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Bambi Hu

Publications and source records attributed to Bambi Hu.

At least 19 recordsLinked to original sources
PubMed2006

Asymmetric heat conduction in nonlinear lattices.

In this Letter, we conduct an extensive study of the two-segment Frenkel-Kontorova model. We show that the rectification effect of the heat flux reported in recent literature is possible only in the weak interfacial coupling limit. The rectification effect will be reversed when the properties of the interface and the system size change. These two types of asymmetric heat conduction are governed by different mechanisms though both are induced by nonlinearity. An intuitive physical picture is proposed to interpret the reversal of the rectification effect. Since asymmetric heat conduction depends critically on the properties of the interface and the system size, it is probably not an easy task to fabricate a thermal rectifier or thermal diode in practice.

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PubMed2005

Dynamical mode locking in commensurate structures with an asymmetric deformable substrate potential.

The overdamped dynamics in the commensurate structures of the one-dimensional Frenkel-Kontorova model subjected to a parametrized deformable periodic substrate potential and driven by a periodic force is examined. It was found that when the shape of the substrate potential starts to deviate from the standard one, new subharmonic steps appear in the response function even in the structures with an integer value of average interparticle distance while the critical depinning force can even decrease for some values of system parameters. These novel phenomena could be particularly relevant for the charge-density wave systems, vortex lattices, and systems of Josephson-junction arrays.

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PubMed2005

Synchronized chaotic intermittent and spiking behavior in coupled map chains.

We study phase synchronization effects in a chain of nonidentical chaotic oscillators with a type-I intermittent behavior. Two types of parameter distribution, linear and random, are considered. The typical phenomena are the onset and existence of global (all-to-all) and cluster (partial) synchronization with increase of coupling. Increase of coupling strength can also lead to desynchronization phenomena, i.e., global or cluster synchronization is changed into a regime where synchronization is intermittent with incoherent states. Then a regime of a fully incoherent nonsynchronous state (spatiotemporal intermittency) appears. Synchronization-desynchronization transitions with increase of coupling are also demonstrated for a system resembling an intermittent one: a chain of coupled maps replicating the spiking behavior of neurobiological networks.

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PubMed2005

Clustering of atoms in a model with multiple thermostats.

We propose a model for a one-dimensional chain of interacting particles in an external periodic potential. In this model the particles have a complex structure treated in a mean-field fashion: particle collisions are inelastic and also each particle is considered as having its own thermostat. We derived the Fokker-Planck equation for this model and demonstrated that the model has a truly equilibrium ground state. When an external dc force is applied to the atoms, the model exhibits a hysteresis even at high temperatures due to the clustering of atoms with the same velocity. Another effect of clustering is phase separation in the steady state when the system splits into regions of immobile atoms ("traffic jams") and regions of running atoms.

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PubMed2005

Two-dimensional two-state lattice-gas model.

We propose a two-dimensional lattice-gas (2D LG) model where atoms may be in two different states: the immobile state, in which they jump as usual in the LG model, and the running state, in which the atoms always jump in the driving direction. The model demonstrates a typical behavior of "traffic-jam" models: the system splits into domains of immobile atoms (jams) and running atoms. We considered four variants of the 2D LG model, namely the multilane and truly 2D models, each with "passive" and "active" atomic jumps. The model has the steady state with a power law distribution of jam sizes characterized by a universal exponent 3/2. The phase diagram of the model shows that the mobility of the 2D system is lower than the mobility of the 1D model due to the spreading of jams in the direction transverse to the driving direction.

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PubMed2005

Dynamic phases in the two-dimensional underdamped driven Frenkel-Kontorova model.

We study the nonlinear dc response of a two-dimensional underdamped system of interacting atoms subject to an isotropic periodic external potential with triangular symmetry. When driving force increases, the system transfers from a disorder locked state to an ordered sliding state corresponding to a moving crystal. By varying the values of the effective elastic constant, damping, and temperature, we found different scenarios and intermediate phases during the ordering transition. For a soft atomic layer, the system passes through a plastic-channel regime that appears as a steady-state regime at higher values of the damping coefficient. For high values of the effective elastic constant, when the atomic layer is stiff, the intermediate plastic phase corresponds to a traffic-jam regime with immobile islands in the sea of running atoms. At a high driving of the stiff layer, a solitonlike elastic flow of atoms has been observed.

