Current reversals in an inhomogeneous system with asymmetric unbiased fluctuations

We present a study of the transport of a Brownian particle moving in a periodic symmetric potential in the presence of asymmetric unbiased fluctuations. The particle is considered to move in a medium with periodic space dependent friction. By tuning the parameters of the system, the direction of the current exhibits reversals, both as a function of temperature as well as the amplitude of rocking force. We found that the mutual interplay between the opposite driving factors is the necessary term for current reversals.Received: 11 October 2003, Published online: 9 April 2004PACS: 05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion - 02.50.Ey Stochastic processes - 87.10. + e General theory and mathematical aspects     

Revisiting the quantum group symmetry of diatomic molecules

We propose a q-deformed model of anharmonic vibrations in diatomic molecules. We study the applicability of the model to the phenomenological Dunham expansion by comparison with experimental data. In contrast with other applications where it is difficult to find a physical interpretation for the deformation parameter, q, in our analysis it is directly related to the third-order coefficient in the Dunham expansion. We study the consistency of the parameters that determine the q-deformed system by comparing them with the vibrational terms fitted to 161 electronic states of diatomic molecules. We show how to include both positive and negative anharmonicities in a simple and systematic way.Received: 16 July 2004, Published online: 24 August 2004PACS: 33.15.Mt Rotation, vibration, and vibration-rotation constants - 02.20.Uw Quantum groups - 31.15.Hz Group theory - 03.65.Fd Algebraic methods - 02.20.-a Group theoryV.K. Dobrev: Permanent address, and after 30 April 2004: Institute of Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 1784, Sofia, Bulgaria     

Stationary shear flows of dense granular materials: a tentative continuum modelling

We propose a simple continuum model to interpret the shearing motion of dense, dry and cohesion-less granular media. Compressibility, dilatancy and Coulomb-like friction are the three basic ingredients. The granular stress is split into a rate-dependent part representing the rebound-less impacts between grains and a rate-independent part associated with long-lived contacts. Because we consider stationary flows only, the grain compaction and the grain velocity are the two main variables. The predicted velocity and compaction profiles are in apparent qualitative agreement with most of the experimental or numerical results concerning free-surface shear flows as well as confined shear flows.Received: 24 March 2004, Published online: 29 June 2004PACS: 45.70.Ht Avalanches - 45.70.-n Granular systems - 83.80.Fg Granular solids     

Two-step spin conversion and other effects in the atom-phonon coupling model

We study an atom-phonon coupling model introduced recently for spin-conversion phenomenon. The originality of this model, performed on a linear chain of atoms, is that the elastic force constant values of the spring linking two atoms depends on their electronic states. This leads to introduce naturally in the chain long- and short-range interactions, which appear respectively like a Zeeman and an exchange interactions. The exchange-like interaction can be ferro-, antiferro- or equal to zero. The effects of long-range interactions have already been studied. Here we study those of the short-range interaction. Some parts of the chain phase diagram are analysed and the main features of the experimental behaviours of spin conversion compounds are qualitatively reproduced.Received: 2 February 2004, Published online: 29 June 2004PACS: 63.20.Kr Phonon-electron and phonon-phonon interactions - 63.50. + x Vibrational states in disordered systems - 64.60.-i General studies of phase transitions     

Memory effects in classical and quantum mean-field disordered models

We apply the Kovacs experimental protocol to classical and quantum p-spin models. We show that these models have memory effects as those observed experimentally in super-cooled polymer melts. We discuss our results in connection to other classical models that capture memory effects. We propose that a similar protocol applied to quantum glassy systems might be useful to understand their dynamics.Received: 14 April 2004, Published online: 30 September 2004PACS: 75.10.Nr Spin-glass and other random models - 75.40.Mg Numerical simulation studies - 05.70.Ln Nonequilibrium and irreversible thermodynamics     

Magnetocaloric effect in transition metals based compounds: a theoretical approach

In this paper we study the magnetocaloric effect in transition metals based compounds. For this purpose, we use a microscopical model, based on the band theory of magnetism, where the magnetic lattice is coupled with the crystalline lattice and with the external magnetic field. We apply the model to calculate the magnetocaloric effect in the compound MnAs, which undergoes a first order magnetic phase transition. The theoretically calculated isothermal entropy changes and the adiabatic temperature changes upon magnetic field variation exhibit a good agreement with the available experimental data.Received: 7 April 2004, Published online: 31 August 2004PACS: 75.30.Sg Magnetocaloric effect, magnetic cooling - 75.10.Lp Band and itinerant models - 75.20.En Metals and alloys     

