Gun Sang Jeon

XY model in small-world networks

The phase transition in the XY model on one-dimensional small-world networks is investigated by means of Monte Carlo simulations. It is found that long-range order is present at finite temperatures, even for very small values of the rewiring probability, suggesting a finite-temperature transition for any nonzero rewiring probability. Nature of the phase transition is discussed in comparison with the globally coupled XY model.

Double stochastic resonance peaks in systems with dynamic phase transitions

To probe the connection between the dynamic phase transition and stochastic resonance, we study the mean-field kinetic Ising model and the two-dimensional Josephson-junction array in the presence of appropriate oscillating magnetic fields. Observed in both systems are double stochastic resonance peaks, one below and the other above the dynamic transition temperature, the appearance of which is argued to be a generic property of the system with a continuous dynamic phase transition. In particular, the frequency matching condition around the dynamic phase transition between the external drive frequency and the internal characteristic frequency of the system is identified as the origin of such double peaks.

Microscopic Verification of Topological Electron-Vortex Binding in the Lowest Landau-Level Crystal State

When two-dimensional electrons are subjected to a very strong magnetic field, they are believed to form a triangular crystal. By a direct comparison with the exact wave function, we demonstrate that this crystal is not a simple Hartree-Fock crystal of electrons but an inherently quantum mechanical crystal characterized by a nonperturbative binding of quantized vortices to electrons. It is suggested that this has qualitative consequences for experiment.

Autonomous stochastic resonance in fully frustrated Josephson-junction ladders

We investigate autonomous stochastic resonance in fully frustrated Josephson-junction ladders, which are driven by uniform constant currents. At zero temperature large currents induce oscillations between the two ground states, while for small currents the lattice potential forces the system to remain in one of the two states. At finite temperatures, on the other hand, oscillations between the two states develop even below the critical current; the signal-to-noise ratio is found to display array-enhanced stochastic resonance. It is suggested that such behavior may be observed experimentally through the measurement of the staggered voltage.

COULOMB GAPS IN ONE-DIMENSIONAL SPIN-POLARIZED ELECTRON SYSTEMS

We investigate the density of states (DOS) near the Fermi energy of one-dimensional spin-polarized electron systems in the quantum regime where the localization length is comparable to or larger than the interparticle distance. The Wigner lattice gap of such a system, in the presence of weak disorder, can occur precisely at the Fermi energy, coinciding with the Coulomb gap in position. The interplay between the two is investigated by treating the long-range Coulomb interaction and the random disorder potential in a self-consistent Hartree-Fock approximation. The DOS near the Fermi energy is found to be well described by a power law, the exponent of which decreases with increasing disorder strength. {copyright} {ital 1996 The American Physical Society.}

Effective projection theory in a correlated electron system

We propose an efficient method for nonperturbative calculation of Greens function in a correlated electron system. The key idea of the method is to project out irrelevant operators having zero norm in the ground state, which we refer to as effective projection theory. We apply the method to a mesoscopic Anderson model and show that for a given wavefunction ansatz, equations of motion can be closed only by relevant operators, allowing easy calculation of the zero-temperature Greens function. It turns out that the resulting Greens functions reproduce exact limits of both weak and strong interactions. The accuracy is also verified for small systems by comparison with exact diagonalization results, revealing that effective projection theory captures the essential correlated features in the entire regime of interactions.

Spatiotemporal stochastic resonance in fully frustrated Josephson ladders

We consider a Josephson-junction ladder in an external magnetic field with half flux quantum per plaquette. When driven by external currents, periodic in time and staggered in space, such a fully frustrated system is found to display spatiotemporal stochastic resonance under the influence of thermal noise. Such resonance behavior is investigated both numerically and analytically, which reveals significant effects of anisotropy and yields rich physics.

Boundary effects on dynamic behavior of Josephson-junction arrays

The boundary effects on the current-voltage characteristics in two-dimensional arrays of resistively shunted Josephson junctions are examined. In particular, we consider both the conventional boundary conditions (CBCs) and the fluctuating twist boundary conditions (FTBCs), and make a comparison of the obtained results. It is observed that the CBCs, which have been widely adopted in existing simulations, may give a problem in scaling, arising from rather large boundary effects; the FTBCs in general turn out to be effective in reducing the finite-size effects, yielding results with good scaling behavior. To resolve the discrepancy between the two boundary conditions, we propose that the proper scaling in the CBCs should be performed with the boundary data discarded; this is shown to give results which indeed scale well and are the same as those from the FTBCs.

Properties of the one-dimensional Hubbard model: cellular dynamical mean-field description.

The one-dimensional half-filled Hubbard model is considered at zero temperature within the cellular dynamical mean-field theory (CDMFT). By the computation of the spectral gap and the energy density with various cluster and bath sizes we examine the accuracy of the CDMFT in a systematic way, which proves the accurate description of the one-dimensional systems by the CDMFT with small clusters. We also calculate the spectral weights in a full range of the momentum for various interaction strengths. The results do not only account for the spin-charge separation, but they also reproduce all the features of the Bethe ansatz dispersions, implying that the CDMFT provides an excellent description of the spectral properties of low-dimensional interacting systems.

Dynamic transitions and resonances in Josephson-junction arrays under oscillating magnetic fields

We investigate dynamic transitions and stochastic resonance phenomena in two-dimensional fully frustrated Josephson-junction arrays driven by staggered oscillating magnetic fields. As the temperature is lowered, the dynamic order parameter, defined to be the average staggered magnetization, is observed to acquire nonzero values. The resulting transition is found to belong to the same universality as the equilibrium