Exchange dipole interaction in a multilevel cooperative system of atoms

Atomic systems with three or more equidistant energy levels, in which a cascade process is possible, are considered. Hamiltonians obtained for such systems are analogous to Heisenberg Hamiltonians, but for systems with integral spins. For Dicke states in multilevel systems, quantum-mechanical mean values of the energy of a cooperative system are calculated taking into account weak interactions between atoms. The type of emission preceding superradiant avalanche emission of the system is analyzed. It can be expected that a coherent state may be formed, in which collective processes affect one another not only via population of the intermediate common layer, but also via phasing of pure quantum states. The single superfluorescence pulse that can be formed in this case is not a simple superposition of two or more pulses of sequential superradiant transitions in two-level systems.     

Wilson's renormalization group applied to 2D lattice electrons in the presence of van Hove singularities

The weak coupling instabilities of a two dimensional Fermi system are investigated for the case of a square lattice using a Wilson renormalization group scheme to one loop order. We focus on a situation where the Fermi surface passes through two saddle points of the single particle dispersion. In the case of perfect nesting, the dominant instability is a spin density wave but d-wave superconductivity as well as charge or spin flux phases are also obtained in certain regions in the space of coupling parameters. The low energy regime in the vicinity of these instabilities can be studied analytically. Although saddle points play a major role (through their large contribution to the single particle density of states), the presence of low energy excitations along the Fermi surface rather than at isolated points is crucial and leads to an asymptotic decoupling of the various instabilities. This suggests a more mean-field like picture of these instabilities, than the one recently established by numerical studies using discretized Fermi surfaces. Received 11 April 2001 and Received in final form 6 September 2001     

Molecular dynamics simulations on the wet/dry self-latching and electric fields triggered wet/dry transitions between nanosheets: A non-volatile memory nanostructure

We design a nanostructure composing of two nanoscale graphene sheets parallelly immersed in water.Using molecular dynamics simulations,we demonstrate that the wet/dry state between the graphene sheets can be self-latched;moreover,the wet→dry/dry→wet transition takes place when applying an external electric field perpendicular/parallel to the graphene sheets(E;/E;).This structure works like a flash memory device(a non-volatile memory):the stored information(wet and dry states)of the system can be kept spontaneously,and can also be rewritten by external electric fields.On the one hand,when the distance between the two nanosheets is close to a certain distance,the free energy barriers for the transitions dry→wet and wet→dry can be quite large.As a result,the wet and dry states are self-latched.On the other hand,an E;and an E;will respectively increase and decrease the free energy of the water located in-between the two nanosheets.Consequently,the wet→dry and dry→wet transitions are observed.Our results may be useful for designing novel information memory devices.     

Rate processes in condensed media: Basic equations

A general theory of rate processes is developed. Starting from the first principles, the non-Markovian and Markovian type equations governing relaxation processes are derived. Under certain conditions (which are specified) these equations may be approximately reduced to master equations.The theory is applied to two specific models. In one of them the electron-nuclear system is represented by two intersecting electronic energy hyper-surfaces with a continuum of degrees of freedom plus a small perturbation causing transitions between these electronic states. The equations determining the time behaviour of the electronic subsystem in the general case do not coincide with master equations and the time evolution of the system has mixed oscillatory decaying behaviour.Another model takes into account a possible competition between electronic and vibrational relaxations. The corresponding kinetic equations are derived.     

Mobility edges and reentrant localization in one-dimensional dimerized non-Hermitian quasiperiodic lattice

The mobility edges and reentrant localization transitions are studied in one-dimensional dimerized lattice with nonHermitian either uniform or staggered quasiperiodic potentials.We find that the non-Hermitian uniform quasiperiodic disorder can induce an intermediate phase where the extended states coexist with the localized ones,which implies that the system has mobility edges.The localization transition is accompanied by the PT symmetry breaking transition.While if the non-Hermitian quasiperiodic disorder is staggered,we demonstrate the existence of multiple intermediate phases and multiple reentrant localization transitions based on the finite size scaling analysis.Interestingly,some already localized states will become extended states and can also be localized again for certain non-Hermitian parameters.The reentrant localization transitions are associated with the intermediate phases hosting mobility edges.Besides,we also find that the non-Hermiticity can break the reentrant localization transition where only one intermediate phase survives.More detailed information about the mobility edges and reentrant localization transitions are presented by analyzing the eigenenergy spectrum,inverse participation ratio,and normalized participation ratio.     

