Unconventional Energy Bands and Chirality of Ultracold Atoms in Trilayer Honeycomb Lattice

We investigate ultracold fermionic atoms in the trilayer honeycomb lattice. In the low energy approximation, we derive an effective Hamiltonian for pseudospins. The energy spectrum shows a cubic form of the wavevector and is gapless. The quasiparticles and quasiholes are chiral and show Berry's phase π when thewavevector adiabatically evolves along a closed circle. Furthermore, the experimental detection of the energy spectrum is proposed with Bragg scattering techniques.     

Semiclassical description of inelastic atom scattering by surfaces

Inelastic scattering of atoms of moderate energies (say<5 eV) by solid surfaces is almost entirely due to energy exchange with lattice vibrations. It can give valuable information about the atom-surface interaction potential and the vibrational dynamics at surfaces. Theoretically this process represents a challenging many-body problem, calling for suitable approximation methods. Work in progress (K. Burke, L. D. Chang, and W. Kohn) is described. (1) We have solved a simple model problem in which the normal modes of the lattice are schematized by a single one-dimensional harmonic oscillator, initially in its ground state (T=0). The classical solution gives a unique energy loss. We have calculated the leading quantum correction and find a Gaussian final energy distribution whose width is proportional toh ^1/2. Our exact results are in general different from the so-called trajectory approximation. (2) We are about to propose a new type of atom-surface scattering experiment, which will provide a direct measure of the quantum corrections to classical scattering.     

有限元法计算GaN/AlN量子点结构中的电子结构

根据有效质量理论单带模型，采用有限元方法(FEM)计算了GaN/AlN量子点结构中的电子结构，分析了应变和极化对电子结构的影响，计算了不同尺寸的量子点的能级，分析了量子点的大小对电子能级的影响. 结果表明，形变势和压电势提升了电子能级，而且使简并能级分裂. 随着量子点尺寸的增大，量子限制能减小，而压电势能起到更显著的作用，使电子的能级降低，吸收峰发生红移. 关键词： GaN/AlN量子点结构 有效质量理论 电子能级     

First-principles calculation of the phonon frequencies in γ Fe

The total energy, the equilibrium lattice constant, and the bulk modulus of the fcc phase of iron have been calculated by the full-potential LMTO method. The use of a generalized gradient approximation in the calculation of the electronic structure and lattice properties of γ-Fe is discussed. Next, the transverse phonon frequency at the W point of the Brillouin zone of a fcc lattice is calculated by the “frozen-phonon” method in the harmonic approximation. A local minimum has been found in the curve of the variation of the total energy of the system as a function of the amplitude of the atomic displacements corresponding to the chosen normal mode. To take account of anharmonic effects, a pseudoharmonic approximation is used and an effective potential that approximates the curve of the variation of the total energy of the system and depends on the temperature via the correlation function of the mean-square displacement of atoms from their equilibrium positions is constructed. The theoretical temperature dependence of the effective frequency of the phonon mode responsible for the structural phase transition corresponds qualitatively to the experimentally observed dependence. A new interpretation is given for the structural phase transition as a transition of the corresponding phonon mode from the excited to the ground state. Fiz. Tverd. Tela (St. Petersburg) 39, 171–175 (January 1997)     

二维光量子阱共振隧穿光谱特性的改善

用时域有限差分法研究了光子晶体量子阱中的量子化能态.研究发现,开腔与闭腔光量子阱结构共振透射峰的数目相同,位置几乎不变,但闭腔光量子阱出射光强更强,透射率更大,频率选择性更好,品质因子Q值更高.同时计算了开腔和闭腔光量子阱光场分布,结果表明,开腔光量子阱为行波阱,闭腔光量子阱为驻波阱,充分证实了闭腔光量子阱更能束缚光场的设想,对其作用机理进行了探讨.     

Scaled-energy spectroscopy of a ｜M｜= 1 Rydberg barium atom in an electric field

We observe strong energy-dependent quantum defects in the scaled-energy Stark spectra for |M| = 1 Rydberg states of barium atoms at three scaled energies: ε = 2.000,ε = 2.500 and ε = 3.000.In an attempt to explain the observations,theoretical calculations of closed orbit theory based on a model potential including core effect are performed for non-hydrogenic atoms.While such a potential has been uniformly successful for alkali atoms with a single valence electron,it fails to match experimental results for barium atoms in the 6snp Rydberg states with two valence electrons.Our study points out that this discrepancy is due to the strong perturbation from the 5d8p state,which voids the simple approximation for constant quantum defects of principle quantum number n.     

