Influence of phase interface properties on mechanical characteristics of metal ceramic composites

The paper reports on theoretical study to elucidate the influence of geometric (width) and mechanical characteristics of phase interfaces on strength, ultimate strain, and fracture energy of metal ceramic composites. The study was performed by computer simulation with the movable cellular automaton method and a well-developed mesoscale structural composite model that takes explicit account of wide transition zones between reinforcing inclusions and the matrix. It is shown that the formation of relatively wide “ceramic inclusions-binder” interfaces with gradual variation in mechanical properties allows a considerable increase in the mechanical properties of the composite. Of great significance is not only the interface width but also the gradient of mechanical properties in the transition zone. The presence of defects and inclusions of nano- and atomic scales in interface regions can increase internal stresses in these regions, induce a steep gradient of mechanical properties in them, and hence decrease strain characteristics and fracture energy of the composite.     

Molecular physics and chemistry applications of quantum Monte Carlo

We discuss recent work with the diffusion quantum Monte Carlo (QMC) method in its application to molecular systems. The formal correspondence of the imaginary-time Schrödinger equation to a diffusion equation allows one to calculate quantum mechanical expectation values as Monte Carlo averages over an ensemble of random walks. We report work on atomic and molecular total energies, as well as properties including electron affinities, binding energies, reaction barriers, and moments of the electronic charge distribution. A brief discussion is given on how standard QMC must be modified for calculating properties. Calculated energies and properties are presented for a number of molecular systems, including He, F, F^?, H₂, N, and N₂. Recent progress in extending the basic QMC approach to the calculation of “analytic” (as opposed to finite-difference) derivatives of the energy is presented, together with an H₂ potential-energy curve obtained using analytic derivatives.     

Macroscopic quantum tunneling in differently loaded Josephson junctions

In relation to the problem of quantum measurement, macroscopic quantum tunneling (MQT) in Josephson junctions is one of the most investigated topics. Although many theoretical studies have been devoted to this argument, the agreement on a single theoretical model has not yet been reached. The purpose of the present work is to analyze the load “seen” by the junction, in order to evaluate the contribution of the dissipation that modifies the decay-rate of the metastable state of the junction. The present work reports some theoretical results in relation to different kinds of loading systems, namely, the cases of capacitive, inductive and purely resistive load. The resistive load is considered also in relation to an experimental test.     

Interacting instantons and quantum mechanical metastability

The decay by tunneling of a quantum mechanical metastable state at finite temperature is described in terms of interacting instantons. The picture of instantons with arbitrary interaction strength is developed using complex classical paths. It is shown that there exists a temperature scale at which multi-instanton configurations “freeze”. Far below this scale the partition function of the theory is that of a gas of weakly interacting particles, while far above it the contribution of only one particle dominates. It is argued that this qualitative behavior is common to all quantum mechanical systems possessing a dilute gas regime at low temperatures.     

Spin valve effect in a magnetic nanoelectromechanical shuttle

We study spin-dependent shuttle phenomena in a nanoelectromechanical single electron transistor (NEM-SET) with magnetic leads by considering the coupling between the transport of spin-polarized electrons and mechanical oscillations of the nanometer quantum dot. It is shown that there are two different bias-voltage thresholds for the shuttle instability in electronic transport through the NEM-SET, respectively, corresponding to parallel (P) and antiparallel (AP) magnetization alignments. In between the two thresholds, the electronic transport is in the shuttling regime for the P alignment but in the tunneling regime for the AP one, resulting in a very large spin valve effect.     

Study of dissipative tunneling controllability in systems of interacting quantum molecules

Controlled 2D dissipative tunneling in systems of coupled quantum molecules is studied in the quasi-classical (instanton) approximation. It is shown that, with allowance for the electron tunneling trajectory bifurcation, the effect of two-particle wave function blocking in a system of coupled quantum molecules is observed when the radii of quantum dots (making up the quantum molecules) coincide. If the influence of a thermostat medium is neglected, blocking is replaced by a “kink” in the tunneling probability as a function of the asymmetry parameter of quantum molecules.     

