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.     

Interference of coherent and incoherent interactions in the Master Equation for open systems

The limits of validity of a well known type of Master Equation for open systems are discussed. Using Zwanzig's projector formalism we derive a “Generalized Master Equation” which describes interference processes between coherent and incoherent interactions. The conditions under which this equation reduces to the known ones is given and discussed for laser systems.     

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.     

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.     

High-order harmonic generation by atoms in a two-color laser field: Phase control of recombination radiation spectrum and duration

The feasibility of phase control over above-threshold tunnel ionization and subsequent recombination emission in two-frequency laser fields is studied. It is shown that, in such fields, we can control the instants of ionization t₀ (within optical cycle T) and recombination t _k . The conditions that minimize the characteristic times δt₀?T and δt _k ?T, within which effective ionization and recombination occur, were found. Phase control allows recombination radiation to be generated with the selection of a narrow spectral range, while additional high-frequency “background illumination” sets up high harmonic “amplification” conditions. It was shown that special two-frequency pumping with elliptically polarized radiation can generate coherent electromagnetic pulses of attosecond width. The width of the pulses decreases as the intensity of pumping increases and can reach subattosecond values. Experimental generation of such pulses may lead to a breakthrough in the development of new methods for femto-and attosecond diagnostics of fast processes.     

On the semiclassical approach to the ionization process during sputtering

The semiclassical “rate equation” approach to the ionization process during sputtering is shown to be the correct one in the limit of high temperatures. Specifically we show that simple “ rate equation” represents a high temperature asymptotic form of the equation of motion for the occupation number 〈n_a(t)〉 of relevant valence level of the sputtered particle.     

Hamiltonian and Brownian systems with long-range interactions: III. The BBGKY hierarchy for spatially inhomogeneous systems

We study the growth of correlations in systems with weak long-range interactions. Starting from the BBGKY hierarchy, we determine the evolution of the two-body correlation function by using an expansion of the solutions of the hierarchy in powers of 1/N in a proper thermodynamic limit N→+∞, where N is the number of particles. These correlations are responsible for the “collisional” evolution of the system beyond the Vlasov regime due to finite N effects. We obtain a general kinetic equation that can be applied to spatially inhomogeneous systems and that takes into account memory effects. These peculiarities are specific to systems with unshielded long-range interactions. For spatially homogeneous systems with short memory time like plasmas, we recover the classical Landau (or Lenard-Balescu) equations. An interest of our approach is to develop a formalism that remains in physical space (instead of Fourier space) and that can deal with spatially inhomogeneous systems. This enlightens the basic physics and provides novel kinetic equations with a clear physical interpretation. However, unless we restrict ourselves to spatially homogeneous systems, closed kinetic equations can be obtained only if we ignore some collective effects between particles. General exact coupled equations taking into account collective effects are also given. We use this kinetic theory to discuss the processes of violent collisionless relaxation and slow collisional relaxation in systems with weak long-range interactions. In particular, we investigate the dependence of the relaxation time with the system size N and try to provide a coherent discussion of all the numerical results obtained for these systems.     

Flow equations for the spin-boson problem

Using continuous unitary transformations recently introduced by Wegner [1], we obtain flow equations for the parameters of the spin-boson Hamiltonian. Interactions not contained in the original Hamiltonian are generated by this unitary transformation. Within an approximation that neglects additional interactions quadratic in the bath operators, we can close the flow equations. Applying this formalism to the case of Ohmic dissipation at zero temperature, we calculate the renormalized tunneling frequency. We find a transition from an untrapped to trapped state at the critical coupling constant α _c =1. We also obtain the static susceptibility via the equilibrium spin correlation function. Our results are both consistent with results known from the Kondo problem and those obtained from mode-coupling theories. Using this formalism at finite temperature, we find a transition from coherent to incoherent tunneling atT ₂ ^* ≈2T ₁ ^* , whereT ₁ ^* is the crossover temperature of the dynamics known from the NIBA.     

