Exchange bias and asymmetric reversal in nanostructured dot arrays

The size dependence of exchange bias field HE and coercivity Hc was studied by measuring exchange biased Fe-FeF2 dot arrays in comparison with an unstructured exchange biased Fe-FeF2 bilayer. The domain sizes in the ferromagnet (FM) and the antiferromagnet (AFM) play an important role for exchange bias (EB), and thus interesting phenomena may be expected when the size of an EB system becomes comparable to these sizes. We observe drastic changes of HE and Hc in nanostructured Fe-FeF2, which are unexpected because they appear even at a structure size which is too large for matching with AFM or FM domain size to play a role. We propose that under certain conditions the hysteresis loop is affected differently in the two branches of the reversal by shape anisotropy due to patterning. This is possible because the EB induces a reversal asymmetry already in the unpatterned bilayer system.     

Directed transport of an asymmetric particle pair induced by non-equilibrium conditions

The hopping motion of a classical bounded pair of two particles along a chain is investigated. It is shown that in the asymmetric case of the system dynamics including excited states which differ from the respective ground states by the barrier to be overcome by one of the two particles, the over- and underpopulation of these excited states leads to a directed motion of the particle pair. Thereby, overpopulation results in one direction of motion, whereas underpopulation results in the opposite direction, and the mean velocity is determined by the amount of over-resp. underpopulation. For small deviations from equilibrium, the system exhibits linear response well known from other ratchet-type models. Possible generalizations and applications are discussed. Received 17 August 2001 and Received in final form 11 October 2001     

Hardening by softening in a flow of chainlike self-driven objects

A two-dimensional square lattice system, on which flexible, chainlike, self-driven objects move randomly but are drifted to a same direction, causing a unidirectional net flow, is investigated by numerical simulations. It is shown that the objects exhibit a freezing transition from a smoothly flowing state to a completely jammed state, in which the objects become immobile and cannot move anymore. Comparison with the flow of rigid objects shows that this complete jamming (hardening) results from the flexibility (softening) of each self-driven object. This is the first report of the freezing transition of a free transport system (without obstacles) where the net flow is not multidirectional (as in the case of opposing flows or crossing flows) but unidirectional.     

Induced growth of asymmetric nanocantilever arrays on polar surfaces

We report that the Zn-terminated ZnO (0001) polar surface is chemically active and the oxygen-terminated (000(-)1) polar surface is inert in the growth of nanocantilever arrays. Longer and wider "comblike" nanocantilever arrays are grown from the (0001)-Zn surface, which is suggested to be a self-catalyzed process due to the enrichment of Zn at the growth front. The chemically inactive (0001;)-O surface typically does not initiate any growth, but controlling experimental conditions could lead to the growth of shorter and narrower nanocantilevers from the intersections between (000(-)1)-O with +/- (01(-)10) surfaces.     

Concentration dependence of the collective dynamics of swimming bacteria

At concentrations near the maximum allowed by steric repulsion, swimming bacteria form a dynamical state exhibiting extended spatiotemporal coherence. The viscous fluid into which locomotive energy of individual microorganisms is transferred also carries interactions that drive the coherence. The concentration dependence of correlations in the collective state is probed here with a novel technique that herds bacteria into condensed populations of adjustable concentration. For the particular thin-film geometry employed, the correlation lengths vary smoothly and monotonically through the transition from individual to collective behavior.     

Fission barriers and asymmetric ground states in the relativistic mean-field theory

The symmetric and asymmetric fission path for ²⁴⁰Pu, ²³²Th and ²²⁶Ra is investigated within the relativistic mean-field model. Standard parametrizations which are well fitted to nuclear ground-state properties are found to deliver reasonable qualitative and quantitative features of fission, comparable to similar nonrelativistic calculations. Furthermore, stable octupole deformations in the ground states of radium isotopes are investigated. They are found in a series of isotopes, qualitatively in agreement with nonrelativistic models. But the quantitative details differ amongst the models and between the various relativistic parametrizations.     