Vertical electronic transport in novel semiconductor heterojunction structures

The investigation of vertical transport in semiconductor heterojunction systems has recently undergone a renaissance due to improved epitaxial techniques in a number of material systems. By using resonant tunneling, we can perform electronic spectroscopy not only of the double barrier structure itself, but of any system (with quantized well states) suitably coupled to a resonant tunneling spectrometer. In designing such systems, an important degree of freedom is introduced by utilizing multi-component structures; for example, a GaAs contact — AlGaAs barrier — InGaAs quantum well. In this structure, the high electron affinity of the quantum well creates a “deep” quantum well, in which we demonstrate that quantum well states can be hidden from transport. Finally, we present results from microfabricated quantum well structures (“quantum dots”) which are sufficiently small in the lateral dimension to introduce size effects. Telegraph noise due to the lateral size of these structures has been observed, and the first indications of lateral quantization in all three dimensions in a semiconductor quantum well are presented.     

Quantum tunneling of ultracold atoms in optical traps

We review our theoretical advances in quantum tunneling of BoseEinstein condensates in optical traps and in microcavities. By employing a real physical system, the frequencies of the pseudo Goldstone modes in different phases between two optical traps are studied respectivdy, which are tile crucial feature of the non-Abelian Joseptmon effect. When the optical lattices are under gravity, we investigate the quantum tummling in the ＂Wannier-Stark localization＂ regime and ＂Lan（lau Zener tunneling＂ regime. We finally get the total decay rate and the rate is valid over the entire range of temperatures. At high temperatures, we show how the decay rate reduces to the appropriate results for the classical thermal activation. At hltermediate temperatures, the results of tile total decay rate are consistent with the thermally assisted tunneling. At low temperatures, we obtain the pure quantmn tunneling ultimately. And we study the alternating-current and direct-current （ac and de） photonic 3osephson effects in two weakly linked microcavities containing ultracold two-level atones, which allows for direct observation of the effects. This enables new investigations of the effect of maw-body physics in strongly coupled atom-cavity systems and provides a strategy for constructing novel interference devices of coherent photons. In addition, we propose the experimental protocols to observe these quantmn tunneling of Bose- Einstein condensates.     

Comparison of classical and quantum lattice properties with the aid of cellular automata

Classical and quantum lattice properties are compared by studying the effect of “local” rules applied simultaneously to all individual sites of a given array on the formation of structures on a more “global” level, where the latter are large scale patterns of evolution maps for such arrays. As a classical model, the rotation-representation matrix is used operating on an array of angular- momentum eigenstates, while for the study of a quantum mechanical lattice the previously introduced quantum mechanical cellular automata are investigated. Apart from a presentation of results which hold for each of the two classes of lattices separately, they are also compared in terms of their reversibility and/or irreversibility properties. In particular, an interesting similarity is found between classical “order through fluctuations” phenomena and properties of simple quantum mechanical lattices, which may eventually be used to model time-reversible quantum systems on the basis of underlying irreversible processes.     

低于再碰撞阈值下强激光场原子非序列双电离

在最近的实验和理论研究中,我们探讨了氩原子和氖原子在红外强激光场中低于再碰撞阈值的非序列双电离问题。在研究中,我们发现在非序列双电离过程中,氖原子的电子关联表现为在激光偏振面内肩并肩出射,而对于氩原子的电子关联行为表现为在激光偏振面内的背对背出射,我们采用三维半经典模型（考虑电子隧道电离）很好地解释了实验结果。在阈值附近,我们发现电子在激光场中的多次散射以及电子再碰撞激发后电子隧道电离是氩原子反关联行为的主要原因,而电子在激光场作用下的单次碰撞是电子关联行为的主要原因。通过测量双电离过程中产生电子的横向电子动量分布,观察到了库伦聚焦效应,我们认为这是非经典的关联行为。最后,我们给出了氩原子和氖原子在激光场中阈值的解析模型,并给出了原子的关联和反关联激光强度区域.     

