Eok Kyun Lee

Melting Behaviors of Icosahedral Metal Clusters Studied by Monte Carlo Simulations

We present an atom‐resolved analysis method that traces physical quantities such as the root‐mean‐square bond length fluctuation and coordination number for individual atoms as functions of temperature or time. This method is applied to explain the temperature‐dependent behaviors of three types of NiN (N=12,13,14) clusters. The detailed studies for the three types of clusters reveal characteristics as follows: (a) as the temperature increases, all three types of clusters undergo two‐stage melting, irrespective of the existence of vacancy or adatom on the icosahedral surfaces, (b) the melting of icosahedral clusters with vacancy starts with vacancy hopping, which has not been observed for any type of small clusters (N<34), (c) the melting of the icosahedral clusters with adatom (N=14) is initiated by adatom hopping, followed by the site exchange between the adatom and surface atoms.

Brownian motion from molecular dynamics

Brownian motion of single particles with various masses M and diameters D is studied by molecular dynamics simulations. Besides the momentum auto-correlation function of the Brownian particle the memory function and the fluctuating force which enter the generalized Langevin equation of the Brownian particle are determined and their dependence on mass and diameter are investigated for two different fluid densities. Deviations of the fluctuating force distribution from a Gaussian form are observed for small particle diameters. For heavy particles the deviations of the fluctuating force from the total force acting on the Brownian particle decrease linearly with the mass ratio m/M where m denotes the mass of a fluid particle.

Quantum cryptography using single-particle entanglement

A quantum cryptography scheme based on entanglement between a single-particle state and a vacuum state is proposed. The scheme utilizes linear optics devices to detect the superposition of the vacuum and single-particle states. Existence of an eavesdropper can be detected by using a variant of Bells inequality.

Effects of nonmagnetic impurities on the spin transport property of a graphene nanoribbon device

Using a nonequilibrium density functional calculation, we investigated the electronic transport properties and fundamental mechanism of spin polarization as a function of the location of impurities from the center to an edge of a graphene nanoribbon device (GND) with zigzag edges. A center-located impurity enables both edges to be enhanced with respect to their spin transports whereas an edge-located impurity results in only the opposite edge channel being dominant. In the case of a center-located impurity, the ferromagnetic ground state induces new spin states near the Fermi level responsible for the spin-polarized current in the GND. We argue that the spin-polarized current can flow through the edge states induced by a nonmagnetic impurity around the Fermi level, especially on a GND with a center-located impurity.

Rate description of Fokker-Planck processes with time-periodic parameters

The large time dynamics of a periodically driven Fokker–Planck process possessing several metastable states is investigated. At weak noise transitions between the metastable states are rare. Their dynamics then represent a discrete Markovian process characterized by time dependent rates. Apart from the occupation probabilities, so-called specific probability densities and localizing functions can be associated to each metastable state. Together, these three sets of functions uniquely characterize the large time dynamics of the conditional probability density of the original process. Exact equations of motion are formulated for these three sets of functions and strategies are discussed how to solve them. These methods are illustrated and their usefulness is demonstrated by means of the example of a bistable Brownian oscillator within a large range of driving frequencies from the slow semiadiabatic to the fast driving regime.

Qubit Geometry and Conformal Mapping

AbstractIdentifying the Bloch sphere with the Riemann sphere (the extended complex plane), we obtain relations between single qubit unitary operations and Möbius transformations on the extended complex plane. PACS: 03.67.-a, 03.67.Lx, 03.67.Hk

Universal melting behaviour of clusters

Abstract We have simulated the melting behaviour of metal and non-metal clusters for the size of 12⩽ n ⩽34, where n is the number of atoms. We find that the non-monotonic variation of melting temperature with size, the dependence of melting, boiling, and sublimation temperatures on the potentials, the existence of a surface-melted phase, and the absence of a premelting peak in heat capacity curves can be explained by the relative stability of the internal atoms to the surface atoms in the clusters.

First-Principles Study of the α–β Phase Transition of Ferroelectric Poly(vinylidene difluoride): Observation of Multiple Transition Pathways

Transition routes from the α (nonpolar) phase to the β (polar) phase of polyvinylidene difluoride (PVDF) are investigated by first-principles simulation methods. Among various possible routes, including complex torsional and rotational motions, we propose two prototypical transition routes and identify important intermediate structures along each transition pathway using the generalized solid-state nudged elastic band (G-SSNEB) method. The effect of the external electric field and mechanical drawing on the transition behavior is investigated by estimating electric enthalpy and stress tensors. Finite-temperature ab initio molecular dynamics (AIMD) simulations and stress tensor analysis reveal the possibility of enhancement of the crystallinity under hydrodynamic compression.

DYNAMICS OF SIMPLE FLUIDS CONFINED IN CYLINDRICAL PORE: EFFECT OF PORE SIZE

We study the thermodynamic and dynamical properties of Weeks–Chandler–Anderson (WCA) fluids confined in a cylindrical pore by means of a canonical molecular dynamics simulation method. The pore model is an infinitely long cylinder consisted of a thermal wall and mimics a typical carbon nanotube. The thermodynamic properties are obtained for relatively high density fluids over a wide range of pore diameters at a given temperature. The size dependence of the self-diffusion coefficients in the cylindrical pore is also investigated. It is found that, as the pore diameter decreases, the potential energy and axial component of the pressure exhibit a sharp rise and the self-diffusion coefficient decreases. The observed behaviors can be understood in terms of the geometrical confinement and attenuation of transport induced by dispersive fluid-wall interaction. In addition, anomalous diffusion is observed at the pore size corresponding to twice the particle diameter.

Thermodynamic properties of trapped, two-dimensional interacting Bose gases in Hartree-Fock approximation

We demonstrate the importance of the consistent treatment of mutual interaction between the condensate and the thermal component by applying the semiclassical Hartree-Fock model to the 2D trapped interacting Bose system. In contrast to the case where the interaction from the thermal component is neglected, the present system shows a lowering of the critical temperature and increase of the critical chemical potential due to the short-range repulsive interaction.