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Search Results: 1 - 10 of 20464 matches for " Youngwook Kim "
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Direction of Arrival Estimation of Humans with a Small Sensor Array Using an Artificial Neural Network
Youngwook Kim;Hao Ling
PIER B , 2011, DOI: 10.2528/PIERB10100510
Abstract: An array processing algorithm based on artificial neural networks (ANNs) is proposed to estimate the directions of arrival (DOAs) of moving humans using a small sensor array. In the approach, software beamforming is first performed on the received signals from the sensor elements to form a number of overlapping beams. The received signals from all the beams produce a unique "signature" in accordance with the target locations as well as the number of targets. The target tracking procedure is handled by two separate ANNs in sequence. The first ANN determines the number of targets, and the second ANN estimates their respective DOAs. The ANNs are trained using simulation data generated based on a point scatterer model in free space. The proposed approach is tested using measurement data from two loudspeakers and two walking humans in line-of-sight and through-wall environments.
Ballistic transport of graphene pnp junctions with embedded local gates
Seung-Geol Nam,Dong-Keun Ki,Jong Wan Park,Youngwook Kim,Jun Sung Kim,Hu-Jong Lee
Physics , 2011, DOI: 10.1088/0957-4484/22/41/415203
Abstract: We fabricated graphene pnp devices, by embedding pre-defined local gates in an oxidized surface layer of a silicon substrate. With neither dielectric-material deposition nor electron-beam irradiation on the graphene, we obtained high-quality graphene pnp devices without degradation of the carrier mobility even in the local-gate region. The corresponding increased mean free path leads to the observation of ballistic and phase-coherent transport across a 130-nm-wide local gate, which is about an order of magnitude wider than reported previously. Furthermore, in our scheme, we demonstrated independent control of the carrier density in the local-gate region, with a conductance map very distinctive from top-gated devices. It was caused as the electric field arising from the global back gate is strongly screened by the embedded local gate. Our scheme allows the realization of ideal multipolar graphene junctions with ballistic carrier transport.
Persistent Topological Surface State at the Interface of Bi2Se3 Film Grown on Patterned Graphene
Namdong Kim,Paengro Lee,Youngwook Kim,Jun Sung Kim,Yongsam Kim,Do Young Noh,Seong Uk Yu,Jinwook Chung,Kwang S. Kim
Physics , 2014,
Abstract: We employed graphene as a patternable template to protect the intrinsic surface states of thin films of topological insulators (TIs) from environment. Here we find that the graphene provides high-quality interface so that the Shubnikov de Haas (SdH) oscillation associated with a topological surface state could be observed at the interface of a metallic Bi2Se3 film with a carrier density higher than ~10^19 cm-3. Our in situ X-ray diffraction study shows that the Bi2Se3 film grows epitaxially in a quintuple layer-by-layer fashion from the bottom layer without any structural distortion by interfacial strain. The magnetotransport measurements including SdH oscillations stemming from multiple conductance channels reveal that the topological surface state, with the mobility as high as ~0.5 m^2/Vs, remains intact from the graphene underneath without degradation. Given that the graphene was prepatterned on arbitrary insulating substrates, the TI-based microelectronic design could be exploited. Our study thus provides a step forward to observe the topological surface states at the interface without degradation by tuning the interface between TI and graphene into a measurable current for device application.
Ordered Growth of Topological Insulator Bi2Se3 Thin Films on Dielectric Amorphous SiO2 by MBE
Sahng-Kyoon Jerng,Kisu Joo,Youngwook Kim,Sang-Moon Yoon,Jae Hong Lee,Miyoung Kim,Jun Sung Kim,Euijoon Yoon,Seung-Hyun Chun,Yong Seung Kim
Physics , 2013, DOI: 10.1039/C3NR03032F
Abstract: Topological insulators (TIs) are exotic materials which have topologically protected states on the surface due to the strong spin-orbit coupling. However, a lack of ordered growth of TI thin films on amorphous dielectrics and/or insulators presents a challenge for applications of TI-junctions. We report the growth of topological insulator Bi2Se3 thin films on amorphous SiO2 by molecular beam epitaxy (MBE). To achieve the ordered growth of Bi2Se3 on amorphous surface, the formation of other phases at the interface is suppressed by Se passivation. Structural characterizations reveal that Bi2Se3 films are grown along the [001] direction with a good periodicity by van der Waals epitaxy mechanism. Weak anti-localization effect of Bi2Se3 films grown on amorphous SiO2 shows modulated electrical property by the gating response. Our approach for ordered growth of Bi2Se3 on amorphous dielectric surface presents considerable advantages for TI-junctions with amorphous insulator or dielectric thin films.
Breakdown of the interlayer coherence in twisted bilayer graphene
Youngwook Kim,Hoyeol Yun,Seung-Geol Nam,Minhyeok Son,Dong Su Lee,Dong Chul Kim,S. Seo,Hee Cheul Choi,Hu-Jong Lee,Sang Wook Lee,Jun Sung Kim
Physics , 2012, DOI: 10.1103/PhysRevLett.110.096602
Abstract: Coherent motion of the electrons in the Bloch states is one of the fundamental concepts of the charge conduction in solid state physics. In layered materials, however, such a condition often breaks down for the interlayer conduction, when the interlayer coupling is significantly reduced by e.g. large interlayer separation. We report that complete suppression of coherent conduction is realized even in an atomic length scale of layer separation in twisted bilayer graphene. The interlayer resistivity of twisted bilayer graphene is much higher than the c-axis resistivity of Bernal-stacked graphite, and exhibits strong dependence on temperature as well as on external electric fields. These results suggest that the graphene layers are significantly decoupled by rotation and incoherent conduction is a main transport channel between the layers of twisted bilayer graphene.
