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Search Results: 1 - 10 of 434497 matches for " S. H. Youn "
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Impacts of aircraft emissions on the air quality near the ground
H. Lee,S. C. Olsen,D. J. Wuebbles,D. Youn
Atmospheric Chemistry and Physics Discussions , 2013, DOI: 10.5194/acpd-13-689-2013
Abstract: The continuing increase in demand for commercial aviation transport raises questions about the effects of resulting emissions on the environment. The purpose of this study is to investigate, using a global chemistry transport model, to what extent aviation emissions outside the boundary layer influence air quality in the boundary layer. The effects of current levels of aircraft emissions were studied through comparison of multiple simulations allowing for the separated effects of aviation emissions occurring in the low, middle and upper troposphere. We show that emissions near cruise altitudes rather than emissions during landing and take-off are responsible for most of the total odd-nitrogen (NOy), ozone (O3) and aerosol perturbations near the ground with a noticeable seasonal difference. Overall, the perturbations of these species are smaller than 1 ppb even in winter when the perturbations are greater than in summer. Based on the widely used air quality standards and uncertainty of state-of-the-art models, we conclude that aviation-induced perturbations have a negligible effect on air quality even in areas with heavy air traffic. Aviation emissions lead to a less than 1% aerosol enhancement in the boundary layer due to a slight increase in ammonium nitrate (NH4NO3) during cold seasons and a statistically insignificant aerosol perturbation in summer. In addition, statistical analysis using probability density functions, Hellinger distance, and p-value indicate that aviation emissions outside the boundary layer do not affect the occurrence of extremely high aerosol concentrations in the boundary layer. An additional sensitivity simulation assuming the doubling of surface ammonia emissions demonstrates that the aviation induced aerosol increase near the ground is highly dependent on background ammonia concentrations whose current range of uncertainty is large.
Hexagonal pnictide SrPtAs: superconductivity with locally broken inversion symmetry
S. J. Youn,M. H. Fischer,S. H. Rhim,M. Sigrist,D. F. Agterberg
Physics , 2011,
Abstract: Unlike other pnictides, SrPtAs has a hexagonal structure, containing layers with As-Pt atoms that form a honeycomb lattice. These layers lack inversion symmetry which allows for a spin-orbit coupling that we show has a dramatic effect on superconductivity in this material. In particular, for conventional s-wave superconductivity in SrPtAs, both the spin susceptibility and the paramagnetic limiting field are enhanced significantly with respect to that usually expected for s-wave superconductors. SrPtAs provides a prime example of a superconductor with locally broken inversion symmetry.
Fabrication and Electrical Properties of Pure VO2 Phase Films
B. G. Chae,D. H. Youn,H. T. Kim,S. Y. Maeng,K. Y. Kang
Physics , 2003,
Abstract: We have grown VO2 thin films by laser ablation for electronic device applications. In obtaining the thin films of the pure VO2 phase, oxygen partial pressure is a critical parameter because vanadium oxides have several phases with the oxygen concentration. It is found that the pure VO2 films are epitaxially grown on Al2O3 substrate in the narrow ranges of 55-60 mTorr in an Ar+10% O2 ambient, and that the mixed phase films are synthesized when the deposition pressure slightly deviates from the optimum pressure. The (100) oriented VO2 films undergo an abrupt metal-insulator transition (MIT) with resistance change of an order of 104 at 338K. In the films of mixed phases, the small change of the resistance is observed at the same temperature. The polycrystalline films grown on SiO2/Si substrate undergo a broaden MIT of the resistance. Furthermore, the abrupt MIT and collective current motion appearing in metal are observed when the electric field is applied to the film.
Role of spin-orbit coupling on the electronic structure and properties of SrPtAs
S. J. Youn,S. H. Rhim,D. F. Agterberg,M. Weinert,A. J. Freeman
Physics , 2012,
Abstract: The effect of spin-orbit coupling on the electronic structure of the layered iron-free pnictide superconductor, SrPtAs, has been studied using the full potential linearized augmented plane wave method. The anisotropy in Fermi velocity, conductivity and plasma frequency stemming from the layered structure are found to be enhanced by spin-orbit coupling. The relationship between spin-orbit interaction and the lack of two-dimensional inversion in the PtAs layers is analyzed within a tight-binding Hamiltonian based on the first-principles calculations. Finally, the band structure suggests that electron doping could increase $T_c$.
