A New Approach of Quantum Mechanics for Neutron Single-Slit Diffraction

Phenomena of electron, neutron, atomic and molecular diffraction have been studied in many experiments, and these experiments are explained by many theoretical works. We study neutron single-slit diffraction with a quantum mechanical approach. It is found that the obvious diffraction patterns can be obtained when the single- slit width a is in the range of 3λ - 60λ. We also lind a new quantum effect of the thickness of single-slit which can make a large impact on the diffraction pattern. The new quantum effect predicted in our work can be tested by the neutron single-slit diffraction experiment.     

Quantum transport of ultracold atoms in an accelerating optical potential

Received: 8 July 1997     

Effects of diffraction conditions and processes on rheed intensity oscillations during the MBE growth of GaAs

The RHEED intensity oscillation technique has received wide-spread attention for the study of MBE growth dynamics, but insufficient consideration has been given to the diffraction conditions and processes involved. We report here a systematic investigation of the intensity oscillation behaviour as a function of diffraction parameters (azimuth, incidence angle, specular and non-specular beams), with constant growth conditions for GaAs films on GaAs (001) substrates.We show that many reported anomalies attributed to growth effects, such as phase differences and periodicity variations, can be accounted for entirely by diffraction events, provided it is realised that multiple scattering processes are the dominant cause of RHEED intensity variations during growth.The technique can provide valuable information on growth behaviour, but only if diffraction-dependent effects are first eliminated.     

The application of RHEED intensity effects to interrupted growth and interface formation during MBE growth of GaAs/(Al,Ga)As structures

There is a diffraction-induced phase effect in the RHEED intensity oscillation technique used in MBE, whereby intensity maxima only correspond to monolayer completion for very restricted conditions. In particular, the angle of incidence of the primary beam is extremely critical. The effect occurs because the total intensity at the measured position of the specular beam is always derived from at least two different diffraction processes, which do not have the same phase relation to monolayer formation. It can be accomodated either by a systematic series of measurements to establish an empirical relationship between incidence angle and phase, or by Fourier transform techniques. Unless full account is taken of this purely diffraction-induced effect, very misleading results can be obtained for the time constants of the recovery period following cessation of growth and this is illustrated for GaAs. The effect also has important implications for the growth-interrupt technique. In addition, it is shown that for heterojunction formation in the GaAs/(Al, Ga)As system, adatom (Ga and Al) migration lengths are of greater importance than the position in the monolayer at which the composition is changed, and that RHEED can provide only limited information on the interface structure.     

LEED structure analysis of a Co{001} surface

A {001} surface of face-centered-cubic cobalt was cleaned to the point of elimination of all impurities except carbon and oxygen, which were reduced to minimum terminal amounts. A LEED structure analysis of this surface, using 12 intensity spectra at 3 angles of incidence, reveals that the atomic arrangement corresponds to truncation of the bulk structure but with about 4% contraction of the first interlayer spacing along 〈001〉 with respect to the bulk.     

X-Ray Photoelectron Spectroscopy and Reflection High Energy Electron Diffraction of Epitaxial Growth SiC on Si（100） Using C60 and Si

The formation of silicon carbide upon deposition of C60 and Si on Si（100） surface at 850^o C is studied via x-ray photoelectron spectroscopy and reflection high energy electron diffraction （RHEED）. The C ls, O ls and Si 2p core-level spectra and the RHEED patterns indicate the formation of 3C-SiC.     

Controlled Growth of Zn-Polar ZnO Films on Al-Terminated α-Al2O3(0001) Surface by Using Wurtzite MgO Buffer

The controlled growth of Zn-polar ZnO fihns on Al-terminated α-Al203 （0001） substrates is investigated by the radio-frequency plasma-assisted molecular beam epitaxy method. Prior to the growth, α-Al2O3 （0001） surface is modified by an ultrathin MgO layer, which serves as a uniform template for epitaxy of Zn-polar ZnO films. The microstructures of ZnO/MgO/Al2O3 interface are investigated by in-situ reflection high-energy electron diffraction observations and ex-situ high-resolution transmission electron microscopy characterization. It is found that under Mg-rich condition, the achievement of the wurtzite MgO ultrathin layer plays a key role in the subsequent growth of Zn-polar ZnO. An interracial atomic model is proposed to explain the mechanism of polarity selection of both MgO and ZnO films.     