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PubMed2005

Heat conduction in the Frenkel-Kontorova model.

Heat conduction is an old yet important problem. Since Fourier introduced the law bearing his name almost 200 years ago, a first-principle derivation of this simple law from statistical mechanics is still lacking. Worse still, the validity of this law in low dimensions, and the necessary and sufficient conditions for its validity are far from clear. In this paper we will review recent works on heat conduction in a simple nonintegrable model called the Frenkel-Kontorova model. The thermal conductivity of this model has been found to be finite. We will study the dependence of the thermal conductivity on the temperature and other parameters of the model such as the strength and the periodicity of the external potential. We will also discuss other related problems such as phase transitions and finite-size effects. The study of heat conduction is not only of theoretical interest but also of practical interest. We will show various recent designs of thermal rectifiers and thermal diodes by coupling nonlinear chains together. The study of heat conduction in low dimensions is also important to the understanding of the thermal properties of carbon nanotubes.

Electronics↗
PubMed2004

Second-harmonic generation in optical fibers on a continuous-wave background.

We predict that a different type of second-harmonic generation (SHG) in nonlinear optical fibers is possible for the waves excited from a continuous-wave background. We show that in a normal dispersion regime and near the zero-dispersion point of a single-mode optical fiber the phase-matching condition of the SHG can be satisfied by suitably choosing the wave vectors and frequencies of fundamental and second-harmonic waves. Using a multiscale method the nonlinearly coupled envelope equations for the SHG are derived and their explicit solutions are provided and checked by numerical simulation.

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PubMed2004

Drift of rigidly rotating spirals under periodic and noisy illuminations.

Under the weak deformation approximation, the motion of rigidly rotating spirals induced by periodic and noisy illuminations are investigated analytically. We derive an approximate but explicit formula of the spiral drift velocity directly from the original reaction-diffusion equation. With this formula we are able to explain the main features in the periodic and noisy illuminations induced spiral drift problems. Numerical computations of the Oregonator model are carried out as well, and they agree with the main qualitative conclusions of our analytical results.

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PubMed2004

Entanglement as a signature of quantum chaos.

We explore the dynamics of entanglement in classically chaotic systems by considering a multiqubit system that behaves collectively as a spin system obeying the dynamics of the quantum kicked top. In the classical limit, the kicked top exhibits both regular and chaotic dynamics depending on the strength of the chaoticity parameter kappa in the Hamiltonian. We show that the entanglement of the multiqubit system, considered for both the bipartite and the pairwise entanglement, yields a signature of quantum chaos. Whereas bipartite entanglement is enhanced in the chaotic region, pairwise entanglement is suppressed. Furthermore, we define a time-averaged entangling power and show that this entangling power changes markedly as kappa moves the system from being predominantly regular to being predominantly chaotic, thus sharply identifying the edge of chaos. When this entangling power is averaged over all states, it yields a signature of global chaos. The qualitative behavior of this global entangling power is similar to that of the classical Lyapunov exponent.

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PubMed2004

Spiral breakup due to mechanical deformation in excitable media.

To address the problem of how cardiac muscle contraction affects the dynamics of rotating spiral waves, spiral breakup induced by mechanical deformation in excitable media is studied in two partial-differential-equation models. It is shown that spirals begin to break up at omega=0.5 omega(0) when we increase the amplitude of the mechanical deformation gradually. Our numerical results point to a new mechanism of transition from spirals to spatiotemporal chaos, in which the anisotropic time-dependent diffusion coefficient is essential.

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PubMed2003

Transition from spiral waves to defect-mediated turbulence induced by gradient effects in a reaction-diffusion system.