Polymorphic transitions in molecular crystals—III.: Transitions exhibiting unusual behavior

Some polymorphic transitions in molecular crystals, among the few classified earlier as displacive or martensitic on the basis of the existence of a definite orientation relationship between the lattices of the phases and the “instantaneous” transformation rate, have been experimentally investigated. The investigation showed no evidence of a high rate of transition. All occurred through nucleation at the lattice imperfections and continuous growth of the new phase at a rate depending on ΔT = T_tr ? T_a, where T_tr is the actual transition temperature and T_o is the temperature of the thermodynamic equilibrium of the phases. The interface can be observed; its motion can be controlled, be done so slow as it is desirable, or be stopped. The “instantaneous transformations” reported in the literature are caused by temperature delays of nucleation in the absence of appropriate lattice defects. In reality these are polymorphic transitions which appear to be instantaneous only when observed through an optical microscope; actually they occur at the rate corresponding to ordinary temperature dependence. It was established that lamination or cleavage of the crystals in a definite direction is a common feature of the substances exhibiting “atypical” behavior. (Typical behavior has been described in [1,2]). The observed characteristics of the polymorphic transitions (uniform direction of the interfaces, definite orientation relationship of the lattices, small temperature hysteresis of the transition, etc.) can be readily explained in terms of epitaxial growth of the new phase on the cleavage planes of the original phase. At the same time the role of the proximity of crystal structural parameters is elucidated. Simple model calculations showed that a coherent interface may occur when the structural differences in the junction plane of the two phases are small, while in the other cases the contact interface described in [2] is energetically advantageous. However, a cooperative rearrangement at the coherent interface is rejected. There is not any evidence that polymorphic transitions in molecular crystals may occur by means of a “shift”, a “displacement”, a “deformation”, an “overturn”, etc. It is concluded that they always occur as a result of growth. The molecular mechanism of the growth in solids has been described in [2].     

强激光场原子隧道电离的研究新进展

光与物质的相互作用一直是科学的主旋律之一.随着超强超短激光技术的快速发展,如今人们可以研究单个原子的内部世界,并调控光与电子的相互作用,从而实现了对原子内电子的超快动力学过程的探索.强激光诱导的原子隧道电离是众多强场物理现象的基石,具有重要的研究意义,也是研究前沿的热点之一.综述了强场原子隧道电离的最新研究进展,基于隧...     

Quasi-distribution of tunneling time

Using real time Feynman histories, a quasi-distribution of tunneling time Q(τ) is introduced. For the tunneling time of resident time type, an explicit expression for Q is shown for square barriers. Q becomes oscillatory as the barrier becomes opaque. Some well-known tunneling times fall within the range of τ where Q takes non-negligible values. The formal “average” and the “variance” of the tunneling time are found to be related to known tunneling times. It is thus demonstrated that the quasi-distribution extracts the temporal information about tunneling from real time Feynman histories.     

Janco’s disciples in Coulombland

As an introduction to the following two papers, we first give an explanation of the above general title. Both authors met Bernard Jancovici as a professor, and he was such an enthusiastic teacher that we felt like doing a thesis with him. This proved to be a very good idea! Indeed, Jancovici treated each of us both as a student who had much to learn and as a true collaborator from the start. Thus, we can say that we were born to scientific research thanks to him. Moreover, “Janco” also taught us skiing, hiking, wine tasting, etc. As he provided us with such a complete education, he deserves the title of our “spiritual father,” a title which he himself recognizes. In these papers, we would like to give an idea of the scientific approach which Janco taught us, and which is based on the first principles of statistical mechanics. We have chosen to exemplify this point of view through two nice problems of classical and quantum Coulomb systems which we studied after Janco addressed them alone or with us. “Coulombland” refers to systems of particles with Coulomb interactions at large distances. The Coulomb potential is defined as the solution of the Poisson equation inD dimensions. In three dimensions, it is the usual 1/r interaction, while in two dimensions, it takes a logarithmic form. The long range and the harmonicity of the Coulomb potential are responsible for a basic phenomenon called screening. A charged particle in a plasma is surrounded by a polarization cloud, whose total charge exactly compensates the charge of the particle it surrounds. Subsequently, the total effective potential created by a charge and its cloud at large distances is no longer the bare Coulomb potential, and the correlations are expected to decay faster. We review exact analytical results for the large-distance behavior of the correlations in two different situations, namely in the Kosterlitz-Thouless phase of the 2D classical Coulomb gas (Part I), and in the 3D quantum plasmas (Part II). Solvable models and systematic expansions starting from first principles exemplify Janco’s rigorous approach.     

