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Search Results: 1 - 10 of 5422 matches for " Seon-Myeong Choi "
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Effects of Strain on Electronic Properties of Graphene
Seon-Myeong Choi,Seung-Hoon Jhi,Young-Woo Son
Physics , 2009, DOI: 10.1103/PhysRevB.81.081407
Abstract: We present first-principles calculations of electronic properties of graphene under uniaxial and isotropic strains, respectively. The semi-metallic nature is shown to persist up to a very large uniaxial strain of 30% except a very narrow strain range where a tiny energy gap opens. As the uniaxial strain increases along a certain direction, the Fermi velocity parallel to it decreases quickly and vanishes eventually, whereas the Fermi velocity perpendicular to it increases by as much as 25%. Thus, the low energy properties with small uniaxial strains can be described by the generalized Weyl's equation while massless and massive electrons coexist with large ones. The work function is also predicted to increase substantially as both the uniaxial and isotropic strain increases. Hence, the homogeneous strain in graphene can be regarded as the effective electronic scalar potential.
Controlling Energy Gap of Bilayer Graphene by Strain
Seon-Myeong Choi,Seung-Hoon Jhi,Young-Woo Son
Physics , 2010, DOI: 10.1021/nl101617x
Abstract: Using the first principles calculations, we show that mechanically tunable electronic energy gap is realizable in bilayer graphene if different homogeneous strains are applied to the two layers. It is shown that the size of energy gap can be simply controlled by adjusting the strength and direction of these strains. We also show that the effect originates from the occurrence of strain-induced pseudo-scalar potentials in graphene. When homogeneous strains with different strengths are applied to each layer of bilayer graphene, transverse electric fields across the two layers can be generated without any external electronic sources, thereby opening an energy gap. The results demonstrate a simple mechanical method of realizing pseudo-electromagnetism in graphene and suggest a maneuverable approach to fabrication of electromechanical devices based on bilayer graphene.
Anomalous Optical Phonon Splittings in Sliding Bilayer Graphene
Seon-Myeong Choi,Seung-Hoon Jhi,Young-Woo Son
Physics , 2013,
Abstract: We study the variations of electron-phonon coupling and their spectroscopic consequences in response to sliding of two layers in bilayer graphene using first-principles calculations and a model Hamiltonian. Our study shows that the long wave-length optical phonon modes change in a sensitive and unusual way depending on the symmetry as well as the parity of sliding atomic structures and that, accordingly, Raman- and infrared-active optical phonon modes behave differently upon the direction and size of the sliding. The renormalization of phonon modes by the interlayer electronic coupling is shown to be crucial to explain their anomalous behavior upon the sliding. Also, we show that the crystal symmetry change due to the sliding affects the polarized Stokes Raman-scattering intensity, which can be utilized to detect tiny misalignment of graphene layers using spectroscopic tools.
Polarization dependence of double resonant Raman scattering band in bilayer graphene
Jae-Ung Lee,Ngor Mbaye Seck,Duhee Yoon,Seon-Myeong Choi,Young-Woo Son,Hyeonsik Cheong
Physics , 2014, DOI: 10.1016/j.carbon.2014.02.007
Abstract: The polarization dependence of the double resonant Raman scattering (2D) band in bilayer graphene (BLG) is studied as a function of the excitation laser energy. It has been known that the complex shape of the 2D band of BLG can be decomposed into four Lorentzian peaks with different Raman frequency shifts attributable to four individual scattering paths in the energy-momentum space. From our polarization dependence study, however, we reveal that each of the four different peaks is actually doubly degenerate in its scattering channels, i.e., two different scattering paths with similar Raman frequency shifts for each peak. We find theoretically that one of these two paths, ignored for a long time, has a small contribution to their scattering intensities but are critical in understanding their polarization dependences. Because of this, the maximum-to-minimum intensity ratios of the four peaks show a strong dependence on the excitation energy, unlike the case of single-layer graphene (SLG). Our findings thus reveal another interesting aspect of electron-phonon interactions in graphitic systems.
Electronic topological transition in sliding bilayer graphene
Young-Woo Son,Seon-Myeong Choi,Yoon Pyo Hong,Sungjong Woo,Seung-Hoon Jhi
Physics , 2010, DOI: 10.1103/PhysRevB.84.155410
Abstract: We demonstrate theoretically that the topology of energy bands and Fermi surface in bilayer graphene undergoes a very sensitive transition when extremely tiny lateral interlayer shift occurs in arbitrary directions. The phenomenon originates from a generation of effective non-Abelian vector potential in Dirac Hamiltonian by the sliding motions. The characteristics of the transition such as pair annihilations of massless Dirac fermions are dictated by the sliding direction owing to a unique interplay between the effective non-Abelian gauge fields and Berry's phases associated with massless electrons. The transition manifests itself in various measurable quantities such as anomalous density of states, minimal conductivity, and distinct Landau level spectrum.