Ultrahigh frequency additional background radiation of the lower ionosphere during strong geomagnetic disturbances

In periods of high solar activity and the formation of geomagnetic storms, additional background incoherent ultrahigh frequency (UHF) radiation with decimeter to millimeter wavelengths in the high E and D layers of the Earth’s ionosphere is generated. This emission is produced by transitions between Rydberg states of atoms and molecules of atmospheric gases, which are excited by electrons and are surrounded by a neutral species of the medium. At present, there is no reliable information on the integrated intensity of UHF radiation in this wavelength range. This problem can be solved on knowledge of the dynamics of collisional and radiative quenching of Rydberg states and of the kinetics of their population in the lower ionosphere. An analysis of the available experimental data shows that the radiation is generated in an atmospheric layer located at altitudes between 50 and 110 km. The current theory is discussed and the ways of its further improvement connected with the development of more rigorous theoretical methods for describing the effect of neutral particles of medium on the collisional and radiative quenching dynamics, including the elementary processes with participation of the nitrogen and oxygen molecules, are suggested. For quantitatively estimates the influence of excited particles on the incoherent UHF radiation of the atmosphere, it is necessary carrying out of the preliminary calculations the potential energy surfaces and dynamics of nonadiabatic transitions between Rydberg states, construction the electronic wave functions, and determination the dipole moments of the allowed transitions and the emission line shapes. Obtained results can be included into the general kinetic scheme which defines of the UHF radiation intensity versus the density and temperature of the atmosphere. Accompanying its infrared (IR) radiation can be used to define of Rydberg states.     

Optical transitions in single-wall boron nitride nanotubes

Optical transitions in single-wall boron nitride nanotubes are investigated by means of optical absorption spectroscopy. Three absorption lines are observed. Two of them (at 4.45 and 5.5 eV) result from the quantification involved by the rolling up of the hexagonal boron nitride (h-BN) sheet. The nature of these lines is discussed, and two interpretations are proposed. A comparison with single-wall carbon nanotubes leads one to interpret these lines as transitions between pairs of van Hove singularities in the one-dimensional density of states of boron nitride single-wall nanotubes. But the confinement energy due to the rolling up of the h-BN sheet cannot explain a gap width of the boron nitride nanotubes below the h-BN gap. The low energy line is then attributed to the existence of a Frenkel exciton with a binding energy in the 1 eV range.     

Stochastic aspects of pattern formation during the catalytic oxidation of CO on Pd(111) surfaces

We present experimental results on rare transitions between two states due to intrinsic noise between two states in a bistable surface reaction, namely the catalytic oxidation of CO on Pd(111) surfaces. The mean time scales involved are typically of order 10⁴ s and the probability distribution shows two peaks over a large part of the bistable regime of this surface reaction. We use measurements of the resulting CO₂ rate as well as photoelectron emission microscopy (PEEM) to characterize these rare transitions. From our dynamic data we can extract probability distributions for the CO₂ rate. We use x-t plots from PEEM measurements to describe the transitions, which are-as we demonstrate-characterized by one wall moving through the field of view in PEEM measurements. The resulting probability distributions for the CO₂ rate are shown to depend strongly on the value, Y, of the CO fraction in the reactant flux inside the bistable regime. We find that the probability distribution is strongly asymmetric indicating that the two basins of attraction are rather different in depth and width. This is also concluded from the PEEM measurements, which show in one case a rather sharp and narrow domain wall going one way, while it is rather wide and diffuse for the motion in the opposite direction. To have two basins of attraction in the bistable regime, which are rather different in nature is reminiscent of other bistable systems such as, for example, optical bistability, although the time scales involved in the present system are entirely different.     