Coherent Destruction of Tunneling of Bosons with Effective Three-Body Interactions

The tunneling dynamics of dilute boson gases with three-body interactions in a periodically driven double wells are investigated both theoretically and numerically.In our findings,when the system is with only repulsive twobody interactions or only three-body interactions,the tunneling will be suppressed;while in the case of the coupling between two- and three-body interactions,the tunneling can be either suppressed or enhanced.Particularly,when attractive three-body interactions are twice large as repulsive two-body interactions,CDT occurs at isolated points of driving force,which is similar to the linear case.Considering different interaction,the system can experience different transformation from coherent tunneling to coherent destruction of tunneling(CDT).The quasi-energy of the system as the function of the periodically driving force shows a triangular structure,which provides a deep insight into the tunneling dynamics of the system.     

Strange Heat Flux in (An)Harmonic Networks

We study the heat transport in systems of coupled oscillators driven out of equilibrium by Gaussian heat baths. We illustrate with a few examples that such systems can exhibit “strange” transport phenomena. In particular, circulation of heat flux may appear in the steady state of a system of three oscillators only. This indicates that the direction of the heat fluxes can in general not be “guessed” from the temperatures of the heat baths. Although we primarily consider harmonic couplings between the oscillators, we explain why this strange behavior persists under weak anharmonic perturbations.     

Strict monotonicity for critical points in percolation and ferromagnetic models

When is the numerical value of the critical point changed by an enhancement of the process or of the interaction? Ferromagnetic spin models, independent percolation, and the contact process are known to be endowed with monotonicity properties in that certain enhancements are capable of shifting the corresponding phase transition in only an obvious direction, e. g., the addition of ferromagnetic couplings can only increase the transition temperature. The question explored here is whether enhancements do indeed change the value of the critical point. We present a generally applicable approach to this issue. For ferromagnetic Ising spin systems, with pair interactions of finite range ind?2 dimensions, it is shown that the critical temperatureT _c is strictly monotone increasing in each coupling, with the first-order derivatives bounded by positive functions which are continuous on the set of fullyd-dimensional interactions. For independent percolation, with 0<p _c<1, we prove that any “essential enhancement” of the process has an effect on the critical probability, a result with applications to the question of the existence of “entanglements” and to invasion percolation with trapping.     

Nanoelectromechanics and single-charge tunneling

The coupling of electronic and mechanical degrees of freedom has important consequences in nanoscale systems, as emphasized in recent theoretical and experimental work. In particular, the electrical properties of composite nanosystems containing elements with quite different abilities to conduct electricity and with different mechanical properties have been found to be strongly affected. Here we briefly review some of our recent work on the nanoelectromechanics of “heteroconducting” and “heteroelastic” Coulomb blockade systems, where single charge tunneling is the dominant conduction mechanism. We examplify nanoelectromechanical effects both in normal and superconducting systems by discussing (i) a self-assembled single-electron tunneling device exhibiting a dynamical instability leading to “shuttling” of electrons by a movable Coulomb dot and (ii) shuttling of Cooper pairs by a movable single-Cooper-pair box.     

Revisiting the hopes for scalable quantum computation

The hopes for scalable quantum computing rely on the “threshold theorem”: once the error per qubit per gate is below a certain value, the methods of quantum error correction allow indefinitely long quantum computations. The proof is based on a number of assumptions, which are supposed to be satisfied exactly, like axioms, e.g., zero undesired interactions between qubits, etc. However, in the physical world no continuous quantity can be exactly zero, it can only be more or less small. Thus the “error per qubit per gate” threshold must be complemented by the required precision with which each assumption should be fulfilled. In the absence of this crucial information, the prospects of scalable quantum computing remain uncertain.     

A study of Fe-doped Bi-Sr-Ca-Cu superconductors

We have substituted 1.5% of Fe for Cu in several “2212” and “2223” Bi-Sr-Ca-Cu superconductors. All of the samples show a reduction ofT _c about 13 K due to the Fe impurities. Mössbauer measurements at room temperature reveal the structural characteristics such as stacking faults and intergrowth of different phases in these Bi-based compounds on the microscopic scale. The susceptibility ofT _c to Fe-doping in the Bi-“2212” or “2223” system is comparable to that of the “123” system but much smaller than that of the “214” system. The interplanar correlation existing in the “123” and the Bi-“2212” and “2223” systems seems to play an important role in sustaining the high temperature superconductivity and weakening the detrimental effect of impurity elements on superconductivity in these two systems.