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Y. Kim

Publications and source records attributed to Y. Kim.

71 records · Page 4Linked to original sources
arXiv2013

Effects of Electron-Electron Interactions on Electronic Raman Scattering of Graphite in High Magnetic Fields

We report the observation of strongly temperature-dependent, asymmetric spectral lines in electronic Raman scattering spectra of graphite in a high magnetic field up to 45 T applied along the c-axis. The magnetic field quantizes the in-plane motion, while the out-of-plane motion remains free, effectively reducing the system dimension from three to one. Optically created electron-hole pairs interact with, or shake up, the one-dimensional Fermi sea in the lowest Landau subbands. Based on the Tomonaga-Luttinger liquid theory, we show that interaction effects modify the van Hove singularity to the form $(ω-Δ)^{2α-1/2}$ at zero temperature. At finite temperature, we predict a thermal broadening factor that increases linearly with the temperature. Our model reproduces the observed temperature-dependent line-shape, determining $α$ to be $\sim$0.05 at 40 T.

cond-mat.mes-hall↗
arXiv2013

Contractivity of the Wasserstein metric for the kinetic Kuramoto equation

We present synchronization and contractivity estimates for the kinetic Kuramoto model obtained from the Kuramoto phase model in the mean-field limit. For identical Kuramoto oscillators, we present an admissible class of initial data leading to time-asymptotic complete synchronization, that is, all measure valued solutions converge to the traveling Dirac measure concentrated on the initial averaged phase. If two initial Radon measures have the same natural frequency density function and strength of coupling, we show that the Wasserstein p-distance between corresponding measure valued solutions is exponentially decreasing in time. This contraction principle is more general than previous L1-contraction properties of the Kuramoto phase model.

math.AP↗
arXiv2012

Magnetophonon resonance in graphite: High-field Raman measurements and electron-phonon coupling contributions

We perform Raman scattering experiments on natural graphite in magnetic fields up to 45 T, observing a series of peaks due to interband electronic excitations over a much broader magnetic field range than previously reported. We also explore electron-phonon coupling in graphite via magnetophonon resonances. The Raman G peak shifts and splits as a function of magnetic field, due to the magnetically tuned coupling of the E2g optical phonons with the K- and H-point inter-Landau-level excitations. The analysis of the observed anticrossing behavior allows us to determine the electron-phonon coupling for both K- and H-point carriers. In the highest field range (>35 T) the G peak narrows due to suppression of electron-phonon interaction.

cond-mat.mes-hall↗
arXiv2011

Temperature dependence of Raman-active optical phonons in Bi_2Se_3 and Sb_2Te_3

Inelastic light scattering spectra of Bi_2Se_3 and Sb_2Te_3 single crystals have been measured over the temperature range from 5 K to 300 K. The temperature dependence of dominant A^{2}_{1g} phonons shows similar behavior in both materials. The temperature dependence of the peak position and linewidth is analyzed considering the anharmonic decay of optical phonons and the material thermal expansion. This work suggests that Raman spectroscopy can be used for thermometry in Bi_2Se_3- and Sb_2Te_3-based devices in a wide temperature range.

cond-mat.mtrl-sci↗
arXiv2010

Thickness dependence of surface diffusion in epitaxial LaAlO3 on SrTiO3(001)

The LaAlO3/SrTiO3 (001) thin film materials system was studied using in situ, simultaneous x-ray diffuse scattering and specular reflectivity during pulsed laser deposition. Using this method, we are able to measure the time dependence of the characteristic surface length scale and the characteristic time for both in-plane and downhill diffusion. These data allow for the determination of the activation energy for various diffusion processes as a function of LaAlO3 thickness. Additionally, we show that the downhill diffusion rate of the first monolayer is distinctly different than subsequent layers. These results are directly compared to previous experimental observations seen during the deposition of homoepitaxial SrTiO3 (001).

cond-mat.mtrl-sci↗
arXiv2009

Two-pion-exchange and other higher-order contributions to the $pp\to ppπ^0$ reaction

Much effort has been invested on effective-field-theoretical studies of the near-threshold NN \to NNπreactions and, in order to deal with the rather large three-momentum transfers involved, the momentum counting scheme was proposed as an alternative to the usual Weinberg counting scheme. To examine the practical utility of the momentum counting scheme, we study here the contributions of higher-order terms to the pp\to ppπ^0 reaction. We discuss the consequences of the different treatments of the nucleon propagators in these two counting schemes. We also investigate certain higher chiral-order pion-rescattering terms and the five-point counter-term, which turn out to give important contributions.

