arXiv subjects

J. Choi

Publications and source records attributed to J. Choi.

63 records · Page 4Linked to original sources

arXiv2004

Dynamic model of fiber bundles

A realistic continuous-time dynamics for fiber bundles is introduced and studied both analytically and numerically. The equation of motion reproduces known stationary-state results in the deterministic limit while the system under non-vanishing stress always breaks down in the presence of noise. Revealed in particular is the characteristic time evolution that the system tends to resist the stress for considerable time, followed by sudden complete rupture. The critical stress beyond which the complete rupture emerges is also obtained.

cond-mat.stat-mech↗

arXiv2003

Stability and ensemble inequivalence in a globally coupled system

We consider a system of globally coupled rotors, described by a set of Langevin equations, and examine stability of the incoherent phase. The corresponding Fokker-Planck equation, providing a unified description of microcanonical and canonical ensembles, bears a few solutions, depending upon the ensemble. It is found that the stability of each solution varies differently with the temperature, revealing the inequivalence between the two ensembles. This also suggests a physical explanation of the quasi-stationarity observed in recent numerical results.

cond-mat.stat-mech↗

arXiv2001

Development of the Simulator for the Global Orbit Feedback System Using EPICS

We have carried out the basic research for the accelerator and tokamak control system based on the Experimental Physics and Industrial Control System (EPICS). We have used the process database and the state notation language (SNL) in the EPICS to develop the simulator which represents as a virtual machine. In this paper, we introduce the simulator of the global orbit feedback system as an example. This simulates the global orbit feedback system under the constraint conditions for Pohang Light Source (PLS) storage ring. We describe the details of the feedback algorithm and the realization of the simulator.

nucl-ex↗

arXiv2001

Topological quantization and degeneracy in Josephson-junction arrays

We consider the conductivity quantization in two-dimensional arrays of mesoscopic Josephson junctions, and examine the associated degeneracy in various regimes of the system. The filling factor of the system may be controlled by the gate voltage as well as the magnetic field, and its appropriate values for quantization is obtained by employing the Jain hierarchy scheme both in the charge description and in the vortex description. The duality between the two descriptions then suggests the possibility that the system undergoes a change in degeneracy while the quantized conductivity remains fixed.

cond-mat.mes-hall↗

arXiv2000

Renormalization-group study of gate charge effects in Josephson-junction chains

We study the quantum phase transition in a chain of superconducting grains, coupled by Josephson junctions, with emphasis on the effects of gate charges induced on the grains. At zero temperature the system is mapped onto a two-dimensional classical Coulomb gas, where the gate charge plays the role of an imaginary electric field. Such a field is found relevant in the renormalization-group transformation and to change the nature of the superconductor-insulator transition present in the system, tending to suppress quantum fluctuations and helping establish superconductivity. On the basis of this observation, we propose the zero-temperature phase diagram on the plane of the gate charge and the energy ratio.

cond-mat↗

arXiv1997

Quantum Phase Transitions in Josephson Junction Chains

We investigate the quantum phase transition in a one-dimensional chain of ultra-small superconducting grains, considering both the self- and junction capacitances. At zero temperature, the system is transformed into a two-dimensional system of classical vortices, where the junction capacitance introduces anisotropy in the interaction between vortices. This leads to the superconductor-insulator transition of the Berezinskii-Kosterlitz-Thouless type, as the ratios of the Josephson coupling energy to the charging energies are varied. It is found that the junction capacitance plays a role similar to that of dissipation and tends to suppress quantum fluctuations; nevertheless the insulator region survives even for arbitrarily large values of the junction capacitance.

cond-mat.mes-hall↗

arXiv1994

Sphalerons in the Standard Model with a real Higgs singlet

Sphaleron energies within the standard model with a real Higgs singlet added on are calculated. The coupled non-linear equations of motion are numerically solved and the sphaleron energy evaluated for a set of parameters in the Higgs potential. I find a small difference in the sphaleron energy compared to the standard model. A slightly stronger constraint on the strength of the first order phase transition thus results for this model.

hep-ph↗

arXiv1993

Real Higgs singlet and the electroweak phase transition in the standard model, (UM-P-93/80, OZ-93/20)

The effective potential at finite temperature is constructed within the minimal standard model when a real Higgs singlet is added on. We find that a region of parameter space exists for which one can find a first order transition strong enough to prevent the erasure due to sphalerons of baryon asymmetry, while keeping the mass of the smallest Higgs boson above the experimental lower bound of about 60 GeV.

hep-ph↗

arXiv1992

The effective potential at finite temperature in the left-right symmetric model

We obtain the effective potential at finite temperature within the left-right symmetric model. We take the Higgs sector to consist of two doublets and calculate its finite temperature effective potential up to the order of ``ring-diagrams'', using analogous techniques to those employed in the derivation of the effective potential in the Standard Model. We find that two first order phase transitions can exist, one at the $SU(2)_R$ breaking scale, the other at the usual $SU(2)_L\otimes U(1)_Y$ breaking scale. Even though more parameters are present than the standard model, we find that the electroweak phase transition is still too weak to allow baryon asymmetry to survive the transition, if the lower bound of the Higgs boson mass from experiment is adhered to.

hep-ph↗