Effects of an applied low frequency field on the dynamics of a two-level atom interacting with a single-modefield

The effects of an applied low frequency field on the dynamics of a two-level atom interacting with a single-mode field are investigated. It is shown that the time evolution of the atomic population is mainly controlled by the coupling constants and the frequency of the low frequency field, which leads to a low frequency modulation function for the time evolution of the upper state population. The amplitude of the modulation function becomes larger as the coupling constants increase. The frequency of the modulation function is proportional to the frequency of the low frequency field, and decreases with increasing coupling constant.     

Antisite defect types and temporal evolution characteristics of D0_(22)-Ni_3V structure:Studied by the microscopic phase field

Microscopic phase field simulation is performed to study antisite defect type and temporal evolution characteristic of D022-Ni3V structure in Ni75AlxV25-x ternary system.The result demonstrates that two types of antisite defect VNi and NiV coexist in D022 structure;however,the amount of NiV is far greater than VNi;when precipitates transform from D022 singe phase to two phases mixture of D022 and L12 with enhanced Al:V ratio,the amount of VNi has no evident response to the secondary L12 phase,while NiV exhi...     

Time-domain interferometry using synchrotron radiation applied to diffusion in ordered alloys

Time-domain interferometry of synchrotron radiation (TDI) has recently been used as a tool for investigating diffusion in glasses. This work deals with an extension of this technique to ordered structures. In a TDI experiment performed on the B2 alloy CoGa at the APS the intensity scattered into Bragg directions showed no detectable quasielastic signal. Experimental lower limits of the elastic contribution are given. They are in accordance with the coherent scattering function derived in this paper. This result indicates that TDI can be applied to diffusion in crystalline solids, e.g. intermetallic alloys, by using diffuse scattering. Requirements and limitations of diffuse scattering experiments are discussed. Received 21 September 2000 and Received in final form 13 December 2000     

The interaction of a screw dislocation with point defects in bcc iron

In this study, we calculate the interaction energy of intrinsic point defects vacancies and interstitials) with screw dislocations in body-centered cubic iron. First (we calculate the dipole tensor of a defect in the bulk crystal using molecular statics. Using a formulation based on linear elasticity theory, we calculate the interaction energy of the defect and the dislocation using both isotropic and anisotropic strain fields. Second, we perform atomistic calculations using molecular statics methods to directly calculate the interaction energy. Results from these two methods are compared. We verify that continuum methods alone are unable to correctly predict the interactions of defects and dislocations near the core. Although anisotropic theory agrees qualitatively with atomistics far from the core, it cannot predict which dumbbell orientations are stable and any continuum calculations must be used with caution. Spontaneous absorption by the core of both vacancies and dumbbells is seen. This paper demonstrates and discusses the differences between continuum and atomistic calculations of interaction energy between a dislocation core and a point defect.     

Demonstration of high conversion efficiency to second harmonic in a wide tuning range

This article discusses the second-harmonic generation (SHG) of tightly focused fs laser pulses in a BBO crystal. It was found theoretically and demonstrated experimentally that the spatial walk-off (SWO) and group-velocity dispersion (GVD) effects can suppress the pulse distortion and gain depression of SHG due to group-velocity mismatch (GVMM), thus keeping a reasonable pulse waveform and conversion efficiency simultaneously in a relatively wide tuning range. Using this approach, the SHG efficiency of a 2 mm long BBO crystal in a reflecting focusing geometry was enhanced significantly.     

Kinetic theory of situated agents applied to pedestrian flow in a corridor

A situated agent-based model for simulation of pedestrian flow in a corridor is presented. In this model, pedestrians choose their paths freely and make decisions based on local criteria for solving collision conflicts. The crowd consists of multiple walking agents equipped with a function of perception as well as a competitive rule-based strategy that enables pedestrians to reach free access areas. Pedestrians in our model are autonomous entities capable of perceiving and making decisions. They apply socially accepted conventions, such as avoidance rules, as well as individual preferences such as the use of specific exit points, or the execution of eventual comfort turns resulting in spontaneous changes of walking speed. Periodic boundary conditions were considered in order to determine the density-average walking speed, and the density-average activity with respect to specific parameters: comfort angle turn and frequency of angle turn of walking agents. The main contribution of this work is an agent-based model where each pedestrian is represented as an autonomous agent. At the same time the pedestrian crowd dynamics is framed by the kinetic theory of biological systems.     

