molecules is believed to be impossible on this surface     

Rotational self correlation functions of N2O dissolved in liquid SF6

Careful study of the I.R. and Raman spectra of N₂O dissolved in inert solvents has shown that, for solutions in SF₆, purely rotational correlation functions can be unambiguously deduced. The comparison with theoretical models usually proposed to describe the rotational motions proved to be inadequate in that case. The reasons of this failure are analysed with the help of a molecular dynamics simulation of a system of 2 Lennard-Jones centres molecules, where the rotational correlation functions are found to be identical to the experimental ones.     

Temperature‐dependent EXAFS study of the local structure and lattice dynamics in cubic Y2O3

The local structure and lattice dynamics in cubic Y₂O₃ were studied at the Y K‐edge by X‐ray absorption spectroscopy in the temperature range from 300 to 1273 K. The temperature dependence of the extended X‐ray absorption fine structure was successfully interpreted using classical molecular dynamics and a novel reverse Monte Carlo method, coupled with the evolutionary algorithm. The obtained results allowed the temperature dependence of the yttria atomic structure to be followed up to ～6 Å and to validate two force‐field models.     

Ultrafast structural dynamics of perovskite superlattices

Femtosecond x-ray diffraction provides direct insight into the ultrafast reversible lattice dynamics of materials with a perovskite structure. Superlattice (SL) structures consisting of a sequence of nanometer-thick layer pairs allow for optically inducing a tailored stress profile that drives the lattice motions and for limiting the influence of strain propagation on the observed dynamics. We demonstrate this concept in a series of diffraction experiments with femtosecond time resolution, giving detailed information on the ultrafast lattice dynamics of ferroelectric and ferromagnetic superlattices. Anharmonically coupled lattice motions in a SrRuO₃/PbZr_0.2Ti_0.8O₃ (SRO/PZT) SL lead to a switch-off of the electric polarizations on a time scale of the order of 1 ps. Ultrafast magnetostriction of photoexcited SRO layers is demonstrated in a SRO/SrTiO₃ (STO) SL.     

Der Einfluß des elastischen Gitters auf den Orientierungsübergang von festem Ortho-Wasserstoff in Molekularfeldnäherung

We calculate the influence of the elastic lattice on the orientational order-disorder transition in fcc solid ortho-hydrogen. The angular momentaJ = 1 of the molecules are coupled by quadrupole-quadrupole interaction which we reduce within the space group Pa 3 to the Ising model approximation. Local mechanical equilibrium requires the lattice to adjust itself to the forces caused by the inhomogeneous fluctuations in the angular momentum configuration. Following Wagner [6] this results in a 4-angular-momentum interaction with long range and short range contributions depending on the one-phonon Green function of the lattice. We use a Debye phonon spectrum and the molecular field approximation in order to obtain numerical results. According to the first order nature of the phase transition we get a pressure drop at the critical pointT ^c of 9.7 bar. The experimental value is estimated to be 8 bar. Compared to the rigid lattice the phase transition on the elastic lattice is smoothed out by the short range contribution. This corresponds to a reduction of the discontinuity of the order parameter at the critical pointT ^c by about 13%. The phase transition will vanish completely if we increase the magnitude of the short range terms by a factor of three.     

Investigation of lattice dynamics by means of vibronic spectra

Electronic transitions at a foreign probe ion in a lattice exhibit vibrational sidebands. They may be used as a new tool to investigate lattice dynamics. To make this too effective the theory of vibronic spectra has been refined. The principal refinement is that of a partial Born-Oppenheimer approximation which establishes a rigid coupling of the electronic wavefunctions of the foreign ion to its own nucleus. The structural form of the one-phonon sideband is given as a projection of the classical Greens' function (imaginary part) onto single multipole-fields in lattice space. Employing a variety of electronic transitions and by careful group-theoretical analysis a complete irreducible sequence of projections can be extracted from the experiments. These functional forms depend sensibly on the eigenvectors of the lattice modes, whence they contain information, which cannot be received from neutron scattering. Moreover, the spectra related to the higher multipoles give insight into the disturbed (local) dynamics, whereas the lower type spectra concern the undisturbed (ideal) one. Systems, where the foreign probe ion has high symmetry, are most suitable for the new method. For systems of low local symmetry approximations are suggested by the low-frequency behaviour of the measured one-phonon band. Because of the high resolution of optical measurements, the information from vibronic spectra is comparable or even superior in detailed richness to neutron scattering data.     

Distinct correlation between CeO2 and YSZ in out-of-plane and in-plane mosaic dispersions of heteroepitaxial CeO2/YSZ/Si(001) films

The structural correlations including the lattice constants and the mosaic dispersions between CeO₂ and yttria-stabilized ZrO₂ (YSZ) in CeO₂/YSZ/Si(001) heteroepitaxial films have been investigated by out-of-plane and in-plane X-ray-diffraction techniques. The distinct linear correlations of the full width at half-maximum (FWHM) of the ω scan between CeO₂ and YSZ have been found in both directions. CeO₂ always has a 0.7° lower FWHM of the ω scan than YSZ in the out-of-plane direction, but has a 2.6° higher FWHM in the in-plane direction. A possible relationship between the out-of-plane and in-plane FWHMs of the ω scans has been demonstrated with a lattice-rotation model. Besides, the lattice constants of CeO₂ are dependent on the FWHMs of the YSZ ω scans: as the FWHM is below 3.5°, CeO₂ has a tetragonal distortion, and as the FWHM is higher than 3.5°, CeO₂ exhibits a cubic structure without distortion. The results are of great interest, both for the fundamental understanding of the film-growth mechanisms and for potential applications. Received: 11 September 2000 / Accepted: 5 June 2001 / Published online: 30 August 2001     

