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207 results on '"Sohn, C. H."'

4. Strong spin-phonon coupling unveiled by coherent phonon oscillations in Ca2RuO4

Author

Lee, Min-Cheol, Kim, Choong H., Kwak, Inho, Seo, C. W., Sohn, C. H., Nakamura, F., Sow, C., Maeno, Y., Kim, E. -A., Noh, T. W., and Kim, K. W.

Subjects
Condensed Matter - Strongly Correlated Electrons
Abstract

We utilize near-infrared femtosecond pulses to investigate coherent phonon oscillations of Ca2RuO4. The coherent Ag phonon mode of the lowest frequency changes abruptly not only its amplitude but also the oscillation-phase as the spin order develops. In addition, the phonon mode shows a redshift entering the magnetically ordered state, which indicates a spin-phonon coupling in the system. Density functional theory calculations reveal that the Ag oscillations result in octahedral tilting distortions, which are exactly in sync with the lattice deformation driven by the magnetic ordering. We suggest that the structural distortions by the spin-phonon coupling can induce the unusual oscillation-phase shift between impulsive and displacive type oscillations., Comment: 6 pages, 4 figures, 1 table

Published
2017
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5. Phonon-assisted optical excitation in the narrow bandgap Mott insulator Sr3Ir2O7

Author

Park, H. J., Sohn, C. H., Jeong, D. W., Cao, G., Kim, K. W., Moon, S. J., Jin, Hosub, Cho, Deok-Yong, and Noh, T. W.

Subjects
Condensed Matter - Strongly Correlated Electrons
Abstract

We examined the temperature (T) evolution of the optical conductivity spectra of Sr$_3$Ir$_2$O$_7$ over a wide range of 10-400 K. The system was barely insulating, exhibiting a small indirect bandgap of $\sim$0.1 eV. The low-energy features of the optical d-d excitation (${\hbar}{\omega}$ $<$ 0.3 eV) evolved drastically, whereas such evolution was not observed for the O K-edge X-ray absorption spectra. This suggests that the T evolution in optical spectra is not caused by a change in the bare (undressed) electronic structure, but instead, presumably originates from an abundance of phonon-assisted indirect excitations. Our results showed that the low-energy excitations were dominated by phonon-absorption processes which involve, in particular, the optical phonons. This implies that phonon-assisted processes significantly facilitate the charge dynamics in barely insulating Sr$_3$Ir$_2$O$_7$., Comment: submitted to Phys. Rev. B

Published
2014
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8. Temperature Evolution of Itinerant Ferromagnetism in SrRuO3 Probed by Optical Spectroscopy

Author

Jeong, D. W., Choi, Hong Chul, Kim, Choong H., Chang, Seo Hyoung, Sohn, C. H., Park, H. J., Kang, T. D., Cho, Deok-Yong, Baek, S. H., Eom, C. B., Shim, J. H., Yu, J., Kim, K. W., Moon, S. J., and Noh, T. W.

Subjects
Condensed Matter - Strongly Correlated Electrons
Abstract

The temperature ($T$) dependence of the optical conductivity spectra $\sigma(\omega)$ of a single crystal SrRuO$_3$ thin film is studied over a $T$ range from 5 to 450 K. We observed significant $T$ dependence of the spectral weights of the charge transfer and interband $d$-$d$ transitions across the ferromagnetic Curie temperature ($T_c$ ~ 150 K). Such $T$ dependence was attributed to the increase in the Ru spin moment, which is consistent with the results of density functional theory calculations. $T$ scans of $\sigma(\Omega, T)$ at fixed frequencies $\Omega$ reveal a clear $T^2$ dependence below $T_c$, demonstrating that the Stoner mechanism is involved in the evolution of the electronic structure. In addition, $\sigma(\Omega, T)$ continues to evolve at temperatures above $T_c$, indicating that the local spin moment persists in the paramagnetic state. This suggests that SrRuO$_3$ is an intriguing oxide system with itinerant ferromagnetism.

Published
2013
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29. Dynamic Contrast-Enhanced MR Imaging of Nonenhancing T2 High-Signal-Intensity Lesions in Baseline and Posttreatment Glioblastoma: Temporal Change and Prognostic Value

Author

Hwang, I., primary, Choi, S.H., additional, Park, C.-K., additional, Kim, T.M., additional, Park, S.-H., additional, Won, J.K., additional, Kim, I.H., additional, Lee, S.-T., additional, Yoo, R.-E., additional, Kang, K.M., additional, Yun, T.J., additional, Kim, J.-H., additional, and Sohn, C.-H., additional

Published
2019
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32. A Numerical Study on the Performance of a Magnesium-Based Automotive Cooling Fan with Bead Structure.

Author

Hur, K. H., Haider, B. A., and Sohn, C. H.

