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Search Results: 1 - 10 of 551631 matches for " S. J. Woo "
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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.
Controlling quasiparticle excitations in a trapped Bose-Einstein condensate
S. J. Woo,S. Choi,N. P. Bigelow
Physics , 2004, DOI: 10.1103/PhysRevA.72.021605
Abstract: We describe an approach to quantum control of the quasiparticle excitations in a trapped Bose-Einstein condensate based on adiabatic and diabatic changes in the trap anisotropy. We describe our approach in the context of Landau-Zener transition at the avoided crossings in the quasiparticle excitation spectrum. We show that there can be population oscillation between different modes at the specific aspect ratios of the trapping potential at which the mode energies are almost degenerate. These effects may have implications in the expansion of an excited condensate as well as the dynamics of a moving condensate in an atomic wave guide with a varying width.
A rapid, non-invasive procedure for quantitative assessment of drought survival using chlorophyll fluorescence
Nick S Woo, Murray R Badger, Barry J Pogson
Plant Methods , 2008, DOI: 10.1186/1746-4811-4-27
Abstract: This article presents a procedure for rapid, inexpensive and non-invasive assessment of the survival of soil-grown plants during drought treatment. The changes in major photosynthetic parameters during increasing water deficit were monitored via chlorophyll fluorescence imaging and the selection of the maximum efficiency of photosystem II (Fv/Fm) parameter as the most straightforward and practical means of monitoring survival is described. The veracity of this technique is validated through application to a variety of Arabidopsis thaliana ecotypes and mutant lines with altered tolerance to drought or reduced photosynthetic efficiencies.The method presented here allows the acquisition of quantitative numerical estimates of Arabidopsis drought survival times that are amenable to statistical analysis. Furthermore, the required measurements can be obtained quickly and non-invasively using inexpensive equipment and with minimal expertise in chlorophyll fluorometry. This technique enables the rapid assessment and comparison of the relative viability of germplasm during drought, and may complement detailed physiological and water relations studies.With the increasing demands of industrial, municipal and agricultural consumption on dwindling water supplies [1], the development of sustainable farming practices has taken higher priority. For this reason, advancement of the current understanding of plant responses to drought stress and the mechanisms involved has become a major target of research and investment, with the ultimate goal of developing crops with improved water use efficiencies and minimized drought-induced loss of yield [2,3]. On a multi-gene scale, analysis of quantitative trait loci allows identification of genetic regions responsible for control of complex responses such as the co-ordination of the whole-plant response to water deficit [4,5]. In parallel to this, as our comprehension of the molecular signaling events leading to drought responses has increased,
Monte-Carlo Simulation of Pulsed Laser Deposition
Pui-Man Lam,S. J. Liu,C. H. Woo
Physics , 2002, DOI: 10.1103/PhysRevB.66.045408
Abstract: Using the Monte Carlo method, we have studied the pulsed laser deposition process at the sub-monolayer regime. In our simulations, dissociation of an atom from a cluster is incorporated. Our results indicate that the pulsed laser deposition resembles molecular beam epitaxy at very low intensity, and that it is characteristically different from molecular beam epitaxy at higher intensity. We have also obtained the island size distributions. The scaling function for the island size distribution for pulsed laser deposition is different from that of molecular beam epitaxy.
A Suprachoroidal Electrical Retinal Stimulator Design for Long-Term Animal Experiments and In Vivo Assessment of Its Feasibility and Biocompatibility in Rabbits
J. A. Zhou,S. J. Woo,S. I. Park,E. T. Kim,J. M. Seo,H. Chung,S. J. Kim
Journal of Biomedicine and Biotechnology , 2008, DOI: 10.1155/2008/547428
Abstract: This article reports on a retinal stimulation system for long-term use in animal electrical stimulation experiments. The presented system consisted of an implantable stimulator which provided continuous electrical stimulation, and an external component which provided preset stimulation patterns and power to the implanted stimulator via a paired radio frequency (RF) coil. A rechargeable internal battery and a parameter memory component were introduced to the implanted retinal stimulator. As a result, the external component was not necessary during the stimulation mode. The inductive coil pair was used to pass the parameter data and to recharge the battery. A switch circuit was used to separate the stimulation mode from the battery recharging mode. The implantable stimulator was implemented with IC chips and the electronics, except for the stimulation electrodes, were hermetically packaged in a biocompatible metal case. A polyimide-based gold electrode array was used. Surgical implantation into rabbits was performed to verify the functionality and safety of this newly designed system. The electrodes were implanted in the suprachoroidal space. Evoked cortical potentials were recorded during electrical stimulation of the retina. Long-term follow-up using OCT showed no chorioretinal abnormality after implantation of the electrodes.
