arXiv subjects

Bambi Hu

Publications and source records attributed to Bambi Hu.

At least 73 records · Page 4Linked to original sources

arXiv2000

Coherence resonance near blowout bifurcation in nonlinear dynamical systems

Previous studies have shown that noise can induce coherence resonance in some nonlinear dynamical systems close to a bifurcation of a periodic motion, such as in excitable systems. We demonstrate that coherence resonance can be observed in systems close to a {\sl blowout bifurcation}. It is shown that for dynamical systems with an invariant subspace in which there is a phase-coherent chaotic attractor, the interplay among the oscillation of local transverse stability, noise and nonlinearity can lead to coherence resonance phenomenon. The mechanism of coherence resonance in this type of system is different from that in previously studied systems.

nlin.CD↗

arXiv2000

Symmetry-breaking on-off intermittency under modulation: Robustness of supersensitivity, resonance and information gain

Nonlinear dynamical systems possessing an invariant subspace in the phase space and chaotic or stochastic motion within the subspace often display on-off intermittency close to the threshold of stability of the subspace. In a class of symmetric systems, the intermittency is symmetry-breaking [Ying-Cheng Lai, Phys. Rev. E {\bf 53}. R4267 (1996)]. We report interesting and practically important universal behavior of robustness of supersensitivity, resonance and information gain in this class of systems when subjected to a weak modulation. While intermittent loss of synchronization may be harmful to application of high-quality synchronization of coupled chaotic systems, the features reported here may lead to interesting application of on-off intermittency.

nlin.CD↗

arXiv2000

Tunneling Dynamics of Bose-Einstein Condensates with Feshbach Resonances

We study tunneling dynamics of atomic pairs in Bose-Einstein condensates with Feshbach resonances. It is shown that the tunneling of the atomic pairs depends on not only the tunneling coupling between the atomic condensate and the molecular condensate, but also the inter-atomic nonlinear interactions and the initial number of atoms in these condensates. It is found that in addition to oscillating tunneling current between the atomic condensate and the molecular condensate, the nonlinear atomic-pair tunneling dynamics sustains a self-locked population imbalance: macroscopic quantum self-trapping effect. Influence of decoherence induced by non-condensate atoms on tunneling dynamics is investigated. It is shown that decoherence suppresses atomic-pair tunneling.

cond-mat.soft↗

arXiv2000

Quantum Tunneling and Quantum-Classical Transitions in Large Spin Systems

We have studied quantum tunneling of large spins in a biaxial spin system and in a single-axial spin system with a transverse magnetic field. The asymptotically exact eigenvalues and eigenstates of the spin systems are obtained by solving the Mathieu equation. When the height $4q$ of energy barrier formed by the anisotropy and the magnetic field exceeds some critical values $4q_{1T}(r)$, the ground states and the excited states split due to tunneling of large spins. We also have presented the phase diagram of these spin systems in the extremely weak spin-phonon coupling limit. The crossover from thermal to quantum regime is second order. With further decreasing temperature, the first order phase transition occurs from a thermally-assisted resonant tunneling to thermal regime as $q<q_{1T}(s)$. Our theory agrees with recent experimental observations well.

cond-mat.mes-hall↗

arXiv2000

Universal statistics of wave functions in chaotic and disordered systems

We study a new statistics of wave functions in several chaotic and disordered systems: the random matrix model, band random matrix model, the Lipkin model, chaotic quantum billiard and the 1D tight-binding model. Both numerical and analytical results show that the distribution function of a generalized Riccati variable, defined as the ratio of components of eigenfunctions on basis states coupled by perturbation, is universal, and has the form of Lorentzian distribution.

chao-dyn↗

arXiv2000

Disturbance spreading in incommensurate and quasiperiodic systems

The propagation of an initially localized excitation in one dimensional incommensurate, quasiperiodic and random systems is investigated numerically. It is discovered that the time evolution of variances $σ^2(t)$ of atom displacements depends on the initial condition. For the initial condition with nonzero momentum, $σ^2(t)$ goes as $t^α$ with $α=1$ and 0 for incommensurate Frenkel-Kontorova (FK) model at $V$ below and above $V_c$ respectively; and $α=1$ for uniform, quasiperiodic and random chains. It is also found that $α=1-β$ with $β$ the exponent of distribution function of frequency at zero frequency, i.e., $ρ(ω)\sim ω^β$ (as $ω\to 0$). For the initial condition with zero momentum, $α=0$ for all systems studied. The underlying physical meaning of this diffusive behavior is discussed.