Bose–Einstein condensates in 1D- and 2D optical lattices

Bose–Einstein condensates of rubidium atoms are transferred into one- and two-dimensional optical lattice potentials. The phase coherence of the condensate wavefunction in the lattice potential is studied by suddenly releasing the atoms from the trapping potential and observing the multiple matter-wave interference pattern of several thousand expanding quantum gases. We show how arbitrary phase gradients can be mapped onto the periodic wavefunction through the application of a potential gradient. Furthermore, the experimentally measured strength of the momentum components is compared to a theoretical model of the condensate wavefunction in the lattice. Received: 3 July 2001 / Revised version: 26 September 2001 / Published online: 23 November 2001     

Coulomb crystals in the harmonic lattice approximation

The dynamic structure factor &Stilde;(k,omega) and the two-particle distribution function g(r,t) of ions in a Coulomb crystal are obtained in a closed analytic form using the harmonic lattice (HL) approximation which takes into account all processes of multiphonon excitation and absorption. The static radial two-particle distribution function g(r) is calculated for classical (T greater, similarPlanck's over 2piomega(p), where omega(p) is the ion plasma frequency) and quantum (T相似文献   

Cold atoms and molecules in self-assembled dipolar lattices

We study the realization of lattice models, where cold atoms and molecules move as extra particles in a dipolar crystal of trapped polar molecules. The crystal is a self-assembled floating mesoscopic lattice structure with quantum dynamics given by phonons. We show that within an experimentally accessible parameter regime extended Hubbard models with tunable long-range phonon-mediated interactions describe the effective dynamics of dressed particles.     

VO_2晶体电子结构及其掺杂的第一性原理研究

运用密度泛函平面波赝势(PWP)和广义梯度近似(GGA)方法,对二氧化钒(VO2)两种不同晶体电子结构进行了计算.研究了低温单斜晶型和高温四方晶型结构的VO2电子态密度(DOS)和能带(energyband)结构,通过分析发现,四方晶的金属性比较明显,这是由于电子态密度和能带结构分析结果表明不同特性产生的原因是V原子的3d电子贡献不同导致的.本文中我们还将部分O原子替换为F原子后对单斜晶替位掺杂进行了的计算讨论,本文的计算结果都较好地符合实验结果,表明密度泛函平面波赝势和广义梯度近似方法可以用来描述VO2的结构和性质.我们认为,这种方法应用于描述氧化物的电子结构和性质是一种新的探索.     

Effect of the degree of ordering of structural vacancies on the fine structure of the valence-band top in cubic boron nitride

The electronic energy structure of the defect system of c-BN_x with ZnS-type structure is calculated in the multiple-scattering theory by the local coherent potential method. The cluster version of the MT approximation is used to calculate the crystal potential. The effect of the relaxation of the crystal lattice on the electronic structure of nonstoichiometric boron nitride c-BN_0.75 is studied and a comparison is made with the electronic energy structure of c-BN in the same approximation. Fiz. Tverd. Tela (St. Petersburg) 39, 1064–1065 (June 1997)     

First-principles calculation of electronic structure of MgxZn1-xO codoped with aluminium and nitrogen

Aiming at developing p-type semiconductors and modulating the band gap for photoelectronic devices and band engineering, we present the ab initio numerical simulation of the effect of codoping ZnO with Al, N and Mg on the crystal lattice and electronic structure. The simulations are based on the Perdew--Burke--Ernzerhof generalised-gradient approximation in density functional theory. Results indicate that electrons close to the Fermi level transfer effectively when Al, Mg, and N replace Zn and O atoms, and the theoretical results were consistent with the experiments. The addition of Mg leads to the variation of crystal lattice, expanse of energy band, and change of band gap. These unusual properties are explained in terms of the computed electronic structure, and the results show promise for the development of next-generation photoconducting devices in optoelectronic information science and technology.     

Crystal structure of nanostructured PbS films at temperatures of 293–423 K

The crystal structure of lead sulfide films prepared by the hydrochemical deposition has been studied by X-ray diffraction. The thickness of the films synthesized is ∼100 nm, the size of coherent scattering regions is ∼70 nm, and the value of microstrains is ∼0.20%. It is established, for the first time, that the as-synthesized PbS films and the same films annealed in the temperature range 293–423 K have a cubic crystal structure (space group Fm3m) different from the B1-type structure. In the crystal lattice of the structure revealed, sulfur atoms are located not only in the 4(b) positions but also in the 8(c) positions. The occupancies of the 4(b) and 8(c) positions by the S atoms are ∼0.84 and 0.08, respectively.     

The O-H distance in ice

The thermodynamic properties of normal and para-hydrogen are computed from multiple time-step path integral hybrid Monte Carlo (PIHMC) simulations. Four different isotropic pair potentials are evaluated by comparing simulation results with experimental data. The Silvera–Goldman potential is found to be the most accurate of the potentials tested for computing the density and internal energy of fluid hydrogen. Using the Silvera–Goldman potential, simulation and experimental data are compared on isobars ranging from 0.1 to 100 MPa and for temperatures from 18 to 300 K. The Gibbs free energy is calculated from the PIHMC simulations by an adaptation of Widom's particle insertion technique to a path integral fluid. A new method is developed for computing phase equilibria for quantum fluids directly by combining PIHMC with the Gibbs ensemble technique. This Gibbs–PIHMC method is used to calculate the vapour–liquid phase diagram of hydrogen from simulations. Agreement with experimental data is good.     

