M - C dipole formation in fcc (face centred cubic) Fe1-yMyCx solid solution (M=Ti,Nb)

Abstract

Available equilibrium carbon solubility data for fcc (face centred cubic) Fe_{1 - y}M_yC_x solid solution (M = Ti,Nb) reported as functions of temperature T and carbon activity a (C) were analysed taking into account the atomic interaction parameters derived through statistical thermodynamic analysis of related systems. In the fcc Fe_1-yM_yC_x under consideration, the affinity between C atom and M atom (M = Ti,Nb) is very much more attractive than that between C and Fe. Correspondingly, the atom configuration model in which M atoms are distributed randomly over the metal sub-lattice Fe_{1 - y}M_y and all the M atoms in the Fe_{1 - y}M_yC_x are bonded to C to form M - C dipoles was concluded to be the most realistic one reproducing the experimentally determined a (C) - T - x relationships at respective y.     

Atom probe characterisation of high temperature materials

Abstract

An atom probe is capable of quantitatively analysing materials at the atomic level. Modern atom probes are derived from the field ion microscope, and are coupled with time-of-flight mass spectrometers, permitting identification of individual atoms. The introduction of position-sensitive detectors enables the reconstruction of a small volume of the sample owing to simultaneous determination of the x, y, and zcoordinates and the mass to charge ratios of individual atoms. This paper focuses on the application of atom probe techniques to the microstructural analysis of high temperature materials. Illustrations include carbide precipitation in creep resistant power plant steels and analyses of model and commercial multicomponent nickel based superalloys. It is demonstrated that atom probe field ion microscopy and atom probe tomography are valuable techniques in the development and understanding of technologically important alloys for high temperature service.     

Interaction energy parameters in hypostoichiometric mono-carbides,TiCx and NbCx,evaluated by statistical thermodynamics

Abstract

Interaction energy E(C-C) between nearest neighbour C atoms and E(C-M) between a C atom and M atom in hypostoichiometric monocarbide MC_x (M = Ti or Nb) were evaluated from the available a(C)-T-x relationships by statistical thermodynamic analysis (a(C): carbon activity, T: temperature, x: C/M atom ratio). Estimated E(C-C) values were positive (i.e. the C-C interaction is repulsive) for both TiC_x and NbC_x and E(C-C) varied with T showing a trend of weakening repulsion with rising T. On the other hand, the C-M interaction appeared to be strongly attractive.     

Decay interference induced high precision localization in a multilevel atom via controlled spontaneous emission

A simple scheme for one-dimensional atom localization is proposed by employing a technique for the formation of the standing-wave regime using two unidirectional standing-wave fields. We consider a four-level atomic system similar to the one used by Paspalakis and Knight [Phys. Rev. Lett. 1998, 81, 293–296], with travelling-wave fields for the study of the phase control of emission in the presence of vacuum-induced interference between two spontaneous decay channels. In the present system precise position information of the atom can be achieved by measuring the frequency of spontaneous emission, which can be efficiently controlled by different system parameters and also by adjustment of the relative phase in the presence of the decay interference effect. The proposed scheme provides a potential technique to attain 100% detection probability of the atom in one wavelength range with generation of a sharp localization peak at low light level.     

The Interaction of Displaced Number State and Squeezed Number State Fields with Two-level Atoms

Abstract

The time-evolution of a single two-level atom in a single-mode high-Q cavity is sensitive to the quantum fluctuations of the cavity radiation field and to its photon statistics: this sensitivity is realizable experimentally in the Rydberg atom micromaser. We study the effects of the interaction of a two-level atom with two new non-classical radiation fields: the squeezed number state and the displaced number state realizable by nonlinear and linear transformations of field number states which have an initially precise occupation number. The time-varying field fluctuations caused by the atomic interaction are described using the Q-function quasi-probability.     

Computational investigation of GanAl (n = 1–15) clusters by the density-functional theory

Low-lying equilibrium geometric structures of aluminum-doped gallium cluster Ga_nAl (n = 1–15) clusters obtained by an all-electron linear combination of atomic orbital approach, within spin-polarized density functional theory, are reported. The binding energy, dissociation energy, and stability of these clusters are studied with the three-parameter hybrid generalized gradient approximation (GGA) due to Becke-Lee–Yang–Parr (B3LYP). Ionization potentials, electron affinities, hardness, and static polarizabilities are calculated for the ground-state structures within the same method. The growth pattern for Ga_nAl (n = 1–15) clusters is Al-substituted Ga_n + 1 clusters and it keeps the similar frameworks of the most stable Ga_n + 1 clusters except for Ga₈Al and Ga₁₃ Al clusters. The Al atom substituted the surface atom of the Ga_n + 1 clusters for n < 12. Starting from n = 12, the Al atom completely falls into the center of the Ga-frame. The Al atom substituted the center atom of the Ga_n + 1 clusters to form the Al-encapsulated Ga_n geometries for n > 12. The odd−even oscillations from Ga_nAl (n = 5) in the dissociation energy, the second-order energy differences, the HOMO–LUMO gaps, the ionization potential, the electron affinity, and the hardness are more pronounced. The stability analysis based on the energies clearly shows the clusters from n = 5 with an even number of valence electrons are more stable than clusters with odd number of valence electrons.     