cond-mat.stat-mech↗

arXiv2000

Heat conduction in one dimensional nonintegrable systems

Two classes of 1D nonintegrable systems represented by the Fermi-Pasta-Ulam (FPU) model and the discrete $ϕ^4$ model are studied to seek a generic mechanism of energy transport in microscopic level sustaining macroscopic behaviors. The results enable us to understand why the class represented by the $ϕ^4$ model has a normal thermal conductivity and the class represented by the FPU model does not even though the temperature gradient can be established.

cond-mat↗

arXiv2000

Phase synchronization in coupled nonidentical excitable systems and array enhanced coherence resonance

We study the dynamics of a lattice of coupled nonidentical Fitz Hugh-Nagumo system subject to independent external noise. It is shown that these stochastic oscillators can lead to global synchronization behavior {\sl without an external signal}. With the increase of the noise intensity, the system exhibits coherence resonance behavior. Coupling can enhance greatly the noise-induced coherence in the system.

nlin.CD↗

arXiv1999

Numerical accuracy of Bogomolny's semiclassical quantization scheme in quantum billiards

We use the semiclassical quantization scheme of Bogomolny to calculate eigenvalues of the Lima\c con quantum billiard corresponding to a conformal map of the circle billiard. We use the entire billiard boundary as the chosen surface of section (SOS) and use a finite approximation for the transfer operator in coordinate space. Computation of the eigenvalues of this matrix combined with a quantization condition, determines a set of semiclassical eigenvalues which are compared with those obtained by solving the Schrödinger equation. The classical dynamics of this billiard system undergoes a smooth transition from integrable (circle) to completely chaotic motion, thus providing a test of Bogomolny's semiclassical method in coordinate space in terms of the morphology of the wavefunction. We analyze the results for billiards which exhibit both soft and hard chaos.

chao-dyn↗

arXiv1999

Ground state wavefunction of the quantum Frenkel-Kontorova model

The wavefunction of an incommensurate ground state for a one-dimensional discrete sine-Gordon model -- the Frenkel-Kontorova (FK) model -- at zero temperature is calculated by the quantum Monte Carlo method. It is found that the ground state wavefunction crosses over from an extended state to a localized state when the coupling constant exceeds a certain critical value. So, although the quantum fluctuation has smeared out the breaking of analyticity transition as observed in the classical case, the remnant of this transition is still discernible in the quantum regime.

cond-mat↗

arXiv1999

Quantum chaos of a kicked particle in a 1D infinite square potential well

We study quantum chaos in a non-KAM system, i.e. a kicked particle in a one-dimensional infinite square potential well. Within the perturbative regime the classical phase space displays stochastic web structures, and the diffusion coefficient D in the regime increases with the perturbative strength K giving a scaling $D \propto K^{2.5}$, and in the large K regime D goes as K^2. Quantum mechanically, we observe that the level spacing statistics of the quasi eigenenergies changes from Poisson to Wigner distribution as the kick strength increases. The quasi eigenstates show power-law localization in the small K region, which become extended one at large K. Possible experimental realization of this model is also discussed.

chao-dyn↗

arXiv1999

Wave transmission, phonon localization and heat conduction of 1D Frenkel-Kontorova chain

We study the transmission coefficient of a plane wave through a 1D finite quasi-periodic system -- the Frenkel-Kontorova (FK) model -- embedding in an infinite uniform harmonic chain. By varying the mass of atoms in the infinite uniform chain, we obtain the transmission coefficients for {\it all} eigenfrequencies. The phonon localization of the incommensurated FK chain is also studied in terms of the transmission coefficients and the Thouless exponents. Moreover, the heat conduction of Rubin-Greer-like model for FK chain at low temperature is calculated. It is found that the stationary heat flux $J(N)\sim N^α$, and $α$ depends on the strength of the external potential.

cond-mat.stat-mech↗

arXiv1998

Parallel updating cellular automaton models of driven diffusive Frenkel-Kontorova-type systems

