A search for high frequency vibrational modes at a stepped surface

In this paper we present a theory of the vibrations of atoms in the vicinity of a stepped surface on a Bravais crystal. The static relaxations in the positions of the atoms in the crystal are determined, and the atomic force constants are then calculated in the relaxed atomic configuration. The general theory is applied to a simple stepped surface, and the local phonon density of states is carried out for atoms at several points on the stepped surface by the real space continued fraction recursion method. No evidence is found for high frequency surface phonons.     

Simple calculation of the mean-square displacements of volume, (110) and (100) surface atoms of chromium

We use an analytic method based on the expansion of the dynamical matrix to determine the thermal dependence of the mean-square displacements 〈u²〉 of volume, and (100) and (110) surface atoms of Cr. For the volume and (110) surface the results are in satisfactory agreement with previous X-ray and low-energy electron diffraction data. In particular, the zero-point thermal motion of Cr-volume atoms is approximately reached at temperatures as high as 170°K in accordance with experiments.A discussion about the temperature at which the atomic vibrations are strongly frozen is carried out for b.c.c. crystals.     

非简谐振动对SiC的热膨胀系数数和介电性能的影响

利用抛物型电子能谱模型,考虑到原子的非简谐振动,求出了SiC中原子振动的简谐系数与非简谐系数,用固体物理理论和方法,得到了SiC的热膨胀系数和格林乃森参量以及介电常数随温度变化的解析式,探讨了原子非简谐振动对的影响。结果表明：的格林乃森参量和热膨胀系数均随温度升高而非线性增大,而介电常数随温度升高而非线性减小;原子振动的非简谐项（特别是第二非简谐项）对的热膨胀等热学性质和介电性能有重要影响,温度愈高,非简谐振动项的影响愈大。     

Predicting macroscopic thermal expansion of metastable liquid metals with only one thousand atoms

Results of thermal expansion prediction from atomic scale for metastable liquid metals are reported herein. Three pure liquid metals Ni, Fe, and Cu together with ternary Ni₆₀Fe₂₀Cu₂₀ alloy are used as models. The pair distribution functions were employed to monitor the atomic structure. This indicates that the simulated systems are ordered in atomic short range and disordered in long range. The thermal expansion coefficient was computed as functions of temperature and atom cutoff radius, which tends to maintain a constant when the cutoff radius increases to approximately 15 Å. In such a case, slightly more than 1000 atoms are required for liquid Ni, Cu, Fe and Ni₆₀Fe₂₀Cu₂₀ alloy, that is, the macroscopic thermal expansion can be predicted from the volume change of such a tiny cell. Furthermore, the expansion behaviors of the three types of atoms in liquid Ni₆₀Fe₂₀Cu₂₀ alloy are revealed by the calculated partial expansion coefficient. This provides a fundamental method to predict the macroscopic thermal expansion from the atomic scale for liquid alloys, especially in the undercooled regime.     

Charge distribution in structurally disordered systems

In this paper we calculate the charge distribution n(Q) for a structurally disordered system of identical atoms. The atoms have non-zero charges associated to them only because the spatial configuration around each atom is different. The systems considered are those for which an atomic basis set is adequate and an iterative tight binding scheme, where the matrix elements depend on the atomic charges, is used. We study the effect of including explicitly the electrostatic interaction among the charges associated to the atoms in the calculation of n(Q). We propose that the atomic positions of a totally random configuration be modified by amounts proportional to the electrostatic forces on the atoms. We call this a relaxation effect. We find that the new atomic configurations give a narrower n(Q) although they have practically the same energy and radial distribution function as the original configuration.     

Pre-roughening dynamics on Si(100) stepped surfaces: a study by molecular dynamics

The thermal evolution of steps on Si(100) is well studied and experiment indicates that at temperatures below the roughening transition (i.e. T? 1000 K) the displacements of atoms at the step-edge are the basic factor of this evolution. However the evaluation of the nature and participants of these displacements is beyond experimental observations and a theoretical approach is therefore needed. The problem addressed by this study is the identification of the properties of atomic motions of step-edge atoms and this investigation is performed applying an isothermal Molecular Dynamics simulation method to simple stepped configurations on Si(100). The calculations describe the functional dependence of the motions of step-edge atoms on the step type, size and temperature and on the nature of the interatomic forces. Possible mechanisms of kink formations are suggested. Received 15 February 2002 Published online 13 August 2002     

Classical theory of desorption rate velocity distribution of desorbed atoms; possibility of a compensation effect

A three-dimensional theory of atomic desorption is formulated using the thermal equilibrium hypothesis. The velocity distribution of desorbed atoms obtained is Maxwellian at the temperature of the crystal, whatever the interaction potential. As expected the desorption rate is equal to the product of a Boltzmann factor, exp(-U/kT), by a so-called preexponential factor. The case of an adatom linked by harmonic forces to four surface atoms of a unit cell is considered in particular. The result obtained, as well as those given by three-dimensional linear theory, show a compensation effect which seems to be too low if one compares calculated and experimental data. However, the calculated preexponential factor is mainly dependent on: (a) the adatom vibrational frequency; (b) the amount of potential energy which can be concentrated in the bond, characterized by its mean thermal value. It increases with respect to this quantity. This result suggest that high preexponential factor values are due to a long range collective interaction between the adatom and surface atoms, corresponding to the formation of a large “quasi molecule”.     