On Algebraic Integrability of the Deformed Elliptic Calogero-Moser Problem

Abstract

We discuss stationary solutions of the discrete nonlinear Schrödinger equation (DNSE) with a potential of the ? ⁴ type which is generically applicable to several quantum spin, electron and classical lattice systems. We show that there may arise chaotic spatial structures in the form of incommensurate or irregular quantum states. As a first (typical) example we consider a single electron which is strongly coupled with phonons on a 1D chain of atoms — the (Rashba)–Holstein polaron model. In the adiabatic approximation this system is conventionally described by the DNSE. Another relevant example is that of superconducting states in layered superconductors described by the same DNSE. Amongst many other applications the typical example for a classical lattice is a system of coupled nonlinear oscillators. We present the exact energy spectrum of this model in the strong coupling limit and the corresponding wave function. Using this as a starting point we go on to calculate the wave function for moderate coupling and find that the energy eigenvalue of these structures of the wave function is in exquisite agreement with the exact strong coupling result. This procedure allows us to obtain (numerically) exact solutions of the DNSE directly. When applied to our typical example we find that the wave function of an electron on a deformable lattice (and other quantum or classical discrete systems) may exhibit incommensurate and irregular structures. These states are analogous to the periodic, quasiperiodic and chaotic structures found in classical chaotic dynamics.     

Statistical mechanics of solid and liquid mixtures of ortho- and para-hydrogen: II

Various refinements are added to a previous theoretical calculation of the configurational free energy of condensed mixtures of o- and p-hydrogen, based on an oversimplified statistical model: (a) account is taken of the fact that the forces acting between o- and p-molecules are somewhat different and (b) the rigid lattice model previously used is replaced by an ensemble of Einstein oscillators. A better agreement with experiment is reached.     

First-principles calculations of the structural,electronic, optical and thermal properties of the BNxAs1–x alloys

The present paper aims to study the structural, electronic, optical and thermal properties of the boron nitride (BN) and BAs bulk materials as well as the BN_xAs_1–x ternary alloys by employing the full-potential-linearised augmented plane wave method within the density functional theory. The structural properties are determined using the Wu–Cohen generalised gradient approximation that is based on the optimisation of the total energy. For band structure calculations, both the Wu–Cohen generalised gradient approximation and the modified Becke–Johnson of the exchange-correlation energy and potential, respectively, are used. We investigated the effect of composition on the lattice constants, bulk modulus and band gap. Deviations of the lattice constants and the bulk modulus from the Vegard’s law and the linear concentration dependence, respectively, were observed for the alloys where this result allows us to explain some specific behaviours in the electronic properties of the alloys. For the optical properties, the calculated refractive indices and the optical dielectric constants were found to vary nonlinearly with the N composition. Finally, the thermal effect on some of the macroscopic properties was predicted using the quasi-harmonic Debye model in which the lattice vibrations are taken into account.     

NiTi合金势能面的第一性原理研究

采用模守恒赝势(Lin方案)和平面波超软赝势对两种NiTi异构体超晶胞势能面进行了理论研究，结果表明：由于在绝对零度下B19′相总能量低于B2相总能量，B19′相更为稳定.对于B2相，合金化元素Au和Fe在Ni原子位处于势能面平衡点而Ni(Ni位)及位于Ti原子位的Ti，Zr 和Al并非处于平衡位置.在B19′相，Ni，Au，Fe，Ti，Zr，Al均处于势能面平衡点.NiTi合金的马氏体相变应该主要是来自Ni、Ti原子位于势能面非平衡位置所致. 关键词： 密度泛函 势能面 NiTi 合金     

Floating Wigner molecules and possible phase transitions in quantum dots

A floating Wigner crystal differs from the standard one by a spatial averaging over positions of the Wigner-crystal lattice. It has the same internal structure as the fixed crystal, but contrary to it, takes into account rotational and/or translational symmetry of the underlying jellium background. We study properties of a floating Wigner molecule in few-electron spin-polarized quantum dots, and show that the floating solid has the lower energy than the standard Wigner crystal with fixed lattice points. We also argue that internal rotational symmetry of individual dots can be broken in arrays of quantum dots, due to degenerate ground states and inter-dot Coulomb coupling. Received 12 September 2001 / Received in final form 24 April 2002 Published online 9 July 2002     

苝四甲酸二酐的真空升华提纯及其光谱测试与分析

纯度为9.75%的有机半导体材料苝四甲酸二酐(PTCDA),在其升华点进行了真空升华提纯,其纯度可达99.9%。利用质谱、红外光谱及X光电子能谱对这种高纯材料进行了测试并详细分析了其分子结构、化学键的形成、原子在晶格平衡位置的振动模式、电子的组态和原子的结合能的变化。由红外光谱分析得出,苝四甲酸二酐的分子结构是中央5个C构所组成的苝核基团及位于苝核两端的两个酸酐组成,它们主要以共价键结合。晶格上的C原子在其平衡位置主要以伸缩振动为主。其分子中有大量可以自由移动的π电子;分子间离域大π键的交叠决定了苝四甲酸二酐的导电性能。由XPS谱分析得出,高纯度的苝四甲酸二酐中有结合能不同的两种C原子,结合能分别为：285.3和288.7 eV。它们对应于苝环及酸酐上的C原子。另外,有两种类型的O原子,即CO和C—O—C,其结合能分别为531.3和533.1 eV。