Three cellular automaton (CA) models of increasing complexity are introduced to model driven diffusive systems related to the generalized Frenkel-Kontorova (FK) models recently proposed by Braun [Phys.Rev.E58, 1311 (1998)]. The models are defined in terms of parallel updating rules. Simulation results are presented for these models. The features are qualitatively similar to those models defined previously in terms of sequentially updating rules. Essential features of the FK model such as phase transitions, jamming due to atoms in the immobile state, and hysteresis in the relationship between the fraction of atoms in the running state and the bias field are captured. Formulating in terms of parallel updating rules has the advantage that the models can be treated analytically by following the time evolution of the occupation on every site of the lattice. Results of this analytical approach are given for the two simpler models. The steady state properties are found by studying the stable fixed points of a closed set of dynamical equations obtained within the approximation of retaining spatial correlations only upto two nearest neighboring sites. Results are found to be in good agreement with numerical data.

cond-mat↗

arXiv1998

Phase slips and phase synchronization of coupled oscillators

The behaviors of coupled oscillators, each of which has periodic motion with random natural frequency in the absence of coupling, are investigated. Some novel collective phenomena are revealed. At the onset of instability of the phase-locking state, simultaneous phase slips of all oscillators and quantized phase shifts in these phase slips are observed. By incresing the coupling, a bifurcation tree from high-dimensional quasiperiodicity to chaos to quasiperiodicity and periodicity is found. Different orders of phase synchronizations of chaotic oscillators and chaotic clusters play the key role for constructing this tree structure.

chao-dyn↗

arXiv1998

Array-induced collective transport in the Brownian motion of coupled nonlinear oscillator systems

Brownian motion of an array of harmonically coupled particles subject to a periodic substrate potential and driven by an external bias is investigated. In the linear response limit (small bias), the coupling between particles may enhance the diffusion process, depending on the competition between the harmonic chain and the substrate potential. An analytical formula of the diffusion rate for the single-particle case is also obtained. In the nonlinear response regime, the moving kink may become phase-locked to its radiated phonon waves, hence the mobility of the chain may decrease as one increases the external force.

cond-mat.stat-mech↗

arXiv1998

Critical Behavior of Period Doublings in Coupled Inverted Pendulums

We study the critical behaviors of period doublings in N (N=2,3,4,...) coupled inverted pendulums by varying the driving amplitude $A$ and the coupling strength $c$. It is found that the critical behaviors depend on the range of coupling interaction. In the extreme long-range case of global coupling, in which each inverted pendulum is coupled to all the other ones with equal strength, the zero-coupling critical point and an infinity of critical line segments constitute the same critical set in the $A-c$ plane, independently of $N$. However, for any other nonglobal-coupling cases of shorter-range couplings, the structure of the critical set becomes different from that for the global-coupling case, because of a significant change in the stability diagram of periodic orbits born via period doublings. The critical scaling behaviors on the critical set are also found to be the same as those for the abstract system of the coupled one-dimensional maps.

chao-dyn↗

arXiv1998

Nonadiabatic Geometric Phase and Hannay Angle: A Squeezed State Approach

The geometric phases of the cyclic states of a generalized harmonic oscillator with nonadiabatic time-periodic parameters are discussed in the framework of squeezed state. A class of cyclic states are expressed as a superposition of an infinte number of squeezed states. Then, their geometric phases are obtained explicitly and found to be $-(n+1/2)$ times the classical nonadiabatic Hannay angle. It is shown that the analysis based on squeezed state approach provide a clear picture of the geometric meaning of the quantal phase.

cond-mat↗

arXiv1998

Non-Monotone Characteristic of Spectral Statistics in the Transition between Poisson and Gauss

We have computed the spectral number variances of an extended random matrix ensemble predicted by Guhr's supersymmetry formula, showing a non-monotone increase of the curves that arises from an "overshoot" of the two-level correlation function above unity. On the basis of the most general form of $N$-level joint distribution that meets sound probabilistic conditions on matrix spaces, the above characteristic may be attributed to the {\it attractiveness} of the pair potential in long range($E >$ Thouless energy) of the underlying level gas. The approach of level dynamics indicates that the result is "anti-screening" of the level repulsion in short-range statistics of the usual random matrix prediction until the joint level distribution undergoes a phase transition (the Anderson transition).

cond-mat↗