New directions in degenerate dipolar molecules via collective association

We survey results on the creation of heteronuclear Fermi molecules by tuning a degenerate Bose-Fermi mixture into the neighborhood of an association resonance, either photoassociation or Feshbach, as well as the subsequent prospects for Cooper-like pairing between atoms and molecules. In the simplest case of only one molecular state, corresponding to either a Feshbach resonance or one-color photoassociation, the system displays Rabi oscillations and rapid adiabatic passage between a Bose-Fermi mixture of atoms and fermionic molecules. For two-color photoassociation, the system admits stimulated Raman adiabatic passage (STIRAP) from a Bose-Fermi mixture of atoms to stable Fermi molecules, even in the presence of particle-particle interactions. By tailoring the STIRAP sequence it is possible to deliberately convert only a fraction of the initial atoms, leaving a finite fraction of bosons behind to induce atom-molecule Cooper pairing via density fluctuations; unfortunately, this enhancement is insufficient to achieve a superfluid transition with present ultracold technology. We therefore propose the use of an association resonance that converts atoms and diatomic molecules (dimers) into triatomic molecules (trimers), which leads to a crossover from a Bose-Einstein condensate of trimers to atom-dimer Cooper pairs. Because heteronuclear dimers may possess a permanent electric dipole moment, this overall system presents an opportunity to investigate novel microscopic physics.Received: 16 June 2004, Published online: 21 September 2004PACS: 03.75.Ss Degenerate Fermi gases - 05.30.Fk Fermion systems and electron gas - 34.10. + x General theories and models of atomic and molecular collisions and interactions (including statistical theories, transition state, stochastic and trajectory models, etc.) - 74.20.Mn Nonconventional mechanisms (spin fluctuations, polarons and bipolarons, resonating valence bond model, anyon mechanism, marginal Fermi liquid, Luttinger liquid, etc.) - 21.10.-k Properties of nuclei; nuclear energy levels     

Random anisotropy nematic model: Connection with experimental systems

We study theoretically the phase behavior of the continuum Random Anisotropy Nematic model. A domain-type pattern is assumed to appear in a distorted nematic liquid crystal (LC) phase. We map the model parameters to physical quantities characterizing LCs confined to Controlled-Pore Glasses and LC-aerosil dispersions. The domain size dependence on the disorder strength is obtained in accordance with the Imry-Ma prediction. The model estimates for temperature shifts of the paranematic-nematic phase transition and for the critical point, where this transition ceases to exist, are compared to the available experimental results.Received: 28 March 2004, Published online: 29 June 2004PACS: 61.30.-v Liquid crystals - 61.30.Dk Continuum models and theories of liquid crystal structure - 61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions     

Signal mixing in a ratchet device: commensurability and current control

Rectification current in overdamped ratchets can be easily controlled by applying two driving signals and tuning either their relative phase or their frequency ratio. The interplay of the two inputs generates intriguing transport mechanisms that can be implemented to optimize shuttling and separation of particles in a variety of physical and technological applications.Received: 11 April 2004, Published online: 13 July 2004PACS: 05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion - 05.60.Cd Classical transport - 87.16.Uv Active transport processes; ion channels     

Enhancement of high-order harmonic generation at tuned wavelengths through adaptive control

An adaptive learning loop enhances the efficiency and tuning of high-order harmonic generation. In comparison with simple chirp tuning, we observe a broader tuning range and a twofold to threefold enhancement in integrated photon flux in the cutoff region. The driving pulse temporal phase varies significantly for different tunings and is more complicated than a simple chirp. We compare our experimental results with a one-dimensional, time-dependent model that incorporates the intrinsic atomic response, the experimental pulse temporal phase, ionization effects, and transverse coherence of the spatial mode of the laser. The model agrees with our experimental results and indicates that a specific quantum path coupled with ionization effects determines the optimized harmonic spectrum.     