质点振速传感器直线阵超指向性波束形成*

声矢量传感器及其组阵方式的研究已成为水声领域的热门课题。该文研究了当单自由度矢量传感器（质点振速传感器）的观测方向沿直线阵轴线方向时的空间均匀噪声场中两阵元的噪声相关系数,然后采用Gram-Schmidt模态波束分解与综合方法推导了高阶超指向性的最优解,得到了直线阵端射方向的指向性指数和稳健性水平。仿真研究表明,当d / λ < 0.5 (d为阵元间距,λ为波长)时,质点振速传感器直线阵（简称质点振速阵）采用超指向性方法在端射方向的指向性指数略高于声压传感器直线阵（简称声压阵）;而当d / λ > 0.5时,质点振速阵的指向性指数远远高于声压阵。该文的研究内容对水下声纳系统中质点振速传感器的有效组阵和实际应用具有一定的参考价值。     

Threshold fragmentation of atomic and molecular systems by charged particle impact

The Wannier theory –originally derived for the ionisation of atoms by electrons —is extended to general fragmentation reactions which produce three charged particles of arbitrary masses and charges. The center of mass fragmentation cross section and the angular correlation are shown to be given in terms of two energy-independent, characteristic indices as in Wannier's case. These indices vary drastically as a function of masses and charges of the fragments. It is also shown that these indices may have singularities at critical however physical values of charges and masses. In such cases a generalized Wannier threshold law with a suitable, fractional exponent does not exist. The theory developed in this article applies to energies up to several eV above the fragmentation threshold.     

分子尺度不对称体系中热噪声引发的单向输运

不对称体系中的单向输运现象广泛存在于自然界中,宏观理论认为,体系的空间反演不对称性和有一定自相关时间的外加振动是引发单向运动的两个必要条件,因而一般认为,在分子尺度的不对称体系中,仅有热噪声也还是不会引发单向运动的。随着实验技术和计算手段的发展,有迹象表明,在分子尺度的不对称体系中,即使没有外加振动也会有单向运动产生。文章介绍在分子尺度由体系热噪声引发的不对称体系中,单向输运的理论进展,以及产生这种单向输运的条件。     

Two-lane totally asymmetric exclusion processes with particle creation and annihilation

This paper studies two-lane totally asymmetric simple exclusion processes (TASEP) coupled with particle creation and annihilation in one of the two lanes (lane 2). The dependence of the density profiles of both lanes on the lane-change rate Ω$\Omega$ is investigated. It is shown that with the increase of Ω$\Omega$, a complex behavior on both lanes occurs. Synchronization of the shocks in both lanes occurs when Ω$\Omega$ exceeds a threshold _Ωc≈10${\Omega }_c\approx 10$. A boundary layer is also observed at the left boundary as a finite-size effect. The situations arising from very large creation/annihilation rate and very small creation/annihilation rate are also investigated. The mean-field analysis is presented and it agrees well with Monte Carlo simulations.     

Multifragmentation of charge asymmetric nuclear systems

The multifragmentation of excited spherical nuclear sources with various N/Z ratios and fixed mass number is studied within dynamical and statistical models. The dynamical model treats the multifragmentation process as a final stage of growth of density fluctuations in unstable expanding nuclear matter. The statistical model makes a choice of the final multifragment configuration according to its statistical weight at a global thermal equilibrium. Similarities and differences in the predictions of the two models on the isotopic composition of the produced fragments are presented and the most sensitive observable characteristics are discussed.     

Intermittency and clustering in a system of self-driven particles

Intermittent behavior is shown to appear in a system of self-driven interacting particles. In the ordered phase, most particles move in the same approximate direction, but the system displays a series of intermittent bursts during which the order is temporarily lost. This intermittency is characterized and its statistical properties are found analytically for a reduced system containing only two particles. For large systems, the particles aggregate into clusters that play an essential role in the intermittent dynamics. The study of the cluster statistics shows that both the cluster sizes and the transition probability between them follow power-law distributions. The exchange of particles between clusters is shown to satisfy detailed balance.     