How Much Multiuser Diversity is Required for Energy Limited Multiuser Systems?
Youngwook Ko,Sergiy~A. ~Vorobyov,Masoud Ardakani
Mathematics , 2010, DOI: 10.1109/TSP.2010.2049108
Abstract: Multiuser diversity (MUDiv) is one of the central concepts in multiuser (MU) systems. In particular, MUDiv allows for scheduling among users in order to eliminate the negative effects of unfavorable channel fading conditions of some users on the system performance. Scheduling, however, consumes energy (e.g., for making users' channel state information available to the scheduler). This extra usage of energy, which could potentially be used for data transmission, can be very wasteful, especially if the number of users is large. In this paper, we answer the question of how much MUDiv is required for energy limited MU systems. Focusing on uplink MU wireless systems, we develop MU scheduling algorithms which aim at maximizing the MUDiv gain. Toward this end, we introduce a new realistic energy model which accounts for scheduling energy and describes the distribution of the total energy between scheduling and data transmission stages. Using the fact that such energy distribution can be controlled by varying the number of active users, we optimize this number by either (i) minimizing the overall system bit error rate (BER) for a fixed total energy of all users in the system or (ii) minimizing the total energy of all users for fixed BER requirements. We find that for a fixed number of available users, the achievable MUDiv gain can be improved by activating only a subset of users. Using asymptotic analysis and numerical simulations, we show that our approach benefits from MUDiv gains higher than that achievable by generic greedy access algorithm, which is the optimal scheduling method for energy unlimited systems.
Power Allocation Strategies across N Orthogonal Channels at Both Source and Relay
Youngwook Ko,Masoud Ardakani,Sergiy A. Vorobyov
Mathematics , 2010,
Abstract: We consider a wireless relay network with one source, one relay and one destination, where communications between nodes are preformed via N orthogonal channels. This, for example, is the case when orthogonal frequency division multiplexing is employed for data communications. Since the power available at the source and relay is limited, we study optimal power allocation strategies at the source and relay in order to maximize the overall source-destination capacity under individual power constraints at the source and/or the relay. Depending on the availability of the channel state information at the source and rely, optimal power allocation strategies are performed at both the source and relay or only at the relay. Considering different setups for the problem, various optimization problems are formulated and solved. Some properties of the optimal solution are also proved.
Design of Quantification Model for Ransom Ware Prevent  [PDF]
Donghyun Kim, Seoksoo Kim
World Journal of Engineering and Technology (WJET) , 2015, DOI: 10.4236/wjet.2015.33C030
Abstract:

The growth of ICT within the society has become increasingly digitized, thus, the overall activity has amounted to various researches for protecting any data from malicious threats. Recently, ransomware has been a rapidly propagated subject for social engineering techniques especially the ransomware. Users can delete a ransomeware code using an antivirus software code. However, the encrypted data would be impossible to recover. Therefore, ransomware must be prevented and must have early detection before it infects any data. In this paper, we are proposing a quantification model to prevent and detect any cryptographic operations in the local drive.

Trends of Noninvasive Radiofrequency and Minimally Invasive Treatment for the Management of Facial Aging  [PDF]
Sunghee Kim, Moonjong Kim
Journal of Cosmetics, Dermatological Sciences and Applications (JCDSA) , 2019, DOI: 10.4236/jcdsa.2019.91003
Abstract: Various treatments for the management of facial aging have been performed among which noninvasive radio-frequency (RF; i.e., thermage) treatment and minimally invasive treatments are on the rise. The purpose of this study was to analyze trends of the treatment of facial aging in Korea and to investigate relationships between the use of noninvasive RF and minimally invasive treatments. A retrospective analysis conducted on data from 4021 patients showed that thermage treatment increased by 134.9% over 5 years. As a person ages, the rate of facial treatment with both the botulinum toxin (for the masseter and lines of the glabella, lateral canthus, and forehead) and the PDO thread lift increases. The use of the treatments, nasolabial fold filler and Silhouette Soft Thread, however, was not associated with aging. The patients receiving thermage treatment were less likely to undergo any of the other treatments including PDO thread lift, Silhouette Soft Thread, nasolabial fold filler, or any of the botulinum toxin treatments. Overall, the results showed that patients who had received noninvasive RF tended to receive less minimally invasive treatment.
The Effect of Prunella on Anti-Inflammatory Activity in RAW264.7 Mouse Macrophage Cells  [PDF]
Meehye Kim
Food and Nutrition Sciences (FNS) , 2012, DOI: 10.4236/fns.2012.39170
Abstract: The extracts of Prunella vulgaris L. (Labiatae), a popular Western and Chinese herbal medicine, was shown to have anti-inflammatory properties, which might be due to partially, their rosmarinic acid content. Inhition of prostaglandine E2 (PGE2) production in lipopolysaccharide (LPS) stimulated RAW264.7 mouse macrophage cells was assessed with an enzyme immunoassay (EIA) following 8-hour treatments with Prunella vulgaris extracts or fractions. Results showed that 95% ethanol extracts from P. vulgaris significantly inhibited PGE2 production. In further studies, fraction 2 from the 95% ethanol extract of P. vulgaris significantly reduced PGE2 production at 66 µg/ml (72% reduction). Cytotoxic-ity did not play a role in the noted reduction of PGE2 seen in either the extracts or fractions from P. vulgaris. High performance liquid chromatography analysis showed that there was 1.4 mM rosmarinic acid in 95% ethanol Prunella extract (201 mg/ml crude extract). Our results suggest that rosmarinic acid may contribute toward the anti-inflammatory activity of Prunella in a dose-response manner. Prunella might have a potential to be used as a functional food for anti-inflammatory activity.
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