nucl-th↗
arXiv2008

Nova-Like Cataclysmic Variable TT Ari: QPO Behaviour Coming Back From Positive Superhumps

We study the variability of the nova-like cataclysmic variable TT Ari, on time-scales of between minutes and months. The observations in the filter R were obtained at the 40-cm telescope of the Chungbuk National University (Korea). TT Ari was in the "negative superhump" state after its return from the "positive superhump" state, which lasted 8 years. The ephemeris for 12 best pronounced minima is $T_{min}=BJD 2453747.0700(47)+0.132322(53)E.$ where numbers in digits are errors in units of the last digit. The phases of minima may reach 0.2, indicating non-eclipse nature of these minima. The quasi-periodic oscillations (QPO) are present with a mean "period" of 21.6 min and mean semi-amplitude of 36 mmag. This value is consistent with the range 15-25 minutes reported for previous "negative superhump" states and does not support the hypothesis of secular decrease of the QPO period. Either the period, or the semi-amplitude show significant night-to-night variations. According to the position at the two-parameter diagrams, the interval of observations was splitted into 5 parts, showing different characteristics which are discussed in this paper. The system is an excellent laboratory to study processes resulting in variations at time-scales from seconds to decades and needs further monitoring at various states of activity.

astro-ph↗
arXiv2008

Multiple time scales in cataclysmic binaries. The low-field magnetic dwarf nova DO Draconis

We study the variability of the cataclysmic variable DO Dra, on time-scales of between minutes and decades. The characteristic decay time dt/dm=0.902(3) days/mag was estimated from our 3 nights of CCD R observations. The quiescent data show a photometric wave with a cycle about 303(15)d. We analyzed the profile of the composite (or mean) outburst. We discovered however, that a variety of different outburst heights and durations had occurred, contrary to theoretical predictions. With increasing maximum brightness, we find that the decay time also increases; this is in contrast to the model predictions, which indicate that outbursts should have a constant shape. This is interpreted as representing the presence of outburst-to-outburst variability of the magnetospheric radius. A presence of a number of missed weak narrow outbursts is predicted from this statistical relationship. A new type of variability is detected, during 3 subsequent nights in 2007: periodic (during one nightly run) oscillations with rapidly-decreasing frequency from 86 to 47 cycles/day and a semi-amplitude increasing from 0.06 to 0. 10, during a monotonic brightness increase from 14. 27 to 14. 13. This phenomenon was observed only during an unusually prolonged event of about 1 mag brightening in 2007 (lasting till autumn), during which no (expected) outburst was detected. We refer to this behaviour as to the transient periodic oscillations (TPO). To study this new and interesting phenomenon, new regular photometric and spectral (in a target of opportunity mode) observations are required.

astro-ph↗
arXiv2007

A Catalog of HI Clouds in the Large Magellanic Cloud

A 21 cm neutral hydrogen interferometric survey of the Large Magellanic Cloud (LMC) combined with the Parkes multi-beam HI single-dish survey clearly shows that the HI gas is distributed in the form of clumps or clouds. The HI clouds and clumps have been identified using a thresholding method with three separate brightness temperature thresholds ($T_b$). Each catalog of HI cloud candidates shows a power law relationship between the sizes and the velocity dispersions of the clouds roughly following the Larson Law scaling $σ_v \propto R^{0.5}$, with steeper indices associated with dynamically hot regions. The clouds in each catalog have roughly constant virial parameters as a function mass suggesting that that the clouds are all in roughly the same dynamical state, but the values of the virial parameter are significantly larger than unity showing that turbulent motions dominate gravity in these clouds. The mass distribution of the clouds is a power law with differential indices between -1.6 and -2.0 for the three catalogs. In contrast, the distribution of mean surface densities is a log-normal distribution.

astro-ph↗
arXiv2007

Two-pion-exchange contributions to the pp\to ppπ^0 reaction

Our previous study of the near-threshold pp\to ppπ^0 reaction based on a hybrid nuclear effective field theory is further elaborated by examining the momentum dependence of the relevant transition operators. We show that the two-pion exchange diagrams give much larger contributions than the one-pion exchange diagram, even though the former is of higher order in the Weinberg counting scheme. The relation between our results and an alternative counting scheme, the momentum counting scheme, is also discussed.

nucl-th↗
arXiv2006

Imaging and optical properties of single core-shell GaAs-AlGaAs nanowires

We study the optical properties of a single core-shell GaAs-AlGaAs nanowire (grown by VLS method) using the technique of micro-photoluminescence and spatially-resolved photoluminescence imaging. We observe large linear polarization anisotropy in emission and excitation of nanowires.