Dissipative control in thermal ensembles using tunneling ionization

It has been observed that non-resonant tunneling ionization of diatomic molecules by strong laser fields can lead to readily observable coherent vibrations in the molecular ground state of both hydrogen and iodine. Moreover, we have shown that this process, called “Lochfrass” or “R-selective ionization,” produces larger amplitude coherent motion in hotter systems than cold. In contrast, reversible interactions, like bond-softening, become less effective as the temperature increases. In this paper, we present a density matrix analysis to demonstrate this unusual temperature dependence and suggest that dissipative interactions, like tunneling ionization, can provide strong-field control in hot systems.     

Application of moiré fringes in investigations of subsurface imperfections — a study of dislocations and strain fields under the reconstructed surface layer of Au(001) by scanning tunneling microscopy

It is demonstrated that the moiré fringes of the incommensurate quasihexagonal reconstructed overlayer and the underlying Au(001) substrate can be used as a “magnifier” to study subsurface imperfections with scanning tunneling microscopy, and that an unprecedented lateral precision of better than 0.1 Å can be achieved in the measurement of atomic shifts by means of scanning tunneling microscopy. As moiré fringes exist in many surfaces and adsorbate systems the method is expected to have wide applications.     

Scattering approach to backaction in coherent nanoelectromechanical systems

We present theoretical results for the backaction force noise and damping of a mechanical oscillator whose position is measured by a mesoscopic conductor. Our scattering approach is applicable to a wide class of systems; in particular, it may be used to describe point contact position detectors far from the weak tunneling limit. We find that the backaction depends not only on the mechanical modulation of transmission probabilities, but also on the modulation of scattering phases, even in the absence of a magnetic field. We illustrate our general approach with several simple examples, and use it to calculate the backaction for a movable, Au atomic point contact modeled by ab initio density functional theory.     

微纳世界的音乐之声——用低维纳米机电器件探索微观世界

纳米机电器件是具有机械运动自由度的纳米电子器件。特别有趣的是,纳米机电器件同时也构成了微纳世界的各种乐器,能够演奏出独特的音乐之声。如同宏观世界中的乐器各有各的特色,这类微纳器件在研究低维及纳米尺度的物理现象时,也展现出一些独到的潜力和优势,例如研究低维体系中的独特相变过程以及纳米材料中的力学各向异性效应等。在这些工作中,通过精密测量低维纳米材料的机械振动,即仔细倾听这些乐器所奏出的音乐之声,能够帮助研究者们观察到一些原来不易被测量的现象,进而探索新的低维物理过程和材料体系,带来新的科学发现。     

Tunneling paths in multi-dimensional semiclassical dynamics

In light of the fundamental importance and renewed interest of the tunnel phenomena, we review the recent development of semiclassical tunneling theory, particularly from the view point of “tunneling path”, beginning from a simple one-dimensional formula to semiclassical theories making use of the analytic continuation, in time, coordinates, or momentum, which are the stationary solutions of semiclassical approximations to the Feynman path integrals. We also pay special attention to the instanton path and introduce various conventional and/or intuitive ideas to generate tunneling paths, to which one-dimensional tunneling theory is applied. Then, we review the recent progress in generalized classical mechanics based on the Hamilton–Jacobi equation, in which both the ordinary Newtonian solutions and the instanton paths are regarded as just special cases. Those new complex-valued solutions are generated along real-valued paths in configuration space. Such non-Newtonian mechanics is introduced in terms of a quantity called “parity of motion”. As many-body effects in tunneling, illustrative numerical examples are presented mainly in the context of the Hamilton chaos and chemical reaction dynamics, showing how the multidimensional tunneling is affected by the system parameters such as mass combination and anisotropy of potential functions.     