Glassy phases in a layered ising model with random interlayer exchange

Thermodynamics of a layered Ising model with infinite-range ferromagnetic intralayer interaction and random nearest-neighbor interlayer coupling is considered. A detailed analysis of the model with vanishing average interlayer coupling is presented. The Gibbs free energy is found in the critical region, and the existence of many metastable states is demonstrated. Thermodynamic parameters of the system are found for periodic states. As the mean square interlayer coupling increases, the equilibrium state of the system undergoes an infinite sequence of first-order phase transitions, the number of magnetic planes and the distance between them change discontinuously, and so do both bulk magnetization and magnetic susceptibility.     

Stochastic resonance for adhesion of membranes with active stickers

The behavior of two membranes that interact by active adhesion molecules or stickers is studied theoretically using mean-field theory and Monte Carlo simulations. The stickers are anchored in one of the membranes and undergo conformational transitions between on and off states. In their on states, the stickers can bind to ligands that are anchored in the other membrane. The transitions between the on and off states arise from the coupling of the stickers to some active, energy-releasing process, which keeps the system out of equilibrium. As one varies the transition rates of this active process, the membrane separation undergoes a stochastic resonance: this separation is maximal at intermediate rates of the sticker transitions and considerably smaller both at high and at low transition rates. This implies that the effective, fluctuation-induced repulsion between the membranes contains a rate-dependent contribution that arises from the switching of the active stickers.     

Statistics of internal excitations of atomic systems

The character of internal excitations is compared for phase transitions and chemical transitions in atomic systems. Although the temperature dependences of some physical parameters of atomic systems have resonance-like structures with maxima in both cases, the dependences of the partition functions on the number of elementary excitations or the excitation energy differ because of the difference in the numbers of interactions that govern the transitions. The phase changes of condensed rare gases are considered in the case where the external pressure is small and the differences between phases are predominantly associated with differences in configurations. Important energy parameters of rare gases are determined by the attractive part of the pairwise interaction potential between atoms. The statistical analysis shows the existence of a “freezing limit” temperature for these systems, below which the liquid state becomes unstable. The kinetics of decay of such unstable states is analyzed in terms of the diffusion of voids.     

Absorption spectrum of a resonantly driven degenerate V-type atom with a dc-field coupling between excited states

We study the absorption spectra of a degenerate V-type atom, where a resonant driving field and a probe field drive different branches of transitions and a dc field is applied to drive the transition between two excited states. The effects of vacuum induced coherence (VIC) on the absorption spectra are investigated. It is demonstrated that in some special cases the VIC can lead to the depression of absorption and narrow resonance. The origin of these features are discussed. When the pump field and the dc field have the same intensity, it is interesting to find that the whole absorption spectrum comes mainly from the absorptions induced by the interferences among different transitions between dressed states.     

On mechanisms of the helix pitch variation in a thin cholesteric layer confined between two surfaces

Two possible mechanisms of the temperature-induced variation and jump of the helix pitch in a spatially bounded planar layer of a cholesteric liquid crystal (LC) was considered within the framework of the continuum theory of elasticity. These mechanisms are related to the existence of two configuration curves of the system free energy. The states with local free energy minima on each of the configuration curves correspond to topologically equivalent configurations of the LC director distribution and are quasi-equivalent in this sense. The transitions between such quasi-equivalent states are especially important in the first mechanism of the helix pitch jump proceeding without participation of defects. The second mechanism is related to transitions between the ground states of different configuration curves corresponding to topologically nonequivalent configurations. This mechanism requires either participation of disclination lines or the formation of defects.     

Study of two‐electron jumps in relaxation of Coulomb glasses

A long‐standing debate in the theory of hopping insulators concerns the role of multi‐electron transitions in the dynamics of the system. The natural assumption is that as temperature is lowered, two‐electron transitions will play an increasingly important role since they provide a way of tunneling through additional energy barriers which would be energetically unfavorable as successive one‐electron transitions. This was disputed in [1], but later it was seen in [2]. The reason for this discrepancy is not clear and deserves further attention. One point where the two approaches diverged was in the selection and weighting of the two‐electron transitions relative to one‐electron transitions. We present calculations of the transition rates to second order in the tunneling matrix element, which will be used in improved numerical studies. We compare results for only one‐electron jumps with results including also two‐electron jumps.     