Quantized charge pumping by surface acoustic waves in ballistic quasi-1D channels

Adiabatic pumping of electrons induced by surface acoustic waves (SAWs) in a ballistic quasi-1D quantum channel is considered using an exactly solvable tight-binding model for non-interacting electrons. The single-electron degrees of freedom, responsible for acoustoelectric current quantization, are related to the transmission resonances. We study the influence of experimentally controllable parameters (SAW power, gate voltage, source-drain bias, amplitude and phase of a secondary SAW beam) on the plateau-like structure of the acoustoelectric current. The results are consistent with existing experimental observations.Received: 24 February 2004, Published online: 12 July 2004PACS: 73.23.-b Electronic transport in mesoscopic systems - 73.50.Rb Acoustoelectric - magnetoacoustic effects - 73.40.Ei Rectification     

Effect of orbital currents on the restricted optical conductivity sum rule

We derive the restricted optical-conductivity sum rule for a model with circulating orbital currents. It is shown that an unusual coupling of the vector potential to the interaction term of the model Hamiltonian results in a non-standard form of the sum rule. As a consequence, the temperature dependence of the restricted spectral weight could be compatible with existing experimental data for high-T _c cuprates above the critical temperature T _c . We extend our results to the superconducting state, and comment on the differences and analogies between these two symmetry-breaking phenomena.Received: 30 April 2004, Published online: 23 July 2004PACS: 71.10.-w Theories and models of many-electron systems - 74.25.Gz Optical properties - 72.15.-v Electronic conduction in metals and alloys - 74.72.-h Cuprate superconductors (high-T _c and insulating parent compounds)S.G. Sharapov: Present address: Institute for Scientific Interchange, via Settimio Severo 65, 10133 Torino, Italy     

Theoretical analysis of a quartz-enhanced photoacoustic spectroscopy sensor

Quartz-enhanced photoacoustic spectroscopy (QEPAS) sensors are based on a recent approach to photoacoustic detection which employs a quartz tuning fork as an acoustic transducer. These sensors enable detection of trace gases for air quality monitoring, industrial process control, and medical diagnostics. To detect a trace gas, modulated laser radiation is directed between the tines of a tuning fork. The optical energy absorbed by the gas results in a periodic thermal expansion which gives rise to a weak acoustic pressure wave. This pressure wave excites a resonant vibration of the tuning fork thereby generating an electrical signal via the piezoelectric effect. This paper describes a theoretical model of a QEPAS sensor. By deriving analytical solutions for the partial differential equations in the model, we obtain a formula for the piezoelectric current in terms of the optical, mechanical, and electrical parameters of the system. We use the model to calculate the optimal position of the laser beam with respect to the tuning fork and the phase of the piezoelectric current. We also show that a QEPAS transducer with a particular 32.8 kHz tuning fork is 2–3 times as sensitive as one with a 4.25 kHz tuning fork. These simulation results closely match experimental data.     

Enthalpy relaxation of polymers: comparing the predictive power of two configurational entropy models extending the AGV approach

The Tool-Narayanaswamy-Moynihan (TNM) phenomenological model is widely accepted in order to describe the structural relaxation of glasses. However several quantitative discrepancies can be found in the literature that cannot be entirely ascribed to the experimental errors. In this work we compare the predictive power of two recently proposed configurational entropy approaches extending the TNM formalism. Both of them change the treatment of non linearity by adding a free parameter. We use Differential Scanning Calorimetry (DSC) experiments in order to test the models in two different polymers. One of them is a commercial PMMA sample, the other is a side chain liquid crystal azo-benzene polymer properly synthesized for optical nanorecording purposes. Different results were found for the two systems. In the PMMA sample only one of the new models was able to improve the agreement between DSC experiments and theory with respect to the TNM model, whereas in the second polymer both the approaches were able to describe the experiments better than TNM model.Received: 25 February 2004, Published online: 21 October 2004PACS: 64.70.Pf Glass transitions - 61.43.Fs Glasses - 61.41. + e Polymers, elastomers, and plastics     

Novel silicon-carbon fullerene-like cages

We present model calculations of kinetic energy releases and fission barriers in asymmetric fission of C₆₀^{r +} ions, using a simple electrostatic model where the fragments are treated as conducting spheres. The kinetic energy releases are calculated using two different approaches for deducing the radii of the spheres. Both approaches give results in qualitative agreement with experimental results. The fission barriers, on the other hand, depend strongly on the model radii and the activation energies for neutral fragment emission. A comparison between the model calculations shows that the choice of the finite size of the smaller fragments become important as r increases and have large influences on the prediction for the C₆₀^{r +} stability limit. The competition between neutral (evaporation) and charged-fragment emission (fission) are discussed within a static over-the-barrier model for electron transfer between conducting spheres.Received: 10 December 2003, Published online: 27 January 2004PACS: 34.70. + e Charge transfer - 36.40.Qv Stability and fragmentation of clusters - 36.40.Wa Charged clusters     