Photocurrent generation at ABA/ABC lateral junction in tri-layer graphene photodetector
Minjung Kim,Seon-Myeong Choi,Ho Ang Yoon,Sun Keun Choi,Jae-Ung Lee,Jungcheol Kim,Sang Wook Lee,Young-Woo Son,Hyeonsik Cheong
Physics , 2015, DOI: 10.1016/j.carbon.2015.09.095
Abstract: Metal-graphene-metal photodetectors utilize photocurrent generated near the graphene/metal junctions and have many advantages including high speed and broad-band operation. Here, we report on photocurrent generation at ABA/ABC stacking domain junctions in tri-layer graphene with a responsivity of 0.18 A/W. Unlike usual metal-graphene-metal devices, the photocurrent is generated in the middle of the graphene channel, not confined to the vicinity of the metal electrodes. The magnitude and the direction of the photocurrent depend on the back-gate bias. Theoretical calculations show that there is a built-in band offset between the two stacking domains, and the dominant mechanism of the photocurrent is the photo-thermoelectric effect due to the Seebeck coefficient difference.
The stability of graphene band structures against an external periodic perturbation; Na on Graphene
Choongyu Hwang,Sunyoung Shin,Seon-Myeong Choi,Namdong Kim,Sanghun Uhm,Hyosang Kim,Chan-cuk Hwang,Doyoung Noh,Seung-Hoon Jhi,Jinwook Chung
Physics , 2011, DOI: 10.1103/PhysRevB.79.115439
Abstract: We report that the $\pi$ band of graphene sensitively changes as a function of an external potential induced by Na especially when the potential becomes periodic at low temperature. We have measured the band structures from the graphene layers formed on the 6H-SiC(0001) substrate using angle-resolved photoemission spectroscopy with synchrotron photons. With increasing Na dose, the $\pi$ band appears to be quickly diffused into background at 85 K whereas it becomes significantly enhanced its spectral intensity at room temperature (RT). A new parabolic band centered at $k\sim$1.15 \AA$^{-1}$ also forms near Fermi energy with Na at 85 K while no such a band observed at RT. Such changes in the band structure are found to be reversible with temperature. Analysis based on our first principles calculations suggests that the changes of the $\pi$ band of graphene be mainly driven by the Na-induced potential especially at low temperature where the potential becomes periodic due to the crystallized Na overlayer. The new parabolic band turns to be the $\pi$ band of the underlying buffer layer partially filled by the charge transfer from Na adatoms. The five orders of magnitude increased hopping rate of Na adatoms at RT preventing such a charge transfer explains the absence of the new band at RT.
Bright stars observed by FIMS/SPEAR
Young-Soo Jo,Kwang-Il Seon,Kyoung-Wook Min,Yeon-Ju Choi,Tae-Ho Lim,Yeo-Myeong Lim,Jerry Edelstein,Wonyong Han
Physics , 2015,
Abstract: In this paper, we present a catalogue of the spectra of bright stars observed during the sky survey using the Far-Ultraviolet Imaging Spectrograph (FIMS), which was designed primarily to observe diffuse emissions. By carefully eliminating the contamination from the diffuse background, we obtain the spectra of 70 bright stars observed for the first time with a spectral resolution of 2--3 {\AA} over the wavelength of 1370--1710 {\AA}. The far-ultraviolet spectra of an additional 139 stars are also extracted with a better spectral resolution and/or higher reliability than those of the previous observations. The stellar spectral type of the stars presented in the catalogue spans from O9 to A3. The method of spectral extraction of the bright stars is validated by comparing the spectra of 323 stars with those of the International Ultraviolet Explorer (IUE) observations.
Effect of Hole Size on Flow Structure and Mixing Characteristic in a Multi-Hole Baffled Micro Combustor  [PDF]
Won Hyun Kim, Young Su Park, Seon Myeong Park, Tae Seon Park
Journal of Applied Mathematics and Physics (JAMP) , 2017, DOI: 10.4236/jamp.2017.51002
Flow structure and mixing properties by the baffle shape are numerically studied for a baffled micro combustor. The baffle shape is changed by various fuel and hole sizes. The numerical simulations based on different geometric conditions are performed by using the Reynolds Stress Model. The fuel-air mixing is greatly affected by flow recirculations. The centrally located flow recirculation has an important role for the entire mixing performance. The results show that this feature depends on the baffle configurations, and the baffle with small air holes represents efficient characters.
A Secure Routing Method for Detecting False Reports and Wormhole Attacks in Wireless Sensor Networks  [PDF]
Hyeon Myeong Choi, Su Man Nam, Tae Ho Cho
Wireless Sensor Network (WSN) , 2013, DOI: 10.4236/wsn.2013.53005

Wireless sensor networks (WSNs) consist of a large number of sensor nodes that monitor the environment and a few base stations that collect the sensor readings. Individual sensor nodes are subject to compromised security because they may be deployed in hostile environments and each sensor node communicates wirelessly. An adversary can inject false reports into the networks via compromised nodes. Furthermore, an adversary can create a wormhole by directly linking two compromised nodes or using out-of-band channels. If these two kinds of attacks occur simultaneously in a network, existing methods cannot defend against them adequately. We thus propose a secure routing method for detecting false report injections and wormhole attacks in wireless sensor networks. The proposed method uses ACK messages for detecting wormholes and is based on a statistical en-route filtering (SEF) scheme for detecting false reports. Simulation results show that the proposed method reduces energy consumption by up to 20% and provide greater network security.

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