Wave and particle pictures of correlated systems

We present a complementarity relation for the branch-distinguishability in an optical experiment with correlated photons. These analysis can be used to propose a two-photon interferometry experiment showing simulataneously noncomplementary wave and particle behaviors.     

Light transmission through metal films perforated with arrays of asymmetric cross-shaped hole

The transmission characteristics of metallic film perforated with an array of asymmetric cross-shaped hole are studied by using the three-dimensional finite difference time-domain method. We find that the wavelengths and intensities of transmission peaks depend strongly on the asymmetric parameters of the cross-shaped hole. The transmission peaks in the structure of asymmetric cross-shaped hole array originate from the splitting of the transmission peak in the corresponding one of symmetric cross-shaped hole array. Moreover, it is also found that the transmission spectra can be adjusted by changing other geometrical parameters of asymmetric cross-shaped hole due to the their effect on the distribution of the oscillating charges on metal surfaces.     

Single particle properties in asymmetric nuclear matter and TBF rearrangement effect

We have developed the formula and the numerical code for calculating the rearrangement contribution to the single particle (s.p.) properties in asymmetric nuclear matter induced by three-body forces within the framework of the Brueckner theory extended to include a microscopic three-body force (TBF). We have investigated systematically the TBF-induced rearrangement effect on the s.p. properties and their isospin-behavior in neutron-rich nuclear medium. It is shown that the TBF induces a repulsive rearrangement contribution to the s.p. potential in nuclear medium. The repulsion of the TBF rearrangement contribution increases rapidly as a function of density and nucleon momentum. It reduces largely the attraction of the BHF s.p. potential and enhances strongly the momentum dependence of the s.p. potential at large densities and high-momenta. The TBF rearrangement effect on symmetry potential is to enhances its repulsion (attraction) on neutrons (protons) in dense asymmetric nuclear matter.     

Self-organized arrays of submicrometer particle of organic dye and its near-field optical study

Organic dye particles of micrometer and submicrometer diameters were prepared by a wetting/dewetting procedure on a hydrophilic glass surface and a self-organized one- or two-dimensional registration was observed. To analyze the molecular assembly in these particles the near-field-excited near-field fluorescence from single particles were detected, while the majority of particles with diameters around 2 μm or less did not show fluorescence. Far-field fluorescence, in contrast, was observed for every particle, and the intensity depended on the excitation polarization when a polarized evanescent field was used for excitation, indicating that the molecules’ transition moment within dye particles orient parallel to the substrate surface. These two observations suggest that the near-field at the tip of the probe was polarized parallel to the probe axis. Another observation, that neighboring particles show similar molecular orientations, suggests that the dewetting process contributed to the alignment of the molecular directions in adjacent particles, which further proves that the dye particles were formed by a self-organizing mechanism.     

Flowrate behavior and clustering of self-driven robots in a channel

In this paper, the collective motion of self-driven robots is studied experimentally and theoretically. In the channel,the flowrate of robots increases with the density linearly, even if the density of the robots tends to 1.0. There is no abrupt drop in the flowrate, similar to the collective motion of ants. We find that the robots will adjust their velocities by a serial of tiny collisions. The speed-adjustment will affect both robots involved in the collision, and will help to maintain a nearly uniform velocity for the robots. As a result, the flowrate drop will disappear. In the motion, the robots neither gather together nor scatter completely. Instead, they form some clusters to move together. These clusters are not stable during the moving process, but their sizes follow a power-law-alike distribution. We propose a theoretical model to simulate this collective motion process, which can reproduce these behaviors well. Analytic results about the flowrate behavior are also consistent with experiments.