Direct observation of local distortion of a crystal lattice with picometer accuracy using atomic resolution neutron holography

We demonstrate that neutron holography permits us to extend the determination of atomic positions beyond nearest neighbors at least up to the fourth neighboring shell around cadmium probe atoms alloyed into a lead crystal. The accuracy achieved is sufficient to allow quantitative determination of displacements of atoms due to elastic distortions induced by impurity atoms. The atomic positions derived from the holographic data are in good agreement with those expected theoretically due to Friedel oscillations in this system. In addition, the atomic positions are in qualitative agreement with results obtained in an independent experiment studying the diffuse distortion scattering around Bragg peaks.     

Electronic,thermal, and optical properties of graphene like SiCx structures: Significant effects of Si atom configurations

We investigate the electronic, thermal, and optical characteristics of graphene like SiC_x structure using model calculations based on density functional theory. The change in the energy bandgap can be tuned by the Si atomic configuration, rather than the dopants ratio. The effects of the concentration of the Si atoms and the shape of supercell are kept constant, and only the interaction effects of two Si atoms are studied by varying their positions. If the Si atoms are at the same sublattice positions, a maximum bandgap is obtained leading to an increased Seebeck coefficient and figure of merit. A deviation in the Wiedemann-Franz ratio is also found, and a maximum value of the Lorenz number is thus discovered. Furthermore, a significant red shift of the first peak of the imaginary part of the dielectric function towards the visible range of the electromagnetic radiation is observed. On the other hand, if the Si atoms are located at different sublattice positions, a small bandgap is seen because the symmetry of sublattice remains almost unchanged. Consequently, the Seebeck coefficient and the dielectric function are only slightly changed compared to pristine graphene. In addition, the electron energy loss function is suppressed in Si-doped graphene. These unique variations of the thermal and the optical properties of Si-doped graphene are of importance to understand experiments relevant to optoelectronic applications.     

Expansion of monomer units in poly(ethylene terephthalate) molecules under conditions of thermal and zero-point atomic vibrations

The temperature dependence of the expansion ?_C of trans conformers of the carbon skeleton of macromolecules in crystallized and amorphous poly(ethylene terephthalate) (PET) due to zero-point and thermal atomic vibrations is investigated by IR spectroscopy. It is found that the thermal expansion coefficient β_C jumpwise increases at the characteristic temperatures T _t and T _b. This increase is associated with the crossover from the quantum to classical statistics of torsional and bending vibration modes. The quantum and classical contributions to the expansion ?_C are determined for each mode. The quantum and classical contributions of the torsional vibrational mode in the amorphous polymer are approximately 1.5 times larger than those in the crystallized polymer. This effect is caused by an increase in the anharmonicity of torsional vibrations in the amorphous polymer.     

Photoemission studies of initial oxidation for ultra-thin zinc film on 6H-SiC(0 0 0 1) surface with synchrotron radiation

The thermal oxidation process of metallic zinc on 6H-SiC(0 0 0 1) surface has been investigated by using atomic force microscopy (AFM), synchrotron radiation photoelectron spectroscopy (SRPES) and XPS methods. The AFM images characterize the surface morphology of ZnO film formed during the thermal oxidation and SRPES record the valence band, Si 2p and Zn 3d spectra at different stages. The O 1s peak is recorded by XPS because of the energy limit of the synchrotron radiation. Our results reveal that the silicon oxides layer of SiC substrate can be reduce by hot metallic zinc atom deposition. The oxygen atoms in the silicon oxides are captured by the zinc atoms to form ZnO_x at the initial stage and as a result, the oxidized SiC surface are deoxidized. After the zinc deposition with the final thickness of 2.5 nm, the sample is exposed in oxygen atmosphere and annealed at different temperatures. According to the evolution of peaks integrated intensities, it is considered that the Zn/SiC system will lose zinc atoms during the annealing in oxygen flux at high temperature due to the low evaporation temperature of pure zinc. After further annealing in oxygen flux at higher temperature, the substrate is also oxidized and finally the interface becomes a stable SiC-SiO_x-ZnO sandwich structure.     