Stress Analysis of ZnO Film with a GaN Buffer Layer on Sapphire Substrate

A 5.35-μm-thick ZnO film is grown by chemical vapour deposition technique on a sapphire （0001） substrate with a GaN buffer layer. The surface of the ZnO film is smooth and shows many hexagonal features. The full width at half maximum of ZnO （0002） u-rocking curve is 161 arcsec, corresponding to a high crystal quality of the ZnO film. From the result of x-ray diffraction 0 - 20 scanning, the stress status in ZnO film is tensile, which is supported by Raman scattering measurement. The reason of the tensile stress in the ZnO film is analysed in detail. The lattice mismatch and thermal mismatch are excluded and the reason is attributed to the coalescence of grains or islands during the growth of the ZnO film.     

Modulational instability of matter waves under strong nonlinearity management

We study modulational instability of matter-waves in Bose-Einstein condensates (BEC) under strong temporal nonlinearity-management. Both BEC in an optical lattice and homogeneous BEC are considered in the framework of the Gross-Pitaevskii equation, averaged over rapid time modulations. For a BEC in an optical lattice, it is shown that the loop formed on a dispersion curve undergoes transformation due to the nonlinearity-management. A critical strength for the nonlinearity-management strength is obtained that changes the character of instability of an attractive condensate. MI is shown to occur below (above) the threshold for the positive (negative) effective mass. The enhancement of number of atoms in the nonlinearity-managed gap soliton is revealed.     

Photo-induced dynamics of Bose-Einstein condensates in optical superlattice

The photo-induced dynamics of cold atoms in a one-dimensional optical superlattice is observed. Steady state distribution of the probability amplitudes and the site population in a one-dimensional optical superlattice is found. It is shown that this solution of the equations, which describes the temporal behavior of a Bose-Einstein condensate in a superlattice, is unstable at the sufficiently high level of boson density. The expression for the increment of modulational instability is obtained on the basis of the linear stability analysis. The numerical examples of non-stationary solutions for boson density in a superlattice for the general model are discussed as applied to both the attraction and repulsion potentials of boson interaction.     

Role of Interactions in Electronic Structure of a Two-Electron Quantum Dot Molecule

We have studied a two-electron quantum dot molecule in a magnetic field. The electron interaction is treated accurately by the direct diagonalization of the Hamiltonian matrix. We calculate two lowest energy levels of the two-electron quantum dot molecule in a magnetic field. Our results show that the electron interactions are significant, as they can change the total spin of the two-electron ground state of the system by adjusting the magnetic field between S = 0 and S = 1. The energy difference AE between the lowest S = 0 and S = 1 states is shown as a function of the axial magnetic field. We found that the energy difference between the lowest S = 0 and S = 1 states in the strong-B S = 0 state varies linearly. Our results provide a possible realization for a qubit to be fabricated by current growth techniques.     

Quantum wave equations and non-radiating electromagnetic sources

A connection between classical non-radiating sources and free-particle wave equations in quantum mechanics is rigorously made. It is proven that free-particle wave equations for all spins have currents which can be defined and which are non-radiating electromagnetic sources. It is also proven that and the advanced and retarded fields are exactly equal for these sources. Implications of these results are discussed.     

Dark Soliton of a Growing Bose--Einstein Condensate in an External Trap

The dynamics of dark soliton in a growing Bose-Einstein condensate with an external magnetic trap are investigated by the variational approach based on the renormalized integrals of motion. The stationary states as physical solutions to the describing equation are obtained, and the evolution of the dark soliton is numerically simulated. The numerical results confirm the theoretical analysis and show that the dynamics depend strictly on the initial condition, the gain coefficient and the external potential.     

