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Search Results: 1 - 10 of 10532 matches for " Young-Woo Son "
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Vortex Dynamics in an Annular Bose-Einstein Condensate
S. J. Woo,Young-Woo Son
Physics , 2012, DOI: 10.1103/PhysRevA.86.011604
Abstract: We theoretically show that the topology of a non-simply-connected annular atomic Bose-Einstein condensate enforces the inner surface waves to be always excited with outer surface excitations and that the inner surface modes are associated with induced vortex dipoles unlike the surface waves of a simply-connected one with vortex monopoles. Consequently, under stirring to drive an inner surface wave, a peculiar population oscillation between the inner and outer surface is generated regardless of annulus thickness. Moreover, a new vortex nucleation process by stirring is observed that can merge the inner vortex dipoles and outer vortex into a single vortex inside the annulus. The energy spectrum for a rotating annular condensate with a vortex at the center also reveals the distinct connection of the Tkachenko modes of a vortex lattice to its inner surface excitations.
Ideal Strength of Doped Graphene
S. J. Woo,Young-Woo Son
Physics , 2012, DOI: 10.1103/PhysRevB.87.075419
Abstract: While the mechanical distortions change the electronic properties of graphene significantly, the effects of electronic manipulation on its mechanical properties have not been known. Using first-principles calculation methods, we show that, when graphene expands isotropically under equibiaxial strain, both the electron and hole doping can maintain or improve its ideal strength slightly and enhance the critical breaking strain dramatically. Contrary to the isotropic expansions, the electron doping decreases the ideal strength as well as critical strain of uniaxially strained graphene while the hole doping increases the both. Distinct failure mechanisms depending on type of strains are shown to be origins of the different doping induced mechanical stabilities. Our findings may resolve a contradiction between recent experimental and theoretical results on the strength of graphene.
Covalent functionalization of strained graphene
Danil W. Boukhvalov,Young-Woo Son
Physics , 2012, DOI: 10.1002/cphc.201100847
Abstract: Enhancement of the chemical activity of graphene is evidenced by first-principles modelling of chemisorption of the hydrogen, fluorine, oxygen and hydroxyl groups on strained graphene. For the case of negative strain or compression, chemisorption of the single hydrogen, fluorine or hydroxyl group is energetically more favourable than those of their pairs on different sublattices. This behaviour stabilizes the magnetism caused by the chemisorption being against its destruction by the pair formations. Initially flat, compressed graphene is shown to buckle spontaneously right after chemisorption of single adatoms. Unlike hydrogenation or fluorination, the oxidation process turns from the endothermic to exothermic for all types of the strain and depends on the direction of applied strains. Such properties will be useful in designing graphene devices utilizing functionalization as well as mechanical strains.
Oxygen reduction reactions on pure and nitrogen-doped graphene: a first-principles modeling
Danil W. Boukhvalov,Young-Woo Son
Physics , 2011, DOI: 10.1039/C1NR11307K
Abstract: Based on first principles density functional theory calculations we explored energetics of oxygen reduction reaction over pristine and nitrogen-doped graphene with different amounts of nitrogen doping. The process of oxygen reduction requires one more step then same reaction catalyzed by metals. Results of calculations evidence that for the case of light doped graphene (about 4% of nitrogen) energy barrier for each step is lower than for the same process on Pt surface. In contrast to the catalysis on metal surface the maximal coverage of doped graphene is lower and depends on the corrugation of graphene. Changes of the energy barriers caused by oxygen load and corrugation are also discussed.
Scattering Theory Approach to Inelastic Transport in Nanoscale Systems
Sejoong Kim,Young-Woo Son
Physics , 2013, DOI: 10.1103/PhysRevB.87.195423
Abstract: We present a scattering-state description for the non-equilibrium multichannel charge transport in the presence of electron-vibration couplings. It is based on an expansion of scattering orders of eigenchannel states. Examining charge transitions between scattering states, we clarifies competing inelastic and elastic scattering processes, and compare with the interpretation based on the non-equilibrium Green's functions formalism. We also derive a general expression for conductance variations in single-channel systems. It provides a comprehensive picture for the variation including the well-known result, the 0.5 rule, from the aspect of interplay between elastic and inelastic scattering processes.