Spectrophotometric Dating of Elliptical Galaxies in the Ultraviolet
Y. -W. Lee,J. -H. Woo,S. Yi,J. -H. Park
Physics , 1999,
Abstract: The UV upturn phenomenon observed in elliptical galaxies is attractive for its potential value as an age indicator of old stellar systems. We present our most recent population models for the UV evolution of elliptical galaxies. We confirm that the dominant UV sources are either metal-poor or metal-rich hot horizontal-branch (HB) stars in local giant ellipticals, but we also note that the contribution from post-asymptotic-giant-branch (PAGB) stars overwhelms the UV spectrum at higher redshifts (look-back times). The model UV spectral energy distribution (SED) is therefore strongly affected by the current uncertainty of the mean mass of PAGB stars at higher redshifts. Fortunately, our models suggest that the far-UV observations at z > 0.35 could produce strong constraint on the PAGB mass, while observations at could produce strong constraint on the PAGB mass, while observations at lower redshifts (0.15 < z < 0.30) would still provide constraints on the models on the origin of the UV upturn. Future observations of ellipticals from the STIS/HST and planned GALEX space UV facility will provide crucial database required for more concrete calibration of our UV dating techniques for old stellar systems.
The MUSE view of QSO PG1307+085: An elliptical galaxy on the $M_{BH}-σ_*$ relation interacting with its group environment
B. Husemann,V. N. Bennert,J. Scharw?chter,J. -H. Woo,O. S. Choudhury
Physics , 2015,
Abstract: We report deep optical integral-field spectroscopy with the MUSE of the luminous radio-quiet QSO PG1307+085 (z=0.154) obtained during the commissioning of the instrument. Given the high sensitivity and spatial resolution delivered by MUSE, we are able to resolve the compact ($r_e$~1.3") elliptical host galaxy. After careful spectroscopic deblending of the QSO and host galaxy emission, we infer a stellar velocity dispersion of $155\pm19$km/s. This places PG1307+085 local $M_{BH}-\sigma_*$ relation within the intrinsic scatter but offset towards a higher black hole mass with respect to the mean relation. The observations with MUSE also reveal a large extended ENLR around PG1307+085 reaching out to 30kpc. In addition, we detect a faint bridge of ionized gas towards the most massive galaxy of the galaxy group being just 20" (50kpc) away. Previous long-slit spectroscopic observations missed most of these extended features due to a miss-aligned slit. The ionized gas kinematics does not show any evidence for gas outflows on kpc scales despite the high QSO luminosity of $L_\mathrm{bol}>10^{46}$ erg/s. Based on the ionized gas distribution, kinematics and metallicity we discuss the origin of the ENLR with respect to its group environments including minor mergers, ram-pressure stripping or filamentary gas accretion as the most likely scenarios. We conclude that PG1307+085 is a normal elliptical host in terms of the scaling relations, but that the gas is most likely affected by the environment through gravity or ambient pressure. It is possible that the ongoing interaction with the environment, mainly seen in the ionized gas, is also be responsible for driving sufficient gas to feed the black hole at the centre of the galaxy.
Origins of the slow and the ubiquitous fast solar wind
S. R. Habbal,R. Woo,S. Fineschi,R. O'Neal,J. Kohl,G. Noci,C. Korendyke
Physics , 1997, DOI: 10.1086/310970
Abstract: We present in this Letter the first coordinated radio occultation measurements and ultraviolet observations of the inner corona below 5.5 Rs, obtained during the Galileo solar conjunction in January 1997, to establish the origin of the slow solar wind. Limits on the flow speed are derived from the Doppler dimming of the resonantly scattered componentof the oxygen 1032 A and 1037 A lines as measured with the UltraViolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO). White light images of the corona from the Large Angle Spectroscopic Coronagraph (LASCO) on SOHO taken simultaneously are used to place the Doppler radio scintillation and ultraviolet measurements in the context ofcoronal structures. These combined observations provide the first direct confirmation of the view recently proposed by Woo and Martin (1997) that the slow solar wind is associated with the axes, also known as stalks, of streamers. Furthermore, the ultraviolet observations also show how the fast solar wind is ubiquitous in the inner corona, and that a velocity shear between the fast and slow solar wind develops along the streamer stalks.
Excitation spectroscopy of vortex lattices in a rotating Bose-Einstein condensate
S. J. Woo,L. O. Baksmaty,S. Choi,N. P. Bigelow
Physics , 2003, DOI: 10.1103/PhysRevLett.92.170402
Abstract: Excitation spectroscopy of vortex lattices in rotating Bose-Einstein condensates is described. We numerically obtain the Bogoliubov-deGenne quasiparticle excitations for a broad range of energies and analyze them in the context of the complex dynamics of the system. Our work is carried out in a regime in which standard hydrodynamic assumptions do not hold, and includes features not readily contained within existing treatments.
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