Koo-Chul Lee

Single-beam atom trap in a pyramidal and conical hollow mirror

We present a novel and simple magneto-optical trap in pyramidal and in conical hollow mirrors, using a single beam. A diode laser having modulation sidebands at microwaves is used for cooling, trapping, and repumping of rubidium atoms in a vapor cell. When the laser is circularly polarized and sent into the hollow region, three pairs of counterpropagating beams are automatically produced therein that have the same polarization configuration as that of a conventional six-beam magneto-optical trap. The fluorescence by the trapped atoms and its mirror image are observed simultaneously. This system may be useful for atom-manipulation applications such as gravitational atom traps and atom waveguides.

Atom trap in an axicon mirror

We have realized a novel atom trap in an axicon (conical hollow) mirror, using a frequency-modulated, single-diode laser. Different spatial distributions of trapped atoms such as a ball and a ring are observed. We show that our numerical simulations are consistent with experimental results. In particular, the ring diameter is found to be approximately the separation between the mirror axis and the magnetic field axis. The axicon trap may be useful as a precooled atom source for cold atomic beams, atom funnels, and atom waveguides.

Nonlinear correction to Darcy's law for a flow through periodic arrays of elliptic cylinders

Nonlinear correction to Darcys law for a two-dimensional flow through periodic arrays of elliptic cylinders is investigated using the lattice Boltzmann equation method. For small Reynolds number, we find that the leading correction for the anisotropic system is a third-degree homogeneous function of the average fluid velocity, and possible terms of the function are determined by symmetry. The dimensionless coefficients of the terms, which depend only on the geometry of the system, are determined for various values of the volume fraction and eccentricity of the cylinders. Also, it is found that small deformation of the shape of the cylinder does not significantly change the coefficients.

Generalized circle theorem on zeros of partition function at asymmetric first order transitions.

We present a generalized circle theorem which includes the Lee-Yang theorem for symmetric transitions as a special case. It is found that zeros of the partition function can be written in terms of discontinuities in the derivatives of the free energy. For asymmetric transitions, the locus of the zeros in tangent to the unit circle if the partition functions of the two phases are added up with unequal prefactors. This conclusion is substantiated by explicit calculation of zeroes of the partition function for the Blume-Capel model near and at the triple line at low temperatures.

PHASE DIAGRAM OF THE RESTRICTED SOLID-ON-SOLID MODEL COUPLED TO THE ISING MODEL

We study the phase transitions of a restricted solid-on-solid model coupled to an Ising model, which can be derived from the coupled XY-Ising model. There are two kinds of phase transition lines. One is a Ising transition line and the other is surface roughening transition line. The latter is a KT transition line from the viewpoint of the XY model. Using a microcanonical Monte Carlo technique, we obtain a very accurate two dimensional phase diagram. The two transition lines are separate in all the parameter space we study. This result is strong evidence that the fully frustrated XY model orders by two separate transitions and that roughening and reconstruction transitions of crystal surfaces occur separately.

Programming physics softwares in Flash

We discuss various aspects of programming physics education software in Adobe Flash. Since the authoring environment for Flash is initially developed for non-programmers, it is easy to learn even for those having no previous knowledge in programming language, although some previous experience in programming may help in mastering advanced features of ActionScript, an object-oriented programming language used for developing Flash programs. The most attractive feature of Flash is its strong graphic capability not available in other standard programming languages.

How to teach statistical thermal physics in an introductory physics course

We report on several simulation programs (available through http://phys.snu.ac.kr/howto/ or http://phya.snu.ac.kr/∼kclee/howto/) which can be used to teach the statistical foundations of thermal physics in introductory college physics courses. These programs are simple applications of a technique for generating random configurations of many dice with a fixed total value. By merely simulating dice throwing we can demonstrate all the important principles of classical thermodynamics.

REJECTION-FREE MONTE CARLO TECHNIQUE

An efficient rejection-free Monte Carlo algorithm for lattice systems is presented. In the microcanonical Monte Carlo technique, the sampling time grows prohibitively large due to a high rejection rate at low energies, as the size of the system increases. In this paper we report on an algorithm which improves the sampling efficiency enormously at low energies by selecting only sites which are acceptable for a trial move, thereby reducing the rejection rate to nil.

What makes chaos border sticky

A “microscopic” investigation of long-time correlations near the chaos border of a particular model system has been carried out. Apparent stickiness of chaos border seems to be due to the influence of nearby hyperbolic orbits which are convergent to the invariant curve which constitutes the chaos border. The diffusion near the chaos border can be described by a one-dimensional biased random walk problem with pausing time which is inversely proportional to the distance from the border. Monte Carlo simulation yields that the first passage time distribution has an algebraic decay tail, of the form ∼t−ν with its exponent ν⋍2.80±0.05.

A new efficient Monte Carlo technique

A new efficient Monte Carlo technique is developed and tested using the 2D Ising model for which exact solutions are known. The new technique calculates continuous thermodynamic functions of continuous thermodynamic variables from independent samples taken within an interval of typically a few hundredths of a Monte Carlo step per spin. The new technique is not only efficient and accurate but also furnishes some new information concerning the relationship between canonical and microcanonical averages for finite systems. It also furnishes primary thermodynamic functions such as free energy directly from the Monte Carlo data, a feature not available in the conventional Monte Carlo method.