Scattering of low-energy atoms by metallic surfaces

The problem of a neutral low-energy atom impinging on a well-defined metallic surface is approached from first principles. The solid and its potential energy of interaction with the incident atom is treated in the most general way, but under the following assumptions: (a) the conduction electrons interact adiabatically with the lattice ions and the gas atom; (b) no chemical reactions occur; (c) the one-phonon approximation is valid. The scattering amplitudes for surface and bulk mode excitations are obtained in terms of the dynamical properties of the metallic surface. Direct collisions of the incident atoms with the lattice ions are shown to give a negligible contribution to the scattering. The most important contribution comes from the interaction of the gas atom with the surface conduction electrons; the excitation of lattice vibrational modes occurs through the electron-phonon term of the Hamiltonian. The general expressions for the scattering amplitudes obtained show that the scattering is incoherent. With further assumptions one obtains a separation of the scattering amplitude into a coherent and incoherent part.     

不同覆盖度下丙硫醇在Au(111)面吸附的理论研究

采用基于密度泛函理论的第一性原理方法研究了丙烷硫醇 (C₃H₇SH)在Au(111)面五种覆盖度(1/16, 2/16, 3/16, 4/16, 1/3) 下的未解离和解离吸附的结构、能量和吸附性质. 发现丙烷硫醇的倾斜角和吸附能均受覆盖度影响, 计算结果显示丙烷硫醇的倾斜角随着覆盖度的增大减小了6°–10°, 吸附能随覆盖度的增大减小了0.21 eV. 特别针对饱和覆盖度, 研究了三种可能的表面结构: (2√3×2√3 ight)R30°, 2√3×3和(3×3). 发现S–H键未解离时三种表面结构的吸附构型和吸附能基本一致; S–H键解离后, (2√3×2√3 ight)R30°和2√3×3结构的吸附能比以(3×3)结构的吸附能约高0.05–0.07 eV, 说明C₃H₇S在Au(111)面吸附时, 倾向于形成(2√3×2√3 ight)R30°和2√3×3结构. 此外, 采用DFT-D2方法对饱和覆盖度下C₃H₇SH分子在Au(111)面的吸附进行了范德华修正, 结果显示分子间相互作用使吸附物和Au表面的距离减小, 该相互作用对吸附能的修正值为0.53 eV, 修正后结果与实验结果接近. 关键词： 第一性原理 覆盖度 表面结构 范德华力     

Orientational order in CO and N2 monolayers on graphite studied by X-ray diffraction

X-ray diffraction has been used to study the structure and orientational phase transitions of CO and N₂ adsorbed on graphite (Papyex). Both form orientationally ordered 2√3 × 2√3 R30° commensurate phases on graphite at low temperatures (10 K). The in-plane herringbone structure of N₂ has been confirmed but CO has more orientational disorder than N₂, which may be associated either with tilting, random static or systematic, of the molecules away from the surface and/or with orientational order of shorter range than the centre of mass order. In the first case the average tilt would have to be about 26° and in the second case the orientational correlation length would have to be 200 Å compared with 450 Å for the translational order. The orientational phase transition is sharp for N₂, occurring over the range 27–30 K, in agreement with previous work. For CO the transition is broad and starts at lower temperatures. This and the structural data indicate that a point quadrupolar interaction is not a suitable model for comparing the properties of N₂ and CO layers. The orientational phase transition in the incommensurate phase of N₂ is found to be broad and occurs below 20 K. For CO it is sharper than for the commensurate phase and occurs at a higher temperature. The lattice parameter changes by 0.75% across the orientational phase transition. For both N₂ and CO there is evidence of translational disorder in the commensurate phases but it cannot be interpreted quantitatively.     

A leed-aes study of thin Pd films on Si(111) and (100) substrates

A system Pd (deposit)-Si (substrate) has been studied by LEED and AES. Pd₂Si formed on Si(111) became epitaxial after a short time of annealing at a temperature between 300 and 700°C, while the Pd₂Si formed on Si(100) did not, in both cases the surfaces of the Pd₂Si being covered with a very thin Si layer. A sequence of superstructures (3√3 × 3√3), (1 × 1), and (2√3 × 2√3) was observed successively in Pd/Si(111) as the annealing temperature was increased. A (√3 × √3) structure was obtained by sputtering the 3√3 surface slightly. It was found that the √3 structure corresponds to Pd²Si(0001)-(1 × 1) grown epitaxially on Si(111), and that the 3√3 structure comes from the thin Si layer accumulated over the silicide surface, while the 2√3 and 1 structures arise from a submonolayer of Pd adsorbed on Si(111). Superstructures observed on a Pd/Si(100) system are also studied.     

Anharmonic effects of periodic molecular systems in the vibron model approximation

The vibron model approximation to take into account anharmonic effects in periodic systems is discussed. This is achieved by considering a simple one-dimensional molecular crystal with four degrees of freedom. In this case the lattice dynamical treatment is separated into two sets of modes, the modes associated to the molecular degrees of freedom and those corresponding to the motions of the centre of mass of the molecules. The non-interacting molecular modes are studied in detail, and analytical expressions for the energies and wave functions for the two-phonon manifold are obtained. A local-normal mode transition as a function of the interaction parameters is identified, similar to that in isolated molecular systems.