Efficiency and reliability of metal-insulator-semiconductor radiative structures based on broad-band semiconductors with unipolar conductivity

This paper discusses the possibility of increasing the efficiency and reliability of radiation sources made from semiconductors with unipolar conductivity and operating in strong electric fields. To this end, the active region of the structure is fabricated in the form of alternating layers of different resistance, so that strong-field regions are spatially separated from luminescence-generation regions. The proposed idea is put into practice on M-i-n structures made from gallium nitride. The properties of fabricated LED structures are presented. Pis’ma Zh. Tekh. Fiz. 23, 17–23 (November 12, 1997)     

Virtual Photons,Causality and Atomic Dynamics in Spontaneous Emission

Abstract

The time-dependent electric energy density surrounding a two-level atom fixed at r = 0 is studied, the atom being taken in its excited state at t = 0 and the field being initially in the vacuum state. The atom-field coupling includes both rotating and counter-rotating terms. The energy density of the spontaneously emitted field in the rotating wave approximation is shown to behave non-causally, while in the presence of the complete coupling it is shown to vanish outside a sphere of radius r = ct centred on the atom. The deviations of atomic dynamics from the exponential Wigner-Weisskopf behaviour during spontaneous decay are shown to be deeply influenced by the counter-rotating terms. It is concluded that the virtual photons induced by the counter-rotating terms in the atom-field coupling are essential in order to ensure causality and cannot be neglected in any accurate treatment of spontaneous emission.     

Spectral and thermodynamic properties of He adsorbed on Ne-plated graphite

The potential energy functionV(r) is constructed for a He atom in the vicinity of a Ne-plated surface of graphite. This function includes He-Ne pair interactions, triple dipole He-Ne-Ne interactions, a McLachlan-type He-Ne-graphite interaction, a theoretical He-graphite potential, and a contribution from the Ne atomic vibrations. The Schrödinger equation is solved to derive the He scattering bound-state resonance positions and the energy-band structureE(K). The specific heat and isosteric heat are computed and compared with data of Crary and Vilches and of Hanono and Lerner, respectively.     

Einstein localization in entangled light scattering

Abstract

We discuss Einstein-Podolsky-Rosen (EPR)-type entanglement in two-particle break-up, using Raman scattering with atom recoil as an example. The Schmidt number K is used as a measure of entanglement and the conditions to acquire high entanglement are obtained. We also illustrate the EPR conditional uncertainty in a situation where Δx Δp reaches a value much smaller than the Heisenberg limit.     

Conditional generation of non-classical states in a non-degenerate two-photon micromaser: Equal-intensity pair-Fock states preparation. I

Abstract

We present the theory underlying a new experimental scheme for the preparation of selected number states of a bimodal high-Q microcavity. The practical feasibility of this method is carefully discussed. In particular we take explicitly into account the existence of fluctuations of the atom–field interaction times as well as the possibility of imperfect atomic detection.     

In situ high resolution transmission electron microscopy investigation of deformation mechanism in sub-10-nm Au crystals

Abstract

In situ high resolution transmission electron microscopy investigations were performed on sub-10-nm Au crystals. The effects of tensile loading direction and crystal size on the deformation mechanism of Au crystals were analysed. For the Au crystals with a width below 2 nm, the surface atom diffusion with a phenomenon of layer by layer peeling is the main deformation mechanism and the tensile loading direction plays negligible effect. For the Au crystals with a width over 7 nm, the dislocations generated form surface and gliding into crystal dominate the plastic deformation and the tensile loading direction plays important role. Lomer dislocations are produced and destructed by dislocation reaction during tensile strain process in <001> oriented Au crystal. The Schmid law is the key intrinsic issue controlling the deformation mechanism for the nanowires with a size larger than 7 nm.     

The enhancement of spontaneous and induced transition rates by a Bose-Einstein condensate

Abstract

In this paper, an exactly solved model for the emission by N atoms is presented, the spontaneous and induced transition rates obtained, are enhanced by a factor which is proportional to the number of atoms n in the volume Λ³(2π²) (A is the transition wavelength of the atom) and dependent on the de-Broglie wavelength Λ_B in a more complicated way.     

