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Search Results: 1 - 10 of 241008 matches for " Y. -P. Shim "
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Tunneling spectroscopy of spin-selective Aharonov-Bohm oscillations in a lateral triple quantum dot molecule
Y. -P. Shim,F. Delgado,P. Hawrylak
Physics , 2009, DOI: 10.1103/PhysRevB.80.115305
Abstract: We present a theory of tunneling spectroscopy of spin-selective Aharonov-Bohm oscillations in a lateral triple quantum dot molecule. The theory combines exact treatment of an isolated many-body system with the rate equation approach when the quantum dot molecule is weakly connected to the leads subject to arbitrary source-drain bias. The tunneling spectroscopy of the many-body complex is analyzed using the spectral functions of the system and applied to holes in a quantum dot molecule. Negative differential conductance is predicted and explained as a result of the redistribution of the spectral weight between transport channels. It is shown that different interference effects on singlet and triplet hole states in a magnetic field lead to spin-selective Aharonov-Bohm oscillations.
Spin-Orbit Interactions in Bilayer Exciton-Condensate Ferromagnets
Y. -P. Shim,A. H. MacDonald
Physics , 2009, DOI: 10.1103/PhysRevB.79.235329
Abstract: Bilayer electron-hole systems with unequal electron and hole densities are expected to have exciton condensate ground states with spontaneous spin-polarization in both conduction and valence bands. In the absence of spin-orbit and electron-hole exchange interactions there is no coupling between the spin-orientations in the two quantum wells. In this article we show that Rashba spin-orbit interactions lead to unconventional magnetic anisotropies, whose strength we estimate, and to ordered states with unusual quasiparticle spectra.
Theory of spin, electronic and transport properties of the lateral triple quantum dot molecule in a magnetic field
F. Delgado,Y. -P. Shim,M. Korkusinski,P. Hawrylak
Physics , 2007, DOI: 10.1103/PhysRevB.76.115332
Abstract: We present a theory of spin, electronic and transport properties of a few-electron lateral triangular triple quantum dot molecule in a magnetic field. Our theory is based on a generalization of a Hubbard model and the Linear Combination of Harmonic Orbitals combined with Configuration Interaction method (LCHO-CI) for arbitrary magnetic fields. The few-particle spectra obtained as a function of the magnetic field exhibit Aharonov-Bohm oscillations. As a result, by changing the magnetic field it is possible to engineer the degeneracies of single-particle levels, and thus control the total spin of the many-electron system. For the triple dot with two and four electrons we find oscillations of total spin due to the singlet-triplet transitions occurring periodically in the magnetic field. In the three-electron system we find a transition from a magnetically frustrated to the spin-polarized state. We discuss the impact of these phase transitions on the addition spectrum and the spin blockade of the lateral triple quantum dot molecule.
FFLO Vortex Lattice States in Cold Fermionic-Atom Systems
Y. -P. Shim,R. A. Duine,A. H. MacDonald
Physics , 2006, DOI: 10.1103/PhysRevA.74.053602
Abstract: Condensation of atom pairs with finite total momentum is expected in a portion of the phase diagram of a two-component fermionic cold-atom system. This unusual condensate can be identified by detecting the exotic higher Landau level (HLL) vortex lattice states it can form when rotated. With this motivation, we have solved the linearized gap equations of a polarized cold atom system in a Landau level basis to predict experimental circumstances under which HLL vortex lattice states occur.
Non-Drude Optical Conductivity of (III,Mn)V Ferromagnetic Semiconductors
S. -R. Eric Yang,Jairo Sinova,T. Jungwirth,Y. P. Shim,A. H. MacDonald
Physics , 2002, DOI: 10.1103/PhysRevB.67.045205
Abstract: We present a numerical model study of the zero-temperature infrared optical properties of (III,Mn)V diluted magnetic semiconductors. Our calculations demonstrate the importance of treating disorder and interaction effects simultaneously in modelling these materials. We find that the conductivity has no clear Drude peak, that it has a broadened inter-band peak near 220 meV, and that oscillator weight is shifted to higher frequencies by stronger disorder. These results are in good qualitative agreement with recent thin film absorption measurements. We use our numerical findings to discuss the use of f-sum rules evaluated by integrating optical absorption data for accurate carrier-density estimates.
Spin selective Aharonov-Bohm oscillations in a lateral triple quantum dot
F. Delgado,Y. -P. Shim,M. Korkusinski,L. Gaudreau,S. A. Studenikin,A. S. Sachrajda,P. Hawrylak
Physics , 2008, DOI: 10.1103/PhysRevLett.101.226810
Abstract: We present a theory for spin selective Aharonov-Bohm oscillations in a lateral triple quantum dot. We show that to understand the Aharonov-Bohm (AB) effect in an interacting electron system within a triple quantum dot molecule (TQD) where the dots lie in a ring configuration requires one to not only consider electron charge but also spin. Using a Hubbard model supported by microscopic calculations we show that, by localizing a single electron spin in one of the dots, the current through the TQD molecule depends not only on the flux but also on the relative orientation of the spin of the incoming and localized electrons. AB oscillations are predicted only for the spin singlet electron complex resulting in a magnetic field tunable "spin valve".
