arXiv subjects

J. W. Park

Publications and source records attributed to J. W. Park.

9 recordsLinked to original sources

arXiv2025

Disentangle Intertwined Interactions in Correlated Charge Density Wave with Magnetic Impurities

Magnetic impurities in strongly correlated electronic systems serve as sensitive probes to a wide range of many-body quantum phenomena. Broken symmetries in such a system can lead to inequivalent lattice sites, and magnetic impurities may interact selectively with particular orbitals or sublattices. However, the microscopic mechanisms behind such site-specific interactions have been poorly understood. Here, we explore the behavior of individual Fe adatoms on a cluster-Mott charge-density-wave (CDW) system of 1T-TaS2 utilizing scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT). Our measurements uncover pronounced site-dependent electronic states of CDW clusters with Fe adatoms, indicating distinct local coupling to cluster-Mott states. DFT calculations identify three distinct types of interactions; hybridization with localized correlated electrons, distorting the CDW cluster, and charge transfer. In particular, the hybridization of Fe 3d and half-filled Ta 5dz2 orbitals suppresses the Mott insulating state for an adatom at the center of a CDW cluster. While the results underscore a crucial role of the direct orbital hybridization and the limitation of the prevailing single-site Kondo impurity model, they suggest the possibility of controlling entangled interactions separately in a cluster Mott insulator.

cond-mat.str-el↗

arXiv2025

Tradeoffs in Biconic Intake Aerodynamic Design Optimization with Sub-optimal Oswatitsch Solutions

The notion of sub-optimal Oswatitsch solutions is introduced in order to systematically conduct a tradeoff between total pressure recovery (TPR) and intake drag coefficient (CDi) for supersonic intakes. It is shown that the Oswatitsch-optimal TPR for a biconic intake may be enhanced by adding a conical flare which modifies the terminal normal shock into a novel Lambda shock structure. The optimization problem is formulated along the lines of Axiomatic Design Theory with the conical angle pair and the cowl fineness ratio as the two design parameters. Reynolds-averaged Navier-Stokes (RANS) simulations are performed to iteratively arrive at the optimal solutions, with and without an intake length constraint, for a fixed value of the intake mass flow rate. The results are used to generate the Pareto front in the space of the objective functions, which yields the set of solutions between which TPR and Cdi may be traded off for one another. Additionally, an off-Oswatitsch solution, where only the second cone angle is altered from its optimal Oswatitsch value, is obtained and is compared with the sub-optimal Oswatitsch solutions that form the Pareto front.

physics.flu-dyn↗

arXiv2022

DeepZipper: A Novel Deep Learning Architecture for Lensed Supernovae Identification

Large-scale astronomical surveys have the potential to capture data on large numbers of strongly gravitationally lensed supernovae (LSNe). To facilitate timely analysis and spectroscopic follow-up before the supernova fades, an LSN needs to be identified soon after it begins. To quickly identify LSNe in optical survey datasets, we designed ZipperNet, a multi-branch deep neural network that combines convolutional layers (traditionally used for images) with long short-term memory (LSTM) layers (traditionally used for time series). We tested ZipperNet on the task of classifying objects from four categories -- no lens, galaxy-galaxy lens, lensed type Ia supernova, lensed core-collapse supernova -- within high-fidelity simulations of three cosmic survey data sets -- the Dark Energy Survey (DES), Rubin Observatory's Legacy Survey of Space and Time (LSST), and a Dark Energy Spectroscopic Instrument (DESI) imaging survey. Among our results, we find that for the LSST-like dataset, ZipperNet classifies LSNe with a receiver operating characteristic area under the curve of 0.97, predicts the spectroscopic type of the lensed supernovae with 79\% accuracy, and demonstrates similarly high performance for LSNe 1-2 epochs after first detection. We anticipate that a model like ZipperNet, which simultaneously incorporates spatial and temporal information, can play a significant role in the rapid identification of lensed transient systems in cosmic survey experiments.

astro-ph.CO↗

arXiv2021

Time Delay Lens Modelling Challenge

In recent years, breakthroughs in methods and data have enabled gravitational time delays to emerge as a very powerful tool to measure the Hubble constant $H_0$. However, published state-of-the-art analyses require of order 1 year of expert investigator time and up to a million hours of computing time per system. Furthermore, as precision improves, it is crucial to identify and mitigate systematic uncertainties. With this time delay lens modelling challenge we aim to assess the level of precision and accuracy of the modelling techniques that are currently fast enough to handle of order 50 lenses, via the blind analysis of simulated datasets. The results in Rung 1 and Rung 2 show that methods that use only the point source positions tend to have lower precision ($10 - 20\%$) while remaining accurate. In Rung 2, the methods that exploit the full information of the imaging and kinematic datasets can recover $H_0$ within the target accuracy ($ |A| < 2\%$) and precision ($< 6\%$ per system), even in the presence of poorly known point spread function and complex source morphology. A post-unblinding analysis of Rung 3 showed the numerical precision of the ray-traced cosmological simulations to be insufficient to test lens modelling methodology at the percent level, making the results difficult to interpret. A new challenge with improved simulations is needed to make further progress in the investigation of systematic uncertainties. For completeness, we present the Rung 3 results in an appendix, and use them to discuss various approaches to mitigating against similar subtle data generation effects in future blind challenges.