Nonradiative relaxation processes in molecular crystals

Internal conversion is the dominant relaxation channel from higher lying excited states in molecular crystals and involves the transfer of energy from the electronic system to the lattice. In this work, we present results from simulations of the nonradiative relaxation process with an emphasis on both intra- and interband transitions. We find the internal conversion process to be strongly nonadiabatic and the associated relaxation time in the case of large energy excitations to be limited by the transitions made between states of different bands.     

Towards quantum cellular automata operation in silicon: transport properties of silicon multiple dot structures

The transport properties of two adjacent double dots, realized in silicon is studied by DC-measurements at a temperature of 4.2 K. From the measured charging diagrams the capacitances within the structure are estimated and using this information as input for a simple electrostatic model the energy scales for electron transitions between two dots are calculated. The change of energy in one pair introduced by an interdot electron transition in the other, the basic transition for the operation of quantum cellular automata, is shown to be less than the thermal energy under the present conditions. Adding an extra electron into the system causes larger energy changes in the double dots and for such transitions a clear correlation in electron transport for both double dots can be observed. The size of this effect complies with our model and the estimated capacitances and can therefore be used as a base for future cell design.     

Stochastic resonance and energy optimization in spatially extended dynamical systems

We investigate a class of nonlinear wave equations subject to periodic forcing and noise, and address the issue of energy optimization. Numerically, we use a pseudo-spectral method to solve the nonlinear stochastic partial differential equation and compute the energy of the system as a function of the driving amplitude in the presence of noise. In the fairly general setting where the system possesses two coexisting states, one with low and another with high energy, noise can induce intermittent switchings between the two states. A striking finding is that, for fixed noise, the system energy can be optimized by the driving in a form of resonance. The phenomenon can be explained by the Langevin dynamics of particle motion in a double-well potential system with symmetry breaking. The finding can have applications to small-size devices such as microelectromechanical resonators and to waves in fluid and plasma.     

The heat and work of quantum thermodynamic processes with quantum coherence

Energy is often partitioned into heat and work by two independent paths corresponding to the change in the eigenenergies or the probability distributions of a quantum system. The discrepancies of the heat and work for various quantum thermodynamic processes have not been well characterized in literature. Here we show how the work in quantum machines is differentially related to the isochoric, isothermal, and adiabatic processes. We prove that the energy exchanges during the quantum isochoric and isothermal processes are simply depending on the change in the eigenenergies or the probability distributions. However, for a time-dependent system in a non-adiabatic quantum evolution, the transitions between the different quantum states representing the quantum coherence can affect the essential thermodynamic properties, and thus the general definitions of the heat and work should be clarified with respect to the microscopic generic time-dependent system. By integrating the coherence effects in the exactly-solvable dynamics of quantum-spin precession, the internal energy is rigorously transferred as the work in the thermodynamic adiabatic process. The present study demonstrates that the quantum adiabatic process is sufficient but not necessary for the thermodynamic adiabatic process.     

Quantum collinear reaction probabilities for vibrationally excited reactants : F+H2(v≤2)→FH(v′≤5)+H

Quantum mechanical reaction probabilities are reported for the collinear reaction F+H₂(v)→FH(v′)+H in an energy range where two or three vibrational H₂ states v are open. Rotated Morse-cubic spline representations of the extended LEPS surfaces Muckerman I and V, and an adaptation of the BOPS SCFCI surface have been used. The scattering is dominated by resonances. A detailed investigation of the different kinds of resonance behaviour is presented. For the two LEPS surfaces, overall features of the reaction probability curves can be correlated qualitatively in a one-to-one manner. Differences between the BOPS and LEPS reaction probabilities are more pronounced for v=1 than for v=0. For all surfaces, effects of vibrational excitation show a much more systematic behaviour in terms of the reverse reaction than for the forward reaction. A multi-step mechanism is deduced for the reaction, and an attempt is made to give an interpretation in terms of physical concepts including centrifugal effects, Franck-Condon transitions and quasibound states. No obvious simple trends emerge from a surprisal analysis. Most surprisal plots are markedly non-linear in the energy range considered.