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Y. H. Kim

Publications and source records attributed to Y. H. Kim.

117 records · Page 7Linked to original sources
arXiv2010

Joint maximum likelihood estimation of carrier and sampling frequency offsets for OFDM systems

In orthogonal-frequency division multiplexing (OFDM) systems, carrier and sampling frequency offsets (CFO and SFO, respectively) can destroy the orthogonality of the subcarriers and degrade system performance. In the literature, Nguyen-Le, Le-Ngoc, and Ko proposed a simple maximum-likelihood (ML) scheme using two long training symbols for estimating the initial CFO and SFO of a recursive least-squares (RLS) estimation scheme. However, the results of Nguyen-Le's ML estimation show poor performance relative to the Cramer-Rao bound (CRB). In this paper, we extend Moose's CFO estimation algorithm to joint ML estimation of CFO and SFO using two long training symbols. In particular, we derive CRBs for the mean square errors (MSEs) of CFO and SFO estimation. Simulation results show that the proposed ML scheme provides better performance than Nguyen-Le's ML scheme.

cs.IT↗
arXiv2010

Far-infrared measurements of oxygen-doped polycrystalline La2CuO4.0315 superconductor under slow-cooled and fast-cooled conditions

We have studied the far-infrared (far-IR) charge dynamics of an equilibrated pure oxygen doped La2CuO4+0.0315 under slow-cooled and fast-cooled conditions. The superconducting transition temperature (Tc) for the slow-cooled and that for the fast-cooled processes were respectively found to be close to the two intrinsic Tc's: One at 30 K and the other at 15 K. Direct comparison with our previous results and other far-IR and Raman studies on single crystalline La2-xSrxCuO4, we conclude that the topology of the pristine electronic phases that are responsible for the two intrinsic Tc's is holes arranged into two-dimensional (2D) square lattices.

cond-mat.supr-con↗
arXiv2009

Fluctuation-Induced Heat Release from Temperature-Quenched Nuclear Spins near a Quantum Critical Point

At a quantum critical point (QCP) -- a zero-temperature singularity in which a line of continuous phase transition terminates -- quantum fluctuations diverge in space and time, leading to exotic phenomena that can be observed at non-zero temperatures. Using a quantum antiferromagnet, we present calorimetric evidence that nuclear spins frozen in a high-temperature metastable state by temperature quenching are annealed by quantum fluctuations near the QCP. This phenomenon, with readily detectable heat release from the nuclear spins as they are annealed, serves as an excellent marker of a quantum critical region around the QCP and provides a probe of the dynamics of the divergent quantum fluctuations.

cond-mat.str-el↗
arXiv2007

Variable stars in the Open Cluster M11 (NGC 6705)

V-band time-series CCD photometric observations of the intermediate-age open cluster M11 were performed to search for variable stars. Using these time-series data, we carefully examined light variations of all stars in the observing field. A total of 82 variable stars were discovered, of which 39 stars had been detected recently by Hargis et al. (2005). On the basis of observational properties such as variable period, light curve shape, and position on a color-magnitude diagram, we classified their variable types as 11 delta Scuti-type pulsating stars, 2 gamma Doradus-type pulsating stars, 40 W UMa-type contact eclipsing binaries, 13 Algol-type detached eclipsing binaries, and 16 eclipsing binaries with long period. Cluster membership for each variable star was deduced from the previous proper motion results (McNamara et al. 1977) and position on the color-magnitude diagram. Many pulsating stars and eclipsing binaries in the region of M11 are probable members of the cluster.

astro-ph↗
arXiv2007

Variable Stars in the Open Cluster NGC 2099 (M37)

Time-series CCD photometric observations of the intermediate-age open cluster NGC 2099 were performed to search for variable stars. We also carried out BV photometry to study physical properties of variables in the cluster. Using V-band time-series data, we carefully examined light variations of about 12,000 stars in the range of 10 < V < 22 mag. A total of 24 variable stars have been identified; seven stars are previously known variables and 17 stars are newly identified. On the basis of observational properties such as light curve shape, period, and amplitude, we classified the new variable stars as nine delta Scuti-type pulsating stars, seven eclipsing binaries, and one peculiar variable star. Judging from the position of delta Scuti-type stars in the color-magnitude diagram, only two stars are likely to have the cluster membership. One new variable KV10 shows peculiar light variations with a delta Scuti-type short period of about 0.044 day as well as a long period of 0.417 day.

astro-ph↗
arXiv2006

The "Bohr" Model for the High Tc Superconductivity

We propose a charge crystal model that captures all the essential physics of the high temperature superconductivity (HTS) in the long wavelength limit. Based on the recent transport and the far-infrared (far-IR) experiments, we argue that the three-dimensional (3D) ordering of the pinned two-dimensional (2D) square electronic lattice (EL) in each CuO2 plane is the building block of HTS. Incorporating the physical picture derived from the neutron scattering experiments, we demonstrate that our model presents a coherent picture of the HTS. We suggest that the charge crystal model serves as a model for the microscopic theory and, hence, offers the key to the mechanism for the HTS.

cond-mat.supr-con↗
arXiv2005

Evidence of Coherent C-Axis Charge Transport in Underdoped La2-xSrxCuO4 Superconductors

We have observed the plasma edge in the normal state c-axis far-infrared (far-IR) reflectivity of La2-xSrxCuO4 (LSCO) single crystals at an unlikely Sr-doping concentration x = 0.07 but not in x = 0.09. We find that the c-axis scattering rate (Gc) is surprisingly small, Gc = 13 (+_) 2 cm-1 for x = 0.07 and Gc = 24 (+_) 1 cm-1 for x = 0.09 and nearly temperature independent but increases linearly with doping up to x = 0.16. We suggest that the apparent absence of the normal state plasma edge in the previous measurements is due to the larger Gc than the screened plasma frequency of the coherent free carriers. We conclude that the c-axis charge transport in LSCO in the underdoped regime is intrinsically coherent.

cond-mat.supr-con↗
arXiv2003

The composite picture of the charge carriers in La2-xSrxCuO4 (0.063 < x < 0.11) superconductors

Through far-infrared studies of La2-xSrxCuO4 single crystals for x = 0.063, 0.07, 0.09, and 0.11, we found that only ~ 0.2 % of the total holes participated in the nearly dissipationless normal state charge transport and superconductivity. We have also observed characteristic collective modes at w ~ 18 cm-1 and 22 cm-1 due to the bound carriers in an electronic lattice (EL) state and the free carriers are massively screened by the EL. Our findings lead us to propose a composite picture of the charge system where the free carriers are coupled to and riding on the EL. This unique composite system of charge carriers may provide further insights into the understanding of the cuprate physics.

cond-mat.supr-con↗