astro-ph.CO↗

arXiv2015

Measurement of the quenching and channeling effects in a CsI crystal used for a WIMP search

We have studied channeling effects in a Cesium Iodide (CsI) crystal that is similar in composition to the ones being used in a search for Weakly Interacting Massive Particles (WIMPs) dark matter candidates, and measured its energy-dependent quenching factor, the relative scintillation yield for electron and nuclear recoils. The experimental results are reproduced with a GEANT4 simulation that includes a model of the scintillation efficiency as a function of electronic stopping power. We present the measured and simulated quenching factors and the estimated effects of channeling.

physics.ins-det↗

arXiv2014

Search for Low-Mass Dark Matter with CsI(Tl) Crystal Detectors

We present a search for low-mass ($\leq 20 GeV/c^{2}$) weakly interacting massive particles(WIMPs), strong candidates of dark matter particles,using the low-background CsI(Tl) detector array of the Korea Invisible Mass Search (KIMS) experiment. With a total data exposure of 24,324.3kg$\cdot$days,we search for WIMP interaction signals produced by nuclei recoiling from WIMP-nuclear elastic scattering with visible energies between 2 and 4keV. The observed energy distribution of candidate events is consistent with null signals, and upper limits of the WIMP-proton spin-independent interaction are set with a 90% confidence level. The observed limit rejects most of the low mass region of parameter space favored by the DAMA annual modulation signal.

astro-ph.CO↗

arXiv2010

Tailoring a two-dimensional electron gas at the LaAlO3/SrTiO3 (001) interface by epitaxial strain

Recently a metallic state was discovered at the interface between insulating oxides, most notably LaAlO3 and SrTiO3. Properties of this two-dimensional electron gas (2DEG) have attracted significant interest due to its potential applications in nanoelectronics. Control over this carrier density and mobility of the 2DEG is essential for applications of these novel systems, and may be achieved by epitaxial strain. However, despite the rich nature of strain effects on oxide materials properties, such as ferroelectricity, magnetism, and superconductivity, the relationship between the strain and electrical properties of the 2DEG at the LaAlO3/SrTiO3 heterointerface remains largely unexplored. Here, we use different lattice constant single crystal substrates to produce LaAlO3/SrTiO3 interfaces with controlled levels of biaxial epitaxial strain. We have found that tensile strained SrTiO3 destroys the conducting 2DEG, while compressively strained SrTiO3 retains the 2DEG, but with a carrier concentration reduced in comparison to the unstrained LaAlO3/SrTiO3 interface. We have also found that the critical LaAlO3 overlayer thickness for 2DEG formation increases with SrTiO3 compressive strain. Our first-principles calculations suggest that a strain-induced electric polarization in the SrTiO3 layer is responsible for this behavior. It is directed away from the interface and hence creates a negative polarization charge opposing that of the polar LaAlO3 layer. This both increases the critical thickness of the LaAlO3 layer, and reduces carrier concentration above the critical thickness, in agreement with our experimental results. Our findings suggest that epitaxial strain can be used to tailor 2DEGs properties of the LaAlO3/SrTiO3 heterointerface.

cond-mat.mtrl-sci↗

arXiv2006

Mean Field Theoretical Structure of He and Be Isotopes

The structures of He and Be even-even isotopes are investigated using an axially symmetric Hartree-Fock approach with a Skyrme-IIIls mean field potential. In these simple HF calculations, He and Be isotopes appear to be prolate in their ground states and Be isotopes have oblate shape isomeric states. It is also shown that there exists a level crossing when the nuclear shape changes from the prolate state to the oblate state. The single neutron levels of Be isotopes exhibit a neutron magic number 6 instead of 8 and show that the level inversion between 1/2- and 1/2+ levels occurs only for a largely deformed isotope. Protons are bound stronger in the isotope with more neutrons while neutron levels are somewhat insensitive to the number of neutrons and thus the nuclear size and also the neutron skin become larger as the neutron number increases. In these simple calculations with Skyrme-IIIls interaction no system with a clear indication of neutron halo was found among He and Be isotopes. Instead of it we have found 8He+2n, 2n+8He+2n, and 16Be+2n like chain structures with clusters of two correlated neutrons. It is also shown that 8He and 14Be in their ground states are below the neutron drip line in which all nucleons are bound with negative energy and that 16Be in its ground state is beyond the neutron drip line with two neutrons in positive energy levels.

nucl-th↗

arXiv2006

Molecular Hydrogen Fluorescence in the Eridanus Superbubble

The first FUV (lambda lambda 1350-1750 A) spectral imaging observations of the Eridanus superbubble, obtained with the SPEAR/FIMS mission, have revealed distinct fluorescent emission from molecular hydrogen. In this study, the observed emission features were compared with those from a photo-dissociation region model with assumed illuminating stellar fields. The result showed rather high line ratios of I_{1580}/I_{1610}, which may imply the existence of high-temperature molecular clouds in the region. The H_2 fluorescence intensity showed a proportional correlation with H-alpha emission, indicating that the fluorescence and the recombination emission have similar physical origins.

astro-ph↗