Subjects
MODAL analysis, FLOW velocity, PERFORMANCE theory, THREE-dimensional printing, MAGNESIUM
Abstract

This paper presents the numerical analysis of three types of magnesium-based, axial-flow automotive cooling fans. The numerical modeling is conducted for geometrically modified fan designs with and without bead structure. The effect of geometric modifications of the fan blades on the fan performances (P-Q curve), fan efficiency, and energy efficiency is investigated using unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with the sliding mesh methodology. The baseline fan having no-beads is fabricated using 3D printing technology and tested to measure the flow velocity and volumetric flow rate which shows good agreement to the numerical results. Subsequently, fans with beads are further optimized to achieve a significant increase in fan performances. To investigate the fan vibrations, modal analysis is also carried out using magnesium-alloy AZ31 as the fan material. The modal analysis gives natural frequencies of all types of fans which are beyond the fan rotational frequency and seems satisfactory. [ABSTRACT FROM AUTHOR]

Published
2021
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33. Prediction of airfoil stall using Navier-Stokes equations in streamline coordinates

Author

Choi, D. H, Sohn, C. H, and Oh, C. S

Subjects
Aerodynamics
Abstract

A Navier-Stokes procedure to calculate the flow about an airfoil at incidence was developed. The parabolized equations are solved in the streamline coordinates generated for an arbitrary airfoil shape using conformal mapping. A modified k-epsilon turbulence model is applied in the entire domain, but the eddy viscosity in the laminar region is suppressed artificially to simulate the region correctly. The procedure was applied to airfoils at various angles of attack, and the results are quite satisfactory for both laminar and turbulent flows. It is shown that the present choice of the coordinate system reduces the error due to numerical diffusion, and that the lift is accurately predicted for a wide range of incidence.

Published
1992

38. Comparison between the Prebolus T1 Measurement and the Fixed T1 Value in Dynamic Contrast-Enhanced MR Imaging for the Differentiation of True Progression from Pseudoprogression in Glioblastoma Treated with Concurrent Radiation Therapy and Temozolomide Chemotherapy

Author

Nam, J.G., primary, Kang, K.M., additional, Choi, S.H., additional, Lim, W.H., additional, Yoo, R.-E., additional, Kim, J.-H., additional, Yun, T.J., additional, and Sohn, C.-H., additional

Published
2017
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48. Functional dysconnectivity of corticostriatal circuitry and differential response to methylphenidate in youth with attention-deficit/hyperactivity disorder

Author

Hong, S-B, Harrison, BJ, Fornito, A, Sohn, C-H, Song, I-C, Kim, J-W, Hong, S-B, Harrison, BJ, Fornito, A, Sohn, C-H, Song, I-C, and Kim, J-W

Abstract

BACKGROUND: Brain frontostriatal circuits have been implicated in the pathophysiology of attention-deficit/hyperactivity disorder (ADHD). However, effects of methylphenidate on circuit-level functional connectivity are as yet unclear. The aim of the present study was to comprehensively investigate the functional connectivity of major striatal subregions in children with ADHD, including subanalyses directed at mapping cognitive and treatment response characteristics. METHODS: Using a comprehensive seeding strategy, we examined resting-state functional connectivity of dorsal and ventral subdivisions of the caudate nucleus and putamen in children and adolescents with ADHD and in age- and sex-matched healthy controls. RESULTS: We enrolled 83 patients with ADHD and 22 controls in our study. Patients showed significantly reduced dorsal caudate functional connectivity with the superior and middle prefrontal cortices as well as reduced dorsal putamen connectivity with the parahippocampal cortex. These connectivity measures were correlated in opposite directions in patients and controls with attentional performance, as assessed using the Continuous Performance Test. Patients showing a good response to methylphenidate had significantly reduced ventral caudate/nucleus accumbens connectivity with the inferior frontal cortices compared with poor responders. LIMITATIONS: Possible confounding effects of age-related functional connectivity change were not excluded owing to the wide age range of participants. CONCLUSION: We observed a region-specific effect of methylphenidate on resting-state functional connectivity, suggesting the pretreatment level of ventral frontostriatal functional connectivity as a possible methylphenidate response biomarker of ADHD.

Published
2015

49. Optical Spectroscopic Studies of the Metal-Insulator Transition Driven by All-In–All-Out Magnetic Ordering in5dPyrochloreCd2Os2O7

Author

Sohn, C. H., primary, Jeong, Hogyun, additional, Jin, Hosub, additional, Kim, Soyeon, additional, Sandilands, L. J., additional, Park, H. J., additional, Kim, K. W., additional, Moon, S. J., additional, Cho, Deok-Yong, additional, Yamaura, J., additional, Hiroi, Z., additional, and Noh, T. W., additional

Published
2015
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50. Design Optimization of Ceiling Fan Blades with Nonlinear Sweep Profile.

Author

Adeeb, E., Sohn, C. H., Maqsood, A., and Afaq, M. A.

Subjects
CEILING fans, ENERGY consumption, ENERGY conservation
Abstract

This study pertains to the design optimization of a four-blade ceiling fan to enhance air circulation and energy efficiency. The sweep angle of the blade profile is nonlinear. The design of experiment (DOE) computational fluid dynamics (CFD) and response surface method (RSM) methods were used in parallel to find the optimal design solution. The design variables considered were inboard angle of attack, outboard angle of attack, blade sweep, and tip-chord length. Numerical simulations were conducted using steady state Reynolds-averaged Navier-Stokes (RANS) equations and the Spalart-Allmaras turbulence model. The baseline results were validated through experimental data. Subsequently, the DOE method was employed to generate the blade design which reduce the number of simulations without losing the influence of different geometric parameter interactions. The response variables studied were volume flow rate, mass flow rate, torque, and energy efficiency. The simulations exhibited that flow pattern has a distinct feature and is further classified into three groups. In the end, the optimal blade design was identified using response surface methodology (RSM). [ABSTRACT FROM AUTHOR]

Published
2018
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