Finsler geometry of topological singularities for multi‐valued fields: Applications to continuum theory of defects

Topological singularity in a continuum theory of defects and a quantum field theory is studied from a viewpoint of differential geometry. The integrability conditions of singularity (Clairaut‐Schwarz‐Young theorem) are expressed by a torsion tensor and a curvature tensor when a Finslerian intrinsic parallelism holds for the multi‐valued function. In the context of the quantum field theory, the singularity called an extended object is expressed by the torsion when the intrinsic parallelism is related to the spontaneous breakdown of symmetry. In the continuum theory of defects, the path‐dependency of point and line defects within a crystal is interpreted by the non‐vanishing condition of torsion tensor in a non‐Riemannian space osculated from the Finsler space, and the domain is not simply connected. On the other hand, for the rotational singularity, an energy integral (J‐integral) around a disclination field is path‐independent when a nonlinear connection is single‐valued. This means that the topological expression for the sole defect (Gauss‐Bonnet theorem with genus ) is understood by the integrability of nonlinear connection.

     

条形耦合波导对光子晶体耦合缺陷模的影响

光子晶体的耦合缺陷模可以通过透射谱形状来表征,而透射谱形状则与连接缺陷的条形耦合波导相关.不同于直接耦合时信号相移只依赖于缺陷模的共振频率,当存在耦合波导时信号相移也决定于波导的长度和色散关系.随着波导长度的增加,透射谱形状发生从三峰到两峰,再到平项,最后又回到三峰的周期性变化.时间耦合模的分析表明,在缺陷模频率附近理论结果与时域有限差分法的计算结果完全一致,表明在此复杂情况下理论模型依然有效.     

Investigation on the conformational changes of bovine serum albumin in a wide pH range from 2 to 12

The conformation of bovine serum albumin largely depends on its microenvironment pH and affects its physical functions and applications. In this study, we investigated the effects of pH (wide range 2–12) on the conformation of bovine serum albumin based on spectroscopic signals by various spectroscopic means including fluorescence, synchronous fluorescence, resonance light scattering, UV-visible absorption, and circular dichroism. The changes in spectroscopic signals, such as peak position and intensity, showed that the structure of bovine serum albumin varied significantly with pH. The conformation of bovine serum albumin was compact at pH 6–7, as indicated by the largest peak position values and peak intensities in the fluorescence, synchronous fluorescence, and RLS spectra. The structure of bovine serum albumin became loose in the acidic or alkaline environment, as indicated by the decreased peak position values and peak intensities. The microenvironment of the amino acid residues of bovine serum albumin also varied with pH, as indicated by the changing peak position values. At pH 7, the hydrophobicity of the tyrosine and tryptophan residues was the weakest, as indicated by the minimum synchronous fluorescence signals. In addition, the secondary structure of bovine serum albumin, especially α-helix, varied with pH. The content of α-helix reached the maximum value of 68% at pH 6–8, whereas it decreased in the acidic or alkaline environment. The study provides valuable details for studying the physiological function and applications of serum albumins.     

Spontaneous excitation of a circularly accelerated atom coupled to electromagnetic vacuum fluctuations

We study, using the formalism proposed by Dalibard, Dupont-Roc and Cohen-Tannoudji, the contributions of the vacuum fluctuation and radiation reaction to the rate of change of the mean atomic energy for a circularly accelerated multilevel atom coupled to vacuum electromagnetic fields in the ultrarelativistic limit. We find that the balance between vacuum fluctuation and radiation reaction is broken, which causes spontaneous excitations of accelerated ground state atoms in vacuum. Unlike for a circularly accelerated atom coupled to vacuum scalar fields, the contribution of radiation reaction is also affected by acceleration, and this term takes the same form as that of a linearly accelerated atom coupled to vacuum electromagnetic fields. For the contribution of vacuum fluctuations, we find that in contrast to the linear acceleration case, terms proportional to the Planckian factor are replaced by those proportional to a non-Planck exponential term, and this indicates that the radiation perceived by a circularly orbiting observer is no longer thermal as is in the linear acceleration case. However, for an ensemble of two-level atoms, an effective temperature can be defined in terms of the atomic transition rates, which is found to be dependent on the transition frequency of the atom. Specifically, we calculate the effective temperature as a function of the transition frequency and find that in contrast to the case of circularly accelerated atoms coupled to the scalar field, the effective temperature in the current case is always larger than the Unruh temperature.     