The transition from spiral waves to defect-mediated turbulence was studied in a spatial open reactor using Belousov-Zhabotinsky reaction. The experimental results show a new mechanism of the transition from spirals to spatiotemporal chaos, in which the gradient effects in the three-dimensional system are essential. The transition scenario consists of two stages: first, the effects of gradients in the third dimension cause a splitting of the spiral tip and a deletion of certain wave segments, generating new wave sources; second, the waves sent by the new wave sources undergo a backfire instability, and the back waves are laterally unstable. As a result, defects are automatically generated and fill all over the system. The result of numerical simulation using the FitzHugh-Nagumo model essentially agrees with the experimental observation.

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PubMed2003

Suppression of spiral waves and spatiotemporal chaos by generating target waves in excitable media.

A method for suppressing spiral waves and spatiotemporal chaos in excitable media is proposed. Applying suitable periodic force to a single point, we can successfully suppress spiral waves as well as spatiotemporal chaos by generating target waves. After we turn off the external force, target waves finally disappear and the whole system which was in the state of spiral wave or spatiotemporal chaos goes to the spatially homogeneous steady state. It is shown that our control method is not sensitively model dependent. It works for a model for catalytic CO oxidation on platinum as well as for a model for cardiac muscle.

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PubMed2003

Three types of transitions to phase synchronization in coupled chaotic oscillators.

We study the effect of noncoherence on the onset of phase synchronization of two coupled chaotic oscillators. Depending on the coherence properties of oscillations characterized by the phase diffusion, three types of transitions to phase synchronization are found. For phase-coherent attractors this transition occurs shortly after one of the zero Lyapunov exponents becomes negative. At rather strong phase diffusion, phase locking manifests a strong degree of generalized synchronization, and occurs only after one positive Lyapunov exponent becomes negative. For intermediate phase diffusion, phase synchronization sets in via an interior crises of the hyperchaotic set.

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PubMed2003

Dispersive anomalous diffusive transport in ratchets with long-range correlated spatial disorder.

The effects of quenched disorder with long-range spatial correlations on the transport properties of an overdamped periodic ratchet are investigated. The modified Fourier-filtering method is applied to generate the long-range correlated spatial disorder with statistical properties eta(x)eta(x') approximately /x'-x/(-gamma), where gamma is the correlation exponent. Small amounts of this kind of quenched disorder are introduced in the equation of overdamped motion of a continuous time system, and the first two moments C1(t)= and C2(t)=<(x(t)- )(2) are calculated. We show that the drift velocity is almost independent of gamma. However, as a consequence of the long-range spatial correlations, the dispersive anomalous diffuse motion [C2(t) approximately t(H)] appears in ratchets, with the difussion exponent H (1<H<2) being dependent on gamma. Moreover, we show that both the amount of quenched disorder and the correlation degree can enhance the anomalous diffusive transport.

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PubMed2003

Oscillatory and rotatory synchronization of chaotic autonomous phase systems.

The existence of rotatory, oscillatory, and oscillatory-rotatory synchronization of two coupled chaotic phase systems is demonstrated in the paper. We find four types of transition to phase synchronization depending on coherence properties of motions, characterized by phase variable diffusion. When diffusion is small the onset of phase synchronization is accompanied by a change in the Lyapunov spectrum; one of the zero Lyapunov exponents becomes negative shortly before this onset. If the diffusion of the phase variable is strong then phase synchronization and generalized synchronization, occur simultaneously, i.e., one of the positive Lyapunov exponents becomes negative, or generalized synchronization even sets in before phase synchronization. For intermediate diffusion the phase synchronization appears via interior crisis of the hyperchaotic set. Soft and hard transitions to phase synchronization are discussed.

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PubMed2003

Driven kinks in the anharmonic Frenkel-Kontorova model.

Multiple and supersonic topological excitations (kinks) driven by an external dc force in the Frenkel-Kontorova model (a chain of atoms subjected to a periodic substrate potential) with the exponential interatomic interaction are studied with the help of numerical simulation. The simulation results are interpreted in terms of dynamics of two limiting cases, the exactly integrable sine-Gordon equation and the Toda chain. The stability of driven kinks and scenarios of their destruction are described for a wide range of model parameters.

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