Poisson's Ratio of Layered Two-dimensional Crystals
Sungjong Woo,Hee Chul Park,Young-Woo Son
Physics , 2015,
Abstract: We present first-principles calculations of elastic properties of multilayered two-dimensional crystals such as graphene, h-BN and 2H-MoS2 which shows that their Poisson's ratios along out-of-plane direction are negative, near zero and positive, respectively, spanning all possibilities for sign of the ratios. While the in-plane Poisson's ratios are all positive regardless of their disparate electronic and structural properties, the characteristic interlayer interactions as well as layer stacking structures are shown to determine the sign of their out-of-plane ratios. Thorough investigation of elastic properties as a function of the number of layers for each system is also provided, highlighting their intertwined nature between elastic and electronic properties.
Half-Metallic Graphene Nanoribbons
Young-Woo Son,Marvin L. Cohen,Steven G. Louie
Physics , 2006, DOI: 10.1038/nature05180
Abstract: Electrical current can be completely spin polarized in a class of materials known as half-metals, as a result of the coexistence of metallic nature for electrons with one spin orientation and insulating for electrons with the other. Such asymmetric electronic states for the different spins have been predicted for some ferromagnetic metals - for example, the Heusler compounds- and were first observed in a manganese perovskite. In view of the potential for use of this property in realizing spin-based electronics, substantial efforts have been made to search for half-metallic materials. However, organic materials have hardly been investigated in this context even though carbon-based nanostructures hold significant promise for future electronic device. Here we predict half-metallicity in nanometre-scale graphene ribbons by using first-principles calculations. We show that this phenomenon is realizable if in-plane homogeneous electric fields are applied across the zigzag-shaped edges of the graphene nanoribbons, and that their magnetic property can be controlled by the external electric fields. The results are not only of scientific interests in the interplay between electric fields and electronic spin degree of freedom in solids but may also open a new path to explore spintronics at nanometre scale, based on graphene.
Energy Gaps in Graphene Nanoribbons
Young-Woo Son,Marvin L. Cohen,Steven G. Louie
Physics , 2006, DOI: 10.1103/PhysRevLett.97.216803
Abstract: Based on a first-principles approach, we present scaling rules for the band gaps of graphene nanoribbons (GNRs) as a function of their widths. The GNRs considered have either armchair or zigzag shaped edges on both sides with hydrogen passivation. Both varieties of ribbons are shown to have band gaps. This differs from the results of simple tight-binding calculations or solutions of the Dirac's equation based on them. Our {\it ab initio} calculations show that the origin of energy gaps for GNRs with armchair shaped edges arises from both quantum confinement and the crucial effect of the edges. For GNRs with zigzag shaped edges, gaps appear because of a staggered sublattice potential on the hexagonal lattice due to edge magnetization. The rich gap structure for ribbons with armchair shaped edges is further obtained analytically including edge effects. These results reproduce our {\it ab initio} calculation results very well.
Dynamic Jahn-Teller Mechanism of Superconductivity in MgB$_2$
Young-Woo Son,Jaejun Yu,Jisoon Ihm
Physics , 2002,
Abstract: We propose a novel mechanism of superconductivity in MgB$_2$ based on the dynamic electronic structure of the $p\sigma$-orbitals coupled with $e_{2g}$ phonons. A nonconventional superconducting state is found to arise frome lectron-phonon interactions in the presence of additional pairing channels made available by the dynamic Jahn-Teller effects. A partially broken pseudo-spin symmetry in this Jahn-Teller system, together with two-phonon exchange pairing, naturally gives rise to two distinct gaps both of which are basically isotropic in the ($k_x$, $k_y$) space. Important experimental observations including high $T_c$ and the anomalous specific heat are explained using this theory.
Negative Thermal Expansion Coefficient of Graphene Measured by Raman Spectroscopy
Duhee Yoon,Young-Woo Son,Heonsik Cheong
Physics , 2011, DOI: 10.1021/nl201488g
Abstract: The thermal expansion coefficient (TEC) of single-layer graphene is estimated with temperature-dependent Raman spectroscopy in the temperature range between 200 and 400 K. It is found to be strongly dependent on temperature but remains negative in the whole temperature range, with a room temperature value of -8.0x10^{-6} K^{-1}. The strain caused by the TEC mismatch between graphene and the substrate plays a crucial role in determining the physical properties of graphene, and hence its effect must be accounted for in the interpretation of experimental data taken at cryogenic or elevated temperatures.
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