The Virial Expansion of a Dilute Bose Gas in Two Dimensions

In terms of the s-wave phase shift of the two-body scattering at thermal wavelength, a systematic perturbative expansion of the Virial coefficients is developed for a two-dimensional dilute system of bosons in its gaseous phase at low temperature. The thermodynamic functions are calculated to the second order of the expansion parameter. The observability of the universal low energy limit of the two dimensional phase shift with a quasi-two dimensional atomic gas in an anisotropic trap is discussed.     

硅纳米晶体薄膜热膨胀性质的分子动力学研究

基于Stillinger-Weber势对硅纳米晶体薄膜的热膨胀性质进行了分子动力学模拟。研究表明,硅纳米晶体薄膜表面层原子的二聚现象引起薄膜收缩,而原子之间的非和谐势能引起薄膜膨胀;在约400K以下的低温段,由于硅纳米晶体薄膜表面层原子发生二聚的原子数目随温度的升高而明显增多,而原子间非和谐势能很小,故此时二聚主导热膨胀性质,热膨胀系数为负;在高温段（约400K以上）,由于发生二聚的原子数目随温度升高不再显著地增加并渐趋于稳定,而原子间非和谐势能逐渐显著并主导热膨胀性质,故热膨胀系数为正。     

超细Pt纳米线结构和熔化行为的分子动力学模拟研究

基于EAM原子嵌入势, 对临界尺寸下的自由Pt纳米线的奇异结构和熔化行为进行分子动力学模拟. 模拟结果显示, 超细Pt纳米线的熔点随径向尺寸和结构的不同而发生明显改变; 引入林德曼因子, 令其临界值为0.03, 以此得到对应熔点值大小与通过势能-温度变化曲线找出的一致, 又比较了纳米线各层粒子平均林德曼指数的大小, 对各层纳米结构的热稳定性进行定量标度; 综合分析发现螺旋结构纳米线的熔化从内核开始, 而多边形结构的纳米线的熔化从外壳层开始.     

Coherent emission of a photon by many atoms

The theory ofWeisskopf andWigner of the natural line width is extended to a case where many atoms at given positions interact with one common quantized radiation field. At timet=0, one quantum of radiation energy is stored by the atoms, which however does not mean necessarily that exactly one of the atoms must be excited. We study the process of the creation of a photon in dependence on the positions of the atoms and on the state of the system of atoms att=0. The dimensions of the emitted wave train and the decay time of this excited atomic state depend sensitively on these parameters.     

Effect of gettering on the atomic structure of gallium arsenide

No changes in the lattice constant or standard parameters due to statistical displacements of atoms from their ideal positions at room temperature are revealed during a study of the atomic structure of gallium arsenide crystals gettered by yttrium. Nonetheless, the effect observed for conventional, ungettered crystals and associated with the influence of point defects on the phonon spectrum disappears.     

Manipulation of atoms and molecules with a low temperature scanning tunneling microscope

Apart from its ability to image surfaces with atomic resolution the scanning tunneling microscope has evolved as a tool to manipulate single atoms and molecules. In this paper we present several examples of atomic manipulation and the formation of nanostructures with this technique. As examples for lateral manipulation, i.e. the sliding of atoms and molecules along the surface, we show CO, C₂H₄ and Pb on a stepped copper surface, the Cu(211). As examples for vertical manipulation, i.e. the transfer of atoms and molecules between the tip and the surface, we discuss Xe and C₃H₆. The design of our low temperature scanning tunneling microscope is shortly described.     

A high-resolution neutron powder diffraction study on high-Tc superconductor YBa2Cu3O7

A high-resolution neutron time-of-flight diffraction measurement of superconductive Y-Ba-Cu-O (T_co = 91 K) has been performed both at room temperature and at 77 K. Profile refinements of the measured data resulted in a unit cell of stoichiometry YBa₂Cu₃O₇. The positions of the oxygen deficiencies were also determined. Refinement of the anisotropic thermal parameters for the oxygen atoms seem to indicate some changes in the form of the thermal ellipsoids of these atoms when cooled below T.     

Geometrical and chemical environment effects on the magnetism of stepped surfaces of V and V over Fe

By means of a real-space tight-binding calculation we obtain the spin-polarization of vicinal (1, 0, 2n?1) vanadium surface, and of one V layer deposited on vicinal (1, 0, 2n?1) iron surfaces (n=1,2,3,4). These geometries can be viewed as a staircase on the (001) surface with steps of monoatomic height andn-atoms width. Forn≤2, the pure V stepped surfaces do not show magnetism which is consistent with the absence of spinpolarization at the (101) surface. By contrast, magnetism is always obtained when a V monolayer is deposited on Fe stepped substrate, which is due to the hybridization of d-orbitals of V and Fe. Furthermore, in all the cases where magnetism is obtained, the surface V atoms at the edge of the step result to be antiferromagnetically coupled with all kink atoms. The effect of this local defect on the magnetic structure seems to remain when approaching the (001) surface (n→∞).