External Modulation and Switching of Acoustic Phonons: Comparative Roles of Potential Distributions
H. Jeong,Y. D. Jho,S. H. Rhim,K. J. Yee,J. P. Shim,D. S. Lee,J. W. Ju,J. H. Baek,C. J. Stanton
Physics , 2015,
Abstract: Acoustic phonons can be coherently generated by ultrafast displacive screening of potential gradients, often enhanced by the strong built-in piezoelectric fields, in wurtzite semiconductors. In such structures, transverse symmetry within the c plane hinders both the generation and detection of the transverse acoustic (TA) modes, and only longitudinal acoustic (LA) mode is generated. We show that even for c-GaN, the application of asymmetric potential distributions in the c plane can break the symmetry and selection rules, thus switching on the normally forbidden TA mode. This is in contrast to the LA mode, the strength of which varies with the symmetric potential distributions. By comparing transient differential reflectivity spectra in structures with and without asymmetric potential distributions, the role of the electrically attained anisotropy was further revealed by the digitized appearance of the TA mode, in clear contrast to the monotonic LA mode, and by modulations in the propagation velocities, optical birefringence, and geometrically varying sensitivities, the underlying mechanisms of which are modeled by electric-field-dependent perturbations of the dielectric tensors, incorporating the results of elastic modulations.
Evaluation of model-simulated source contributions to tropospheric ozone with aircraft observations in the factor-projected space
C. Shim, Y. Wang,Y. Yoshida
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2008,
Abstract: Trace gas measurements of TOPSE and TRACE-P experiments and corresponding global GEOS-Chem model simulations are analyzed with the Positive Matrix Factorization (PMF) method for model evaluation purposes. Specially, we evaluate the model simulated contributions to O3 variability from stratospheric transport, intercontinental transport, and production from urban/industry and biomass burning/biogenic sources. We select a suite of relatively long-lived tracers, including 7 chemicals (O3, NOy, PAN, CO, C3H8, CH3Cl, and 7Be) and 1 dynamic tracer (potential temperature). The largest discrepancy is found in the stratospheric contribution to 7Be. The model underestimates this contribution by a factor of 2–3, corresponding well to a reduction of 7Be source by the same magnitude in the default setup of the standard GEOS-Chem model. In contrast, we find that the simulated O3 contributions from stratospheric transport are in reasonable agreement with those derived from the measurements. However, the springtime increasing trend over North America derived from the measurements are largely underestimated in the model, indicating that the magnitude of simulated stratospheric O3 source is reasonable but the temporal distribution needs improvement. The simulated O3 contributions from long-range transport and production from urban/industry and biomass burning/biogenic emissions are also in reasonable agreement with those derived from the measurements, although significant discrepancies are found for some regions.
Evaluation of model-simulated source contributions to tropospheric ozone with aircraft observations in the factor-projected space
C. Shim,Y. Wang,Y. Yoshida
Atmospheric Chemistry and Physics Discussions , 2007,
Abstract: Trace gas measurements of TOPSE and TRACE-P experiments and corresponding global GEOS-CHEM model simulations are analyzed with the Positive Matrix Factorization (PMF) method for model evaluation purposes. Specially, we evaluate the model simulated contributions to O3 variability from stratospheric transport, intercontinental transport, and production from urban/industry and biomass burning/biogenic sources. We select a suite of relatively long-lived tracers, including 7 chemicals (O3, NOy, PAN, CO, C3H8, CH3Cl, and 7Be) and 1 dynamic tracer (potential temperature). The largest discrepancy is found in the stratospheric contribution to 7Be. The model underestimates this contribution by a factor of 2–3, corresponding well to a reduction of 7Be source by the same magnitude in the default setup of the standard GEOS-CHEM model. In contrast, we find that the simulated O3 contributions from stratospheric transport are in reasonable agreement with those derived from the measurements. However, the springtime increasing trend over North America derived from the measurements are largely underestimated in the model, indicating that the magnitude of simulated stratospheric O3 source is reasonable but the temporal distribution needs improvement. The simulated O3 contributions from long-range transport and production from urban/industry and biomass burning/biogenic emissions are also in reasonable agreement with those derived from the measurements, although significant discrepancies are found for some regions.
Evolutionary Repercussions of Avian Culling on Host Resistance and Influenza Virulence
Eunha Shim, Alison P. Galvani
PLOS ONE , 2009, DOI: 10.1371/journal.pone.0005503
Abstract: Background Keeping pandemic influenza at bay is a global health priority. Of particular concern is the continued spread of the influenza subtype H5N1 in avian populations and the increasing frequency of transmission to humans. To decrease this threat, mass culling is the principal strategy for eradicating influenza in avian populations. Although culling has a crucial short-term epidemiological benefit, evolutionary repercussions on reservoir hosts and on the viral population have not been considered. Methods and Findings To explore the epidemiological and evolutionary repercussions of mass avian culling, we combine population genetics and epidemiological influenza dynamics in a mathematical model parameterized by clinical, epidemiological, and poultry data. We model the virulence level of influenza and the selection on a dominant allele that confers resistance against influenza [1], [2] in a poultry population. Our findings indicate that culling impedes the evolution of avian host resistance against influenza. On the pathogen side of the coevolutionary race between pathogen and host, culling selects for heightened virulence and transmissibility of influenza. Conclusions Mass culling achieves a short-term benefit at the expense of long-term detriments: a more genetically susceptible host population, ultimately greater mortality, and elevated influenza virulence.
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