Elastic response of a free half-space to random force excitations applied on the boundary

A response of an elastic half-space to random forces applied normally to the free boundary is studied. This paper is the second part of the study we presented in [I.A. Shalimova, K.K. Sabelfeld, The response of an elastic three-dimensional half-space to random correlated displacement perturbations on the boundary, Physica A 389 (21) (2010) 4436–4449] where the case of random displacements on the boundary was considered. We analyze the white noise excitations in detail, and derive explicitly the mean of the elastic energy, the strain and displacement correlation tensors. Simulation algorithms are constructed both for displacement and strain random fields, which enables us to calculate any desired statistical characteristics.     

Antisite defect types and temporal evolution characteristics of D0<Subscript>22</Subscript>-Ni<Subscript>3</Subscript>V structure: Studied by the microscopic phase field

Microscopic phase field simulation is performed to study antisite defect type and temporal evolution characteristic of D0₂₂-Ni₃V structure in Ni₇₅Al _x V_25−x ternary system. The result demonstrates that two types of antisite defect V_Ni and Ni_V coexist in D0₂₂ structure; however, the amount of Ni_V is far greater than V_Ni; when precipitates transform from D0₂₂ singe phase to two phases mixture of D0₂₂ and L1₂ with enhanced Al:V ratio, the amount of V_Ni has no evident response to the secondary L1₂ phase, while Ni_V exhibits a definitely contrary variation tendency: Ni_V rises without L1₂ structure precipitating from matrix but declines with it; temporal evolution characteristic and temperature dependent antisite defect V_Ni, Ni_V are also studied in this paper: The concentrations of the both defects decline from high antistructure state to equilibrium level with elapsed time but rise with elevated temperature; the ternary alloying element aluminium atom occupies both α and β sublattices of D0₂₂ structure with a strong site preference of substituting α site. Supported by the National Natural Science Foundation of China (Grant Nos. 50671084 and 50875217), the Doctorate Foundation of Northwestern Polytechnical University of China (Grant No. CX200806), the China Postdoctoral Science Foundation Funded Project (Grant No. 20070420218), and the Natural Science Foundation of Shaanxi Province of China     

Analytic equation of state for solids with multi-exponential potential based on analytic mean field potential approach and applied to the epsilon phase of solid oxygen

The thermodynamic properties of the ε phase of solid oxygen are studied by using the analytic mean field approach （AMFP）. Analytic expressions for the Helmholtz free energy, internal energy and equation of state of solid oxygen have been derived based on the multi-exponential potential. The formulism for the case of double-exponential （DE） model is applied to the ε phase of solid oxygen. Its four potential parameters are determined through fitting the experimental compression data of the ε phase of solid oxygen. Numerical results of the pressure dependence of the volume calculated by using the AMFP are in good agreement with the original experimental data. This suggests that the AMFP is a useful approach to study the thermodynamic properties of the ε phase of solid oxygen. Furthermore, we predict the variation of the volume, lattice parameters and intermolecular distances with pressure, and some thermodynamic quantities versus volume, at several higher temperatures.     

Response of a uniformly accelerated detector to massless Rarita–Schwinger fields in vacuum

We study the response of a uniformly accelerated detector modeled by a two-level atom nonlinearly coupled to vacuum massless Rarita–Schwinger fields. We first generalize the formalism developed by Dalibard, Dupont-Roc, and Cohen-Tannoudji in the linear coupling case, and we then calculate the mean rate of change of the atomic energy of the accelerated atom. Our result shows that a uniformly accelerated atom in its ground state interacting with vacuum Rarita–Schwinger field fluctuations would spontaneously transition to an excited state and the unique feature in contrast to the case of the atom coupled to the scalar, electromagnetic and Dirac fields is the appearance of terms in the excitation rate which are proportional to the sixth and eighth powers of acceleration.     

