Byounghak Lee

Theory of t2g electron-gas Rashba interactions

(Received 24 January 2013; published 8 July 2013)Usingqualitativeconsiderationsbasedonatwo-centerapproximationfortight-binding-modelmatrixelements,we demonstrate that Rashba interactions in complex-oxide two-dimensional electron gases are due primarily tochanges in metal-oxygen-metal bond angles at surfaces and interfaces. We verify this conclusion by comparingour picture with illustrative

Ferromagnetism in diluted magnetic semiconductor heterojunction systems

Diluted magnetic semiconductors (DMSs), in which magnetic elements are substituted for a small fraction of host elements in a semiconductor lattice, can become ferromagnetic when doped. In this paper we discuss the physics of DMS ferromagnetism in systems with semiconductor heterojunctions. We focus on the mechanisms that cause of magnetic and magnetoresistive properties to depend on doping profiles, defect distributions, gate voltage and other system parameters that can in principle be engineered to yield desired results. We emphasize that hole densities at the Mn ion locations that exceed 1020 cm−3 are a necessary condition for high ferromagnetic transition temperatures in any geometry.

Electronic structure of ZnTe:O and its usability for intermediate band solar cell

The electronic structures of lattice highly mismatched ZnTe1−xOx alloys are calculated with the linear scaling three-dimensional fragment method. We found that the intermediate band states should be described as a result of the coupling between O impurity states rather than the coupling of the impurity state with the conduction band states. We also found that this system can be used for intermediate band state solar cell with a theoretical efficiency of 63%.

Determining factors of thermoelectric properties of semiconductor nanowires

It is widely accepted that low dimensionality of semiconductor heterostructures and nanostructures can significantly improve their thermoelectric efficiency. However, what is less well understood is the precise role of electronic and lattice transport coefficients in the improvement. We differentiate and analyze the electronic and lattice contributions to the enhancement by using a nearly parameter-free theory of the thermoelectric properties of semiconductor nanowires. By combining molecular dynamics, density functional theory, and Boltzmann transport theory methods, we provide a complete picture for the competing factors of thermoelectric figure of merit. As an example, we study the thermoelectric properties of ZnO and Si nanowires. We find that the figure of merit can be increased as much as 30 times in 8-Å-diameter ZnO nanowires and 20 times in 12-Å-diameter Si nanowires, compared with the bulk. Decoupling of thermoelectric contributions reveals that the reduction of lattice thermal conductivity is the predominant factor in the improvement of thermoelectric properties in nanowires. While the lattice contribution to the efficiency enhancement consistently becomes larger with decreasing size of nanowires, the electronic contribution is relatively small in ZnO and disadvantageous in Si.

Strain induced vortex-to-uniform polarization transitions in soft-ferroelectric nanoparticles

Domain structures of ferroelectric polarization in patterned nanostructures depend sensitively on an interplay between their geometry and crystallographic anisotropy. In the recently predicted layered perovskite PbSr2Ti2O7 [S. M. Nakhmanson and I. Naumov, Phys. Rev. Lett. 104, 097601 (2010)], the in-plane anisotropy can be controlled by strain to be along in-plane [110]-directions or to vanish, in which case the polarization is free to rotate in the perovskite-layer. Using a microscopic Landau-Ginzburg-Devonshire free energy functional theory, we demonstrate that the domain structure in patterned disks of PbSr2Ti2O7 can be varied between uniform polarization and a vortex structure, analogous to vortices in soft magnetic disks. This opens up the possibility of designing nanostructured layered materials whose dielectric response can be manipulated with small elastic distortions.

Field-effect magnetization reversal in ferromagnetic semiconductor quantum wells

Department of Physics, The University of Texas at Austin, Austin, TX 78712(February 6, 2008)We predict that a novel bias-voltage assisted magnetization reversal process will occur in Mn dopedII-VI semiconductor quantum wells or heterojunctions with carrier induced ferromagnetism. Theeffect is due to strong exchange-coupling induced subband mixing that leads to electrically tunablehysteresis loops. Our model calculations are based on the mean-field theory of carrier inducedferromagnetism in Mn-doped quantum wells and on a semi-phenomenological description of thehost II-VI semiconductor valence bands.

Hole–hole correlation effects on magnetic properties of MnxIII1−xV diluted magnetic semiconductors

Abstract The mean-field theory represents a useful starting point for studying carrier-induced ferromagnetism in MnxIII1−xV diluted magnetic semiconductors. A detail description of these systems requires to include correlations in the many-body hole system. We discuss the effects of correlations among itinerant carriers on magnetic properties of bulk MnxIII1−xV and magnetic semiconductor quantum wells. Presented results were obtained using parabolic band approximation and we also derive a many-body perturbation technique that allows to account for hole–hole correlations in realistic semiconductor valence bands.

Quantum Hall Superfluids in Topological Insulator Thin Films

Three-dimensional topological insulators have protected Dirac-cone surface states. In this Letter we argue that gapped excitonic superfluids with spontaneous coherence between top and bottom surfaces can occur in the topological insulator (TI)-thin-film quantum Hall regime. We find that the large dielectric constants of TI materials increase the layer separation range over which coherence survives and decrease the superfluid sound velocity, but have little influence on the superfluid density or on the charge gap. The coherent state at total Landau-level filling factor νT=0 is predicted to be free of edge modes, qualitatively altering its transport phenomenology compared to the widely studied case of νT=1 in GaAs double-quantum wells.

The linearly scaling 3D fragment method for large scale electronic structure calculations

The linearly scaling three-dimensional fragment (LS3DF) method is an O(N) ab initio electronic structure method for large-scale nano material simulations. It is a divide-and-conquer approach with a novel patching scheme that effectively cancels out the artificial boundary effects, which exist in all divide-and-conquer schemes. This method has made ab initio simulations of thousand-atom nanosystems feasible in a couple of hours, while retaining essentially the same accuracy as the direct calculation methods. The LS3DF method won the 2008 ACM Gordon Bell Prize for algorithm innovation. Our code has reached 442 Tflop/s running on 147,456 processors on the Cray XT5 (Jaguar) at OLCF, and has been run on 163,840 processors on the Blue Gene/P (Intrepid) at ALCF, and has been applied to a system containing 36,000 atoms. In this paper, we will present the recent parallel performance results of this code, and will apply the method to asymmetric CdSe/CdS core/shell nanorods, which have potential applications in electronic devices and solar cells.