Optically stimulated proton hopping in oxides at low temperatures

This communication is inspired by recent results on the observation of “giant” rates for proton transfer in rutile TiO₂ at low temperatures in pump-probe experiments. An important point is that this is not a tunneling effect. We show that this classical looking effect has a quantum mechanical origin and may be called lattice-assisted hopping. To explore the possibility of formulating transport properties in terms of mode vibrations, we use a “quantum” fluctuation–dissipation theorem, thus providing a concept of dynamic activation energy for ion hopping, which had been used in the above experimental study, rather heuristically, to fit the low-temperature “over the barrier” motion data. The resulting expression of hopping activation energy is more general than the standard one defined in units of $k_{B}T$ and is able to describe the crossover from the high to low-temperature regime of proton jumps.     

Phase transitions in random Potts systems and the community detection problem: spin-glass type and dynamic perspectives

Phase transitions in spin-glass type systems and, more recently, in related computational problems have gained broad interest in disparate arenas. In the current work, we focus on the “community detection” problem when cast in terms of a general Potts spin-glass type problem. As such, our results apply to rather broad Potts spin-glass type systems. Community detection describes the general problem of partitioning a complex system involving many elements into optimally decoupled “communities” of such elements. We report on phase transitions between solvable and unsolvable regimes. A solvable region may further split into “easy” and “hard” phases. Spin-glass type phase transitions appear at both low and high temperatures (or noise). Low-temperature transitions correspond to an “order by disorder” type effect wherein fluctuations render the system ordered or solvable. Separate transitions appear at higher temperatures into a disordered (or an unsolvable) phase. Different sorts of randomness lead to disparate behaviors. We illustrate the spin glass character of both transitions and report on memory effects. We further relate Potts type spin systems to mechanical analogs and suggest how chaotic-type behavior in general thermodynamic systems can indeed naturally arise in hard computational problems and spin glasses. The correspondence between the two types of transitions (spin glass and dynamic) is likely to extend across a larger spectrum of spin-glass type systems and hard computational problems. We briefly discuss potential implications of these transitions in complex many-body physical systems.     

On the interpretation of fermionic zero-modes

We examine the effect of fermionic zero modes on tunneling amplitudes within some simple quantum mechanical models. It is shown that the fermionic zero modes do not cause a total suppression of the tunneling, although it may be reduced. With a θ term present, due to a non-trivial topology, it is shown that the θ dependence is not eliminated by the zero modes. Instead the non-trivial topology introduces a “twist” in the fermionic coordinates which breaks a symmetry of the hamiltonian.     

Theory of resonant tunneling through the insulator conduction band of a thin film metal-insulator-metal (MIM) heterojunction

An “atomic” model of an insulating barrier between two free-electron model metals is used to investigate resonant tunneling across the insulator in the presence of a medium to large, externally applied electric field (bias). The exact numerically calculated tunneling current exhibits a pronounced oscillatory bias dependence superposed on the dominant roughly exponential tunneling characteristic. The interpretation of these results in terms of an internal field emission or Fowler-Nordheim type tunneling subject to “periodic deviations” (or interferences) seems plausible and was suggested by Maserjian. To test this conjecture, a trapezoidal barrier model of our “atomic” model analyzed numerically. As expected, the trapezoidal barrier model could only qualitatively reproduce the oscillatory bias dependence of the barrier transmissivity and of the current. Furthermore this limited agreement depends on allowing the effective mass in the barrier to become a strictly adjustable parameter. This failure of the conventional model of the junction can be interpreted as follows: (i) For moderate external (bias) fields the trapezoidal barrier fails to account for the correct position dependence of the Blochwave vector in the insulator's conduction band, hence the correct interference conditions cannot be reproduced. (ii) For large external fields the band model itself begins to fail. An explanation of oscillatory bias dependence at the tunneling current in terms of splitting of the insulator's conduction band into a set of discrete Stark levels is suggested. It is demonstrated that a fit of the oscillatory tunneling characteristics in the “Fowler-Nordheim regime” is not a reliable technique to determine the effective mass in the thin insulating film of tunneling junctions over the energy interval containing the forbidden gap and the adjoining conduction-band.