Formation of macroparticle structures in an rf induction discharge plasma

It was demonstrated experimentally that macroparticles may undergo levitation and form ordered structures in an rf induction discharge plasma. The experiments were carried out using 1.87 μm melamine formaldehyde particles in neon at a pressure of 25–500 Pa. The generator frequency was 100 MHz. Pis’ma Zh. Tekh. Fiz. 24, 62–68 (October 12, 1998)     

Controlling decoherence of a two-level atom in a lossy cavity

Abstract

By use of external periodic driving sources, we demonstrate the possibility of controlling the coherent as well as the decoherent dynamics of a two-level atom placed in a lossy cavity. The control of the coherent dynamics is elucidated for the phenomenon of coherent destruction of tunnelling (CDT), i.e. the coherent dynamics of a driven two-level atom in a quantum superposition state can be brought practically to a complete standstill. We study this phenomenon for different initial preparations of the two-level atom. We then proceed to investigate the decoherence originating from the interaction of the two-level atom with a lossy cavity mode. The loss mechanism is described in terms of a microscopic model that couples the cavity mode to a bath of harmonic field modes. A suitably tuned external cw-laser field applied to the two-level atom slows down considerably the decoherence of the atom. We demonstrate the suppression of decoherence for two opposite initial preparations of the atomic state: a quantum superposition state as well as the ground state. These findings can be used to decrease the influence of decoherence in qubit manipulation processes.     

The Jaynes-Cummings Model with a q Analogue of a Coherent State

Abstract

In this paper we study the time evolution of the atomic inversion of the two-level atom which is coupled to the q analogue of a single mode of the bosonic field. The q field under consideration is supposed to be prepared initially in the q analogue of Glauber's coherent state. We find that q deformation of Heisenberg algebra may correspond to some effective nonlinear interaction of the cavity mode.     

Novel caged clusters of silicon: Fullerenes,Frank-Kasper polyhedron and cubic

We review recent findings of metal (M) encapsulated caged clusters of Si and Ge obtained from computer experiments based on an ab initio pseudopotential method. It is shown that one M atom changes drastically the properties of Si and Ge clusters and that depending upon the size of the M atom, cages of 14, 15, and 16 Si as well as Ge atoms are formed. In particular M@Si₁₆ silicon fullerene has been obtained for M= Zr and Hf, while a Frank-Kasper polyhedron has been obtained for M@X₁₆, X = Si and Ge. These clusters show high stability and large highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) gaps which are likely to make these species strongly abundant. A regular icosahedral M@X₁₂ cluster has also been obtained for X = Ge and Sn by doping a divalent M atom. Interactions between clusters are rather weak. This is attractive for developing self-assembled cluster materials.     

General formalism of interaction of a three-level atom with cavity fields in the Kerr-like medium

Abstract

A general formalism of a three-level atom interacting with one-mode or two-mode cavity fields in a Kerr-like medium is presented. Dynamic behaviours of the atomic occupation probabilities and the transfer phenomena are investigated numerically in two typical cases. Our results show that the transfer phenomena may depend on the initial conditions.     

Kirkendall void formation during room-temperature air-oxidation of thin aluminium and aluminium – lithium alloy films

Abstract

The effect of room-temperature (~20°C) air-oxidation on void formation in sputter-deposited thin films of aluminum and its alloys was investigated using a transmission electron microscope. It was found that after air-oxidation, only lithium-bearing aluminum alloy films exhibited a high (~4 × 10¹⁶ cm^?3) density of small (~2 nm) voids, whereas pure aluminum or lithium-free aluminum alloy films did not contain any voids. In lithium-bearing aluminum alloy films, both aluminum and lithium atoms migrate to the surfaces to form their surface oxide during room-temperature ageing after film deposition. In the course of the atom migration, excess vacancies are generated as a result of the large diffusivity difference existing between aluminum and lithium atoms (D_LiinAl ? D_Al) in the alloy matrix. The agglomeration of these excess vacancies led to the formation of so-called Kirkendall voids inside the alloy. Thus the presence of both aluminum and lithium in the alloys was a key factor for generating these Kirkendall voids in the films.     

N-level Atom Interacting with Single-mode Radiation Field: An Exactly Solvable Model with Multiphoton Transitions and Intensity-dependent Coupling

Abstract

We study the dynamics of an N-level atom coupled in a lossless cavity to a single-mode near-resonant quantized field. The atomic levels are coupled by the multiphoton transitions and the coupling constants between the field and the atomic levels are supposed to be intensity dependent. We find the exact solution for the state vector describing the dynamics of the atom-plus-field system. As an illustration we use the model for studying (i) the time evolution of the atomic occupation probability with the initially coherent field and (ii) the light squeezing, when the cavity field is initially in the vacuum state and the atom is prepared in the atomic ‘coherent state’ (a superposition of atomic states).