Quantum well states in thin (110)-oriented Au films and k-space symmetry

We study the one- and two-dimensional extended Hubbard model by means of the Composite Operator Method within the 2-pole approximation. The fermionic propagator is computed fully self-consistently as a function of temperature, filling and Coulomb interactions. The behaviors of the chemical potential (global indicator) and of the double occupancy and nearest-neighbor density-density correlator (local indicators) are analyzed in detail as primary sources of information regarding the instability of the paramagnetic (metal and insulator) phase towards charge ordering driven by the intersite Coulomb interaction. Very rich phase diagrams (multiple first and second order phase transitions, critical points, reentrant behavior) have been found and discussed with respect to both metal-insulator and charge ordering transitions: the connections with the experimental findings relative to some manganese compounds are analyzed. Moreover, the possibility of improving the capability of describing cuprates with respect to the simple Hubbard model is discussed through the analysis of the Fermi surface and density of states features. We also report about the specific heat behavior in presence of the intersite interaction and the appearance of crossing points.Received: 2 July 2004, Published online: 12 October 2004PACS: 71.10.-w Theories and models of many-electron systems - 71.10.Fd Lattice fermion models (Hubbard model, etc.) - 71.27. + a Strongly correlated electron systems; heavy fermions     

在BBO晶体中获得271—281.5nm和频输出

本文用自制的Q-YAG的倍频光泵浦Rh·6G染料,用调谐的染料激光束(562—598nm)与YAG的倍频光束在B-BaB2O4(BBO)晶体中和频,获得的连续可调谐范围是271-281.5nm,和频输出能量大子200μJ,最高转换效率为5%.我们用陈创天等人和K.Kato所给BB0晶体的Sellmetier方程计算了和频调谐曲线,它们之间的相位匹配角大约有一度之差.实验曲线与两条理论曲线都有不同程度的偏差.但是,与K.Kato的理论曲线更为接近.我们还讨论了BBO晶体的长度对输出能量的影响,并解释了产生这种影响的原因。     

Evolving complex food webs

We describe the properties of a model which links the ecology of food web structure with the evolutionary dynamics of speciation and extinction events; the model describes the dynamics of ecological communities on an evolutionary timescale. Species are defined as sets of characteristic features, and these features are used to determine interaction scores between species. A realistic population dynamics, which incorporates these scores, is used to determine the changes in population sizes on ecological time scales and so determine mean population sizes. We display typical examples of food webs constructed using the model and comment on the good agreement which is found between the model predictions and data on real webs.Received: 18 January 2004, Published online: 14 May 2004PACS: 87.23.Kg Dynamics of evolution - 87.23.Cc Population dynamics and ecological pattern formation - 89.75.Fb Structures and organization in complex systems     

Experimental puzzle of <Superscript>5</Superscript><Emphasis Type="Italic">H</Emphasis>

The experimental evidences for ⁵H are discussed. The results of the recent experiments are controversial. We make a comparison of the experimental data with theoretical calculations (L.V. Grigorenko, N.K. Timofeyuk, M.V. Zhukov, Eur. Phys. J. A 19, 181 (2004)) and try to find a consistent explanation for the current experimental situation. We conclude that more detailed experimental information is required to resolve the existing experimental ambiguity.Received: 22 April 2003, Revised: 8 November 2003, Published online: 18 June 2004PACS: 21.45. + v Few-body systems - 21.60.Gx Cluster models - 25.10. + s Nuclear reactions involving few-nucleon systems     

Electro-optic spectral tuning in a continuous-wave, asymmetric-duty-cycle, periodically poled LiNbO(3) optical parametric oscillator

We demonstrate electro-optic spectral tuning in a continuous-wave periodically poled LiNbO(3) (PPLN) optical parametric oscillator (OPO). We achieve 8.91 cm(-1) of rapid spectral tuning, with a linear tuning rate of 2.89 cm(-1) /(kV/mm), by applying electric fields up to +/-1.5 kV/mm across the crystal while it is operating within the OPO. Intentionally poling the PPLN crystal with an asymmetric domain structure enables tuning, and numerical predictions closely match the experimental observations. The tuning is considerably larger than the typical operational bandwidth of the OPO, indicating that we are in fact shifting the gain curve of the PPLN crystal.