Self-organized in-plane incorporation of Si atoms in GaAs by molecular beam epitaxy

The preferential attachment of Si atoms at misorientation steps on vicinal GaAs(001) surfaces has been studied by RHEED. By analysing the time evolution of the specular beam intensity and the change in surface reconstruction during Si deposition we show that a self-organized Si incorporation along the step edges takes place. The observed (3×2) structure is due to an ordered array of dimerized Si atoms with missing dimer rows. Taking into account the structure of the (3×2) unit mesh and its orientation with respect to the As-terminated or Ga-terminated steps, a characteristic minimum in the RHEED intensity recording corresponds to the number of Ga step-edge sites. Since the preferential path for Ga as well as for Si adatom diffusion is along the [110] direction, the critical terrace width for wirelike Si attachment is much larger for a misorientation toward (111)As than for a misorientation toward (111)Ga. Despite the high local impurity concentration, the Si-modified surface can be overgrown with GaAs without adverse effects on the growth front. This is promising for the fabrication of doping wires.     

SPA-LEED studies of growth of Ag on Ag(111) at low temperatures

The surface morphology after deposition of Ag on Ag(111) at low temperatures (130–200 K) has been studied in detail with SPA-LEED (Spot-Profile Analysis of Low-Energy Electron Diffraction). The surface roughness and the mean terrace size have been quantitatively determined under various conditions. At 130 K the surface roughness increases with coverage exactly according to the relation = ^1/2, which indicates that the inter-layer diffusion can be neglected at 130 K. Although the mean terrace length decreases with increasing coverage (following an approximate power law of ^–2/3) for all studied coverages, it is much larger than expected for a pure random or Poisson-growth mode without any diffusion of the adatoms. Therefore, Ag grows on Ag(111) at this temperature without interlayer diffusion but with intra-layer diffusion. The intralayer diffusion barrierE _d has been determined by measuring the temperature dependence of the two-dimensional island density according to the nucleation theory (supposing a critical nucleus size of one). The obtained valueE _c=0.18 eV agrees with the theoretical calculations and previous measurements. Furthermore, from comparing measured and Monte-Carlo-simulated (MC) surface roughness at different deposition temperatures we obtain E=0.05 eV as a lower limit for the additional barrier at steps.     

Surface phase diagram and temperature induced phase transitions of Sn/Cu(1 0 0)

Room temperature deposition of Sn on Cu(1 0 0) gives rise to a rich variety of surface reconstructions in the submonolayer coverage range. In this work, we report a detailed investigation on the phases appearing and their temperature stability range by using low-energy electron diffraction and surface X-ray diffraction. Previously reported reconstructions in the submonolayer range are p(2 × 2) (for 0.2 ML), p(2 × 6) (for 0.33 ML), ()R45° (for 0.5 ML), and c(4 × 4) (for 0.65 ML). We find a new phase with a structure for a coverage of 0.45 ML. Furthermore, we analyze the temperature stability of all phases. We find that two phases exhibit a temperature induced reversible phase transition: the ()R45° phase becomes ()R45° phase above 360 K, and the new phase becomes p(2 × 2) also above 360 K. The origin of these two-phase transitions is discussed.     

Refit Silver Nanostructures Using a Convergent Electron Beam

Using a superionic conductor AgI thin film and a direct current electric field, we synthesize silver nanowires in diameter of about lOOnm. In order to refit the prepared nanowires, the samples are irradiated by a convergent electron beam （200 k V） inside a transmission electron microscope to prepare new small silver nanostructures. The new nanostructures are investigated in situ by high-resolution transmission electron microscope. This electron- induced crystal growth method is useful for technical applications in fabrication of nanodevices.     

Tomographic approach to the violation of Bell's inequalities for quantum states of two qutrits

The tomographic method is employed to investigate the presence of quantum correlations in two classes of parameter-dependent states of two qutrits. The violation of some Bell's inequalities in a wide domain of the parameter space is shown. A comparison between the tomographic approach and a recent method elaborated by Wu, Poulsen, and Mølmer shows the better adequacy of the former method with respect to the latter one.     

Short range order in high-T c superconductors investigated by electron diffraction

Observation of modulated diffuse streaks in electron diffraction patterns of Tl₂Ba₂CuO₆ and YBa₂Cu₃O_7– ceramic superconductors is reported. These streaks are parallel with the c-axis and the intensity modulation has a period in reciprocal space which is larger than the c*-parameter. This behaviour is interpreted in terms of disordered sublattices. It is shown that the fine structure results from short range order in two successive homologous layers forming one bi-layer and that the intensity modulation in the diffuse streaks therefore allows one to determine which of the bi-layers in the material exhibit disorder.Research assistant of the National Fund for Scientific Research, Belgium