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Search Results: 1 - 10 of 188239 matches for " T. W. Noh "
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Polaron Absorption in a Perovskite Manganite La0.7Ca0.3MnO3
K. H. Kim,J. H. Jung,T. W. Noh
Physics , 1998, DOI: 10.1103/PhysRevLett.81.1517
Abstract: Temperature dependent optical conductivity spectra of a La0.7Ca0.3MnO3 (LCMO) sample were measured. In the metallic regime at very low temperatures, they clearly showed two types of absorption features, i.e., a sharp Drude peak and a broad mid-infrared absorption band, which could be explained as coherent and incoherent bands of a large lattice polaron. This elementary excitation in LCMO was found to be in a strong coupling regime and to have interactions with the spin degree of freedom.
Anomalous Nernst Effects of [CoSiB/Pt] Multilayer Films
O. Kelekci,H. N. Lee,T. W. Kim,H. Noh
Physics , 2013, DOI: 10.4283/JMAG.2013.18.3.225
Abstract: We report a measurement for the anomalous Nernst effects induced by a temperature gradient in [CoSiB/Pt] multilayer films with perpendicular magnetic anisotropy. The Nernst voltage shows a characteristic hysteresis which reflects the magnetization of the film as in the case of the anomalous Hall effects. With a local heating geometry, we also measure the dependence of the anomalous Nernst voltage on the distance d from the heating element. It is roughly proportional to 1/d^1.3, which can be conjectured from the expected temperature gradient along the sample from the heat equation.
Bound Excitons in Sr$_{2}$CuO$_{3}$
K. W. Kim,G. D. Gu,C. C. Homes,T. W. Noh
Physics , 2008, DOI: 10.1103/PhysRevLett.101.177404
Abstract: We investigated temperature dependent optical spectra of the one-dimensional chain compound Sr$_{2}$CuO$_{3}$. The charge transfer transition polarized along the chain direction shows a strongly asymmetric line shape as expected in one-dimensional extended Hubbard model. At low temperature the charge transfer peak shows a large blue shift and reveals additional sharp peaks at the gap. Even though many spectroscopic studies suggest that this material can not have a bound exciton based on the one-dimensional extended Hubbard model, we attribute the additional sharp peaks to excitons, which come to exist due to the long range Coulomb interaction.
Charge Ordering Fluctuation and Optical Pseudogap in La$_{1-x}$Ca$_{x}$MnO$_{3}$
K. H. Kim,S. Lee,T. W. Noh,S-W. Cheong
Physics , 2002, DOI: 10.1103/PhysRevLett.88.167204
Abstract: Optical spectroscopy was used to investigate the optical gap (2$\Delta $) due to charge ordering (CO) and related pseudogap developments with x and temperature (T) in La$_{1-x}$Ca$_{x}$MnO$_{3}$ (0.48 <= x <= 0.67). Surprisingly, we found 2$\Delta $/k_{B}T_{CO} is as large as 30 for x ~0.5, and decreases rapidly with increasing x. Simultaneously, the optical pseudogap, possibly starting from T^* far above T_{CO} becomes drastically enhanced near x=0.5, producing non-BCS T-dependence of 2$\Delta $ with the large magnitude far above T_{CO}, and systematic increase of T^* for x~0.5. These results unequivocally indicate systematically-enhanced CO correlation when x approaches 0.5 even though T_{CO} decreases.
Coercive fields in ultrathin BaTiO3 capacitors
J. Y. Jo,Y. S. Kim,T. W. Noh,Jong-Gul Yoon,T. K. Song
Physics , 2006, DOI: 10.1063/1.2402238
Abstract: Thickness-dependence of coercive field (EC) was investigated in ultrathin BaTiO3 capacitors with thicknesses (d) between 30 and 5 nm. The EC appears nearly independent of d below 15 nm, and decreases slowly as d increases above 15 nm. This behavior cannot be explained by extrinsic effects, such as interfacial passive layers or strain relaxation, nor by homogeneous domain models. Based on domain nuclei formation model, the observed EC behavior is explainable via a quantitative level. A crossover of domain shape from a half-prolate spheroid to a cylinder is also suggested at d~ 15 nm, exhibiting good agreement with experimental results.
