S. T. Chui

Theoretical investigation on the possibility of preparing left-handed materials in metallic magnetic granular composites

We investigate the possibility of preparing left-handed materials in metallic magnetic granular composites. Based on the effective medium approximation, we show that by incorporating metallic magnetic nanoparticles into an appropriate insulating matrix and controlling the directions of magnetization of metallic magnetic components and their volume fraction, it may be possible to prepare a composite medium of low eddy current loss that is left-handed for electromagnetic waves propagating in some special direction and polarization in a frequency region near the ferromagnetic resonance frequency. This composite may be easier to make on an industrial scale. In addition, its physical properties may be easily tuned by rotating the magnetization locally.

Magnetically controllable unidirectional electromagnetic waveguiding devices designed with metamaterials

We demonstrate a sharply asymmetric reflection in a geometrically symmetric magnetic metamaterial (MM) system, which originates from the coupling of electromagnetic (EM) wave to magnetic surface plasmon resonance states and the broken time reversal symmetry in MM. Based on this effect, we have designed unidirectional EM waveguide, wave bender, and beam splitter. Our simulations indicate that the designs possess the merits of robustness against strong defect and disorder, controllability with an external magnetic field, in addition to the finite working bandwidth and the high transmission efficiency.

Formation of robust and completely tunable resonant photonic band gaps

Bartol Research Institute, University of Delaware, Newark, Delaware 19716, USA(Dated: May 8, 2008)We identify different types of the photonic band gaps (PBGs) of two dimensional magnetic pho-tonic crystals (MPCs) consisting of arrays of magnetic cylinders and study the different tunability(by an external static magnetic field) of these PBGs. One type of the band gaps comes from in-finitely degenerate flat bands and is closely related to those in the study of plasmonics. In addition,such PBGs are magnetically tunable and robust against position disorder. We calcualte the trans-mission of the PBG’s and found excellent agreement with the results of the photonic band structurecalculation. Positional disorder of the lattice structure affects the different types of PBGs differently.

SOLITON AND 2D DOMAINS IN ULTRATHIN MAGNETIC FILMS

We show that many two dimensional domain patterns observed in Monte Carlo simulations can be obtained from the many soliton solutions of the imaginary time Sine Gordon equation. This opens the door to analytic physical understanding of the micromagnetics in ultra-thin films.

ac transport in ferromagnetic tunnel junctions

To study possible capacitive effects, we incorporate the effect of electron interaction on the ac spin-polarized tunnelling. Under steady-state nonequilibrium conditions, the voltage-induced charge accumulated at the interface is a sum of two terms, decaying with length scales of the order of the screening length and the spin diffusion length. As a result, the effective width of the capacitor is changed by an additional term. This additional term is a function of the magnetic configurations on opposite sides of the junction and a magnetocapacitance is introduced.

Tunable terahertz radiation from graphene induced by moving electrons

Based on a structure consisting of a single graphene layer situated on periodic dielectric gratings, we show theoretically that terahertz radiation can be generated by low-energy electron bunches moving atop the graphene layer. The THz emission arises from graphene plasmons excited efficiently by the moving electrons. We find that the radiation intensity can be strongly enhanced due to the local field enhancement of graphene plasmons arising from their low losses and high confinement. Importantly, the radiation frequency can be tuned over a wide spectral range by varying the Fermi level of the graphene layer. Our results could find applications in developing tunable and miniature free-electron terahertz radiation sources.

Nucleation of planar magnetization in ultra-thin magnetic films

Abstract We study theoretically the magnetization reversal of Heisenberg spins in a 2D plane under an external reversing field at finite temperatures. Nucleation is observed in a Monte Carlo simulation for an array of spins interacting with exchange, anisotropic and dipolar interactions. The nucleus is elliptical in shape. The eccentricity is smaller the larger the dipolar interaction is with respect to the exchange. The coercive field seems to depend linearly on the temperature.

SWITCHING IN SINGLE-DOMAIN NANOSTRUCTURES AT FINITE TEMPERATURES

We study the finite temperature magnetization reversal of single domain nanostructures (particles and wires) of different materials with Monte Carlo and analytic techniques. The temperature, angular, and shape dependences of the switching field are calculated. For large structure diameters, there are different reversal mechanisms at different orientations of the external field. For small structure diameters growth usually starts with the nucleation and a subsequent depinning of domain walls at the end(s) of the structure. The nucleation energy of the domain wall in a magnetic field approaches zero near the coherent rotation limit at small aspect ratios and at fields less than the coherent rotation limit at large aspect ratios. As the domain wall energy approaches zero the domain wall width can remain finite. For small diameters there is a significant temperature dependence in the coercive field. For structures whose easy crystalline anisotropy axis is not along the major axis, a variety of switching scena...

Reflected wave of finite circulation from magnetic photonic crystals

We study the reflection of electromagnetic waves from a two-dimensional magnetic photonic crystal consisting of a periodic array of magnetic cylinders. At some frequencies the reflected wave is found to exhibit a strong circulation in that, locally, the angular momenta of the components are all of the same sign. As a result of this finite circulation, beams incident from different directions exhibit a dramatic change in their reflected waves. This effect can be used to build subwavelength one way waveguides, nearly perfect beam benders and splitters.

Phase separation and vortex states in the binary mixture of Bose-Einstein condensates

The phase separation and vortex states in a two-component Bose-Einstein condensate consisting of |F = 1, mf = 1〉 and |2, 1〉 internal spin states of 87Rb atoms are considered in the framework of the Thomas-Fermi approximation. It is shown that in the nonrotating system, the atoms in the state |1, −1〉 form a shell around the atoms in the state |2, 1〉. The critical angular velocity for each state is calculated. These velocities depend drastically on the relative concentrations of the components, the critical angular velocity of the outer component being less than the angular velocity of the inner one. It is shown that the atoms in the |1, −1〉 state can form a rotating ring around the resting core of the atoms in the |2, 1〉 state.