cond-mat.mtrl-sci↗
arXiv2006

The secondary star and distance of the polar V1309 Ori

The first phase-resolved JHK light curves of the eclipsing polar (AM Herculis binary) V1309 Ori are presented and interpreted. We separate the contributions from the secondary star and from other sources with the aim of determining a photometric distance. Simple model calculations show that the accretion stream and the cyclotron source on the accreting white dwarf are minor contributors to the infrared light, allowing an accurate determination of spectral type and absolute flux of the secondary star. The unilluminated backside of the secondary star as seen in eclipse has spectral type dM0 to dM0+. Its dereddened magnitude is K = 13.58 at orbital phase phi = 0 (eclipse). Using the calibrated surface brightness of M-stars and the published mass of the secondary, M2 = 0.46 Msun, we obtain a distance d = 600 +/- 25 pc which scales as M2^(1/2). The radius of the Roche-lobe filling secondary exceeds the main-sequence radius of an M0 star by 21 +11/-6 %. The debated origin of the infrared light of V1309 Ori has been settled in favor of the secondary star as the main contributor and an accurate distance has been derived that will place estimates of the luminosity and synchronization time scale on a more secure basis.

astro-ph↗
arXiv2006

Kinetic stabilization of Fe film on GaAs(100): An in situ x-ray reflectivity Study

We study the growth of the Fe films on GaAs(100) at a low temperature, 140 K, by $in$-$situ$ UHV x-ray reflectivity using synchrotron radiation. We find rough surface with the growth exponent, $β_S$ = 0.51$\pm$0.04. This indicates that the growth of the Fe film proceeds via the restrictive relaxation due to insufficient thermal diffusion of the adatoms. The XRR curves are nicely fit by a model with a uniform Fe film, implying that the surface segregation and interface alloying of both Ga and As are negligible. When the Fe film is annealed to 300 K, however, the corresponding XRR can be fit only after including an additional layer of 9 A thickness between the Fe film and the substrate, indicating the formation of ultrathin alloy near the interface. The confinement of the alloy near the interface derives from the fact that the diffusion of Ga and As from the substrate should proceed via the inefficient bulk diffusion, and hence the overlying Fe film is kinetically stabilized.

cond-mat.mtrl-sci↗
arXiv2005

Use of V_{low-k} in a chiral-perturbation-theory description of the $pp\to ppπ^0$ reaction

Previously, the near-threhold $pp\to ppπ^0$ reaction was studied with the use of transition operators derived from chiral perturbation theory ($χ$PT) and the nuclear wave functions generated by high-precision phenomenological potentials. A conceptual problem in that approach was that the transition amplitude receives contributions from very high momentum components (above the cutoff scale of $χ$PT) in the nuclear wave functions. In the present work, we avoid this problem by replacing the "bare" phenomenological potentials with V_{low-k}, which is an effective potential derived from a bare potential by integrating out momentum components higher than a specified cutoff scale. The use of V_{low-k} is found to give an enhancement of the $pp\to ppπ^0$ cross sections over the values obtained with bare potentials. Although this enhancement brings the calculated cross sections closer to the experimental values, the incident-energy dependence of the cross section is not well reproduced, a problem that seems to indicate the necessity of including higher chiral order terms than considered in the present work.

nucl-th↗
arXiv1999

The USA X-ray Timing Experiment

The USA Experiment is a new X-ray timing experiment with large collecting area and microsecond time resolution capable of conducting a broad program of studies of galactic X-ray binaries. USA is one of nine experiments aboard the Advanced Research and Global Observation Satellite which was launched February 23, 1999. USA is a collimated proportional counter X-ray telescope with about 1000 cm$^2$ of effective area per detector with two detectors sensitive to photons in the energy range 1-15 keV. A unique feature of USA is that photon events are time tagged by reference to an onboard GPS receiver allowing precise absolute time and location determination. We will present an overview of the USA instrument, capabilities, and scientific observing plan as well as the current status of the instrument.

astro-ph↗
arXiv1996

RG analysis of magnetic catalysis in Dynamical symmetry breaking

We perform the renormalization group analysis on the dynamical symmetry breaking under strong external magnetic field, studied recently by Gusynin, Miransky and Shovkovy. We find that any attractive four-Fermi interaction becomes strong in the low energy, thus leading to dynamical symmetry breaking. When the four-Fermi interaction is absent, the $β$-function for the electromagnetic coupling vanishes in the leading order in $1/N$. By solving the Schwinger-Dyson equation for the fermion propagator, we show that in $1/N$ expansion, for any electromagnetic coupling, dynamical symmetry breaking occurs due to the presence of Landau energy gap by the external magnetic field.

hep-th↗