Using molecular dynamic simulation data of calcite in a wide pressure range to calculate some of its thermodynamic properties via some universal equations of state

Molecular dynamics, MD, simulation of calcite (CaCO₃) is selected to compare the p-v-T behaviour of some universal equations of state, UEOS, for the temperature range 100 K ≤ T ≤ 800 K, and pressures up to 3000 kbar. The isothermal sets of p-v-T data generated by simulation were each fitted onto some three- and two-parameter EOSs including Parsafar and Mason (PM), Linear Isotherm Regularity (LIR), Birch-Murnaghan (BM), Shanker, Vinet, Baonza and Modified generalized Lennard–Jones (MGLJ) EOSs. It is found that the MD data satisfactorily fit these UEOS with reasonable precision. Some features for a good UEOS criteria such as temperature dependencies of coefficients, pressure deviation, isothermal bulk modulus and its derivative at the zero pressure limit, isobaric thermal expansion, pressure spinodal values and divergence of pseudo critical exponent either near to or far from the spinodal condition, and some regularities for calcite are investigated.     

Magnetic field dependence of the hexagonal to isotropic transition temperature of a single-walled carbon nanotubes dispersed lyotropic liquid crystal

We have carried out electrical conductivity studies on a single-walled carbon nanotubes dispersed lyotropic liquid crystal consisting of 50 wt.% TX-100 in water as a function of magnetic field and temperature. This system exhibits hexagonal and isotropic phases on heating. For all the applied magnetic fields, the temperature dependence of electrical conductivity of the carbon nanotubes dispersed lyotropic liquid crystal system exhibits a discontinuous change at the hexagonal to isotropic transition temperature. We find that the magnetic field dependence of the hexagonal to isotropic transition temperature is similar to that of the viscosity of the system. Using photo images of the sample, we find that the carbon nanotubes in the lyotropic liquid crystal form magnetic field dependent aggregates. We find spherical, rod and hook-like nanotube aggregates for low and high applied magnetic fields respectively. These nanotube aggregates alter the viscosity of our system which in turn alters the transition temperatures.     

An apodized cubic phase mask used in a wavefront coding system to extend the depth of field

The point spread function (PSF) caused by a wavefront coding system with a cubic phase mask has big side-lobes which leads to bad image restoration. This paper proposes a novel apodized cubic phase mask to suppress the side-lobes of the PSF. Simulated annealing algorithm is used to optimize the cubic and the truncation parameter of the phase mask. The system with the novel phase mask has better performance in the modulation transfer function (MTF) especially in low-and-medium spatial frequency region. The simulation results show that the restored images with the novel phase mask are superior to the one with the classic cubic phase mask in contrast and ringing effect. The experimental results show that the side-lobes of the PSF are suppressed by using the apodized cubic phase mask.     

Current transport property in GdBCO coated conductor with artificial pinning centers in a wide range of temperature,magnetic field up to 27 T,and field angle

We have investigated current transport property in Gd₁Ba₂Cu₃O_7−δ coated conductor with artificial pinning centers in a wide range of temperature, magnetic field, B up to 27 T, and field angle. Due to the additional c-axis correlated pins, critical current density, J_c in B//c was enhanced and the improvement was observed in wide range of B. On the other hand, around B⊥c below 65 K, the angular dependence of n-value showed a valley-like behavior, although the J_c was increasing. In addition, the temperature dependence of the pinning force density defined as J_c × B was not scaled on an expected master curve. These results indicate the pinning in B⊥c is governed by different mechanism below 65 K and high magnetic field.     

微观相场法模拟Ni75Al5.3V19.7 中L12和D022结构反位缺陷的演化

运用微观相场法研究Ni₇₅Al_5.3V_19.7合金沉淀过程中L1₂结构和D0₂₂结构反位缺陷发现：在沉淀初期,L1₂结构反位缺陷Al_Ni,V_Ni,Ni_Al,D0₂₂结构反位缺陷V_Ni,Al_Ni关键词： 微观相场 反位缺陷 L1₂结构')" href="#">L1₂结构 D0₂₂结构')" href="#">D0₂₂结构