Phonon mediated drag in double layer two dimensional electron systems
H. Noh,S. Zelakiewicz,T. J. Gramila,L. N. Pfeiffer,K. W. West
Physics , 1999, DOI: 10.1103/PhysRevB.59.13114
Abstract: Experiments studying phonon mediated drag in the double layer two dimensional electron gas system are reported. Detailed measurements of the dependence of drag on temperature, layer spacing, density ratio, and matched density are discussed. Comparisons are made to theoretical results [M. C. Bonsager et al., Phys. Rev. B 57, 7085 (1998)] which propose the existence of a new coupled electron-phonon collective mode. The layer spacing and density dependence at matched densities for samples with layer spacings below 2600 A do not support the existence of this mode, showing behavior expected for independent electron and phonon systems. The magnitude of the drag, however, suggests the alternate limit; one in which electrons and phonons are strongly coupled. The results for still larger layer spacing show significant discrepancies with the behavior expected for either limit.
Initial stages of nickel oxide growth on Ag(001) by pulsed laser deposition
S. H. Phark,Y. J. Chang,T. W. Noh,J. -S. Kim
Physics , 2009, DOI: 10.1103/PhysRevB.80.035426
Abstract: Submonolayers of nickel oxide films were grown on an Ag(001) by pulsed laser deposition, and characterized in-situ by both scanning tunneling microscopy and X-ray photoelectron spectroscopy. We observed quasi-two-dimensional growth of the film, and clearly identified several kinds of defects, such as embedded metallic Ni clusters and, notably, oxygen atoms, even while looking deeply into the substrate. These originated from Ni and O hyperthermal projectiles as well as from NiO clusters that were formed during laser ablation of a NiO target. Those defects played a role of nucleation sites in extending the nucleation stage of thin film growth.
Orbital-selective Mass Enhancements in Multi-band Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ Systems Analyzed by the Extended Drude Model
J. S. Lee,S. J. Moon,T. W. Noh,S. Nakatsuji,Y. Maeno
Physics , 2005, DOI: 10.1103/PhysRevLett.96.057401
Abstract: We investigated optical spectra of quasi-two-dimensional multi-band Ca$% _{2-x} $Sr$_{x}$RuO$_{4}$ systems. The extended Drude model analysis on the ab-plane optical conductivity spectra indicates that the effective mass should be enhanced near $x=0.5$. Based on the sum rule argument, we showed that the orbital-selective Mott-gap opening for the $d_{yz/zx}$ bands, the widely investigated picture, could not be the origin of the mass enhancement. We exploited the multi-band effects in the extended Drude model analysis, and demonstrated that the intriguing heavy mass state near $x=0.5$ should come from the renormalization of the $d_{xy}$ band.
Magnetic Field Dependent Optical Studies of a Layered Antiferromagnet Pr$%_{1/2}$Sr$_{1/2}$MnO$_3$
J. H. Jung,H. J. Lee,T. W. Noh,Y. Moritomo,Y. J. Wang,X. Wei
Physics , 2000, DOI: 10.1103/PhysRevB.62.8634
Abstract: Magnetic field ($H$)-dependent optical conductivity spectra $\sigma (\omega) $ and dielectric constant spectra $\epsilon (\omega)$ of a layered antiferromagnet ($A$-type) Pr$_{1/2}$Sr$_{1/2}$MnO$_3$ were presented. At 0.0 T in 4.2 K, the $\sigma (\omega)$ showed a small Drude-like peak in the far-infrared region and a broad absorption peak in the mid-infrared region. With increasing $H$, large spectral weights were transferred from high to low energy regions with increase of the Drude-like peak. We found that a polaron picture could explain most of the $H$-dependent spectral changes. By comparing with the $H$-dependent spectral weight changes and $\epsilon (\omega)$ of Nd$_{1/2}$Sr$_{1/2}$MnO$_3$, which has the $CE$-type spin ordering, we showed that a dimensional crossover from a 2-dimensional to a 3-dimensional metal occurs in Pr$_{1/2}$Sr$_{1/2}$MnO$_3$.
Missing 2k_F Response for Composite Fermions in Phonon Drag
S. Zelakiewicz,H. Noh,T. J. Gramila,L. N. Pfeiffer,K. W. West
Physics , 1999, DOI: 10.1103/PhysRevLett.85.1942
Abstract: The response of composite Fermions to large wavevector scattering has been studied through phonon drag measurements. While the response retains qualitative features of the electron system at zero magnetic field, notable discrepancies develop as the system is varied from a half-filled Landau level by changing density or field. These deviations, which appear to be inconsistent with the current picture of composite Fermions, are absent if half-filling is maintained while changing density. There remains, however, a clear deviation from the temperature dependence anticipated for 2k_F scattering.
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