Observation of Mott Transition in VO_2 Based Transistors
Hyun-Tak Kim,B. G. Chae,D. H. Youn,S. L. Maeng,K. Y. Kang
Physics , 2003, DOI: 10.1088/1367-2630/6/1/052
Abstract: An abrupt Mott metal-insulator transition (MIT) rather than the continuous Hubbard MIT near a critical on-site Coulomb energy U/U_c=1 is observed for the first time in VO_2, a strongly correlated material, by inducing holes of about 0.018% into the conduction band. As a result, a discontinuous jump of the density of states on the Fermi surface is observed and inhomogeneity inevitably occurs. The gate effect in fabricated transistors is clear evidence that the abrupt MIT is induced by the excitation of holes.
Nonequilibrium Dephasing in an Electronic Mach-Zehnder Interferometer
Seok-Chan Youn,Hyun-Woo Lee,H. -S. Sim
Physics , 2007, DOI: 10.1103/PhysRevLett.100.196807
Abstract: We study nonequilibrium dephasing in an electronic Mach-Zehnder interferometer. We demonstrate that the shot noise at the beam splitter of the interferometer generates an ensemble of nonequilibrium electron density configurations and that electron interactions induce configuration-specific phase shifts of an interfering electron. The resulting dephasing exhibits two characteristic features, a lobe pattern in the visibility and phase jumps of $\pi$, in good agreement with experimental data.
Spatial and temporal characterization of a Bessel beam produced using a conical mirror
K. B. Kuntz,B. Braverman,S. H. Youn,M. Lobino,E. M. Pessina,A. I. Lvovsky
Physics , 2008, DOI: 10.1103/PhysRevA.79.043802
Abstract: We experimentally analyze a Bessel beam produced with a conical mirror, paying particular attention to its superluminal and diffraction-free properties. We spatially characterized the beam in the radial and on-axis dimensions, and verified that the central peak does not spread over a propagation distance of 73 cm. In addition, we measured the superluminal phase and group velocities of the beam in free space. Both spatial and temporal measurements show good agreement with the theoretical predictions.
Electronic interferometer capacitively coupled to a quantum dot
Seok-Chan Youn,Hyun-Woo Lee,H. -S. Sim
Physics , 2009, DOI: 10.1103/PhysRevB.80.113307
Abstract: We theoretically study electron interference in a ballistic electronic interferometer capacitively coupled to a quantum dot. The visibility of the interference is reduced when the dot has degenerate ground states with different excess charges. The degree of the reduction depends on system parameters such as the strength of the capacitive coupling, and the dependence is analyzed in the regime where the dwell time of electrons in the dot is much longer than the electron flight time through the interferometry region coupled to the dot. The result is consistent with recent experimental data.
Temperature dependence and control of the Mott transition in VO_2 based devices
Hyun-Tak Kim,B. G. Chae,D. H. Youn,S. L. Maeng,K. Y. Kang
Physics , 2003,
Abstract: The transition voltage of an abrupt metal-insulator transition (MIT), observed by applying an electric field to two-terminal devices fabricated on a Mott insulator VO_2 film, decreases with increasing temperature up to 334K. The abrupt current jump disappears above 334 K near the MIT temperature. These results suggest that the mechanism of the abrupt MIT induced by temperature is the same as that by an electric field. The magnitude of the current jump (a large current) decreases with increasing external resistance; this is an important observation in terms of applying the abrupt MIT to device applications. Furthermore, the temperature and resistance dependence of the MIT cannot be explained by the dielectric breakdown although a current jump known as breakdown is similar to that observed in an abrupt MIT.
Gate-Induced Mott Transition
Hyun-Tak Kim,B. G. Chae,D. H. Youn,S. L. Maeng,K. Y. Kang
Physics , 2003,
Abstract: For a strongly correlated material, VO2, near a critical on-site Coulomb energy U/Uc=1, the abrupt Mott metal-insulator transition (MIT) rather than the continuous Hubbard MIT is observed by inducing internal optical phonon-coupled holes (hole inducing of 0.018%) into conduction band, with a gate field of fabricated transistors. Observed gate effects, change of the MIT drain-source voltage caused by a gate field, are the effect of measurement due to inhomogeneity of channel material and is an average over the measurement region of the true gate effect based on the large conductivity (or effective mass) near the MIT predicted by the Brinkman-Rice picture. A discontinuous gate effect such as digital is observed, which is a characteristic of the transistor and a possible condition of a very high-speed switching transistor.
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