H. Choi - Kyungpook National Univ.

H. Choi
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H. Choi
Kyungpook National Univ.
South Korea

Pubs By Year

Pub Categories

Physics - Materials Science (8)
Physics - Superconductivity (5)
Computer Science - Robotics (5)
Computer Science - Learning (4)
Physics - Mesoscopic Systems and Quantum Hall Effect (4)
Physics - Strongly Correlated Electrons (4)
High Energy Physics - Phenomenology (3)
Mathematics - Combinatorics (3)
Astrophysics of Galaxies (3)
Mathematics - Functional Analysis (3)
Quantum Physics (2)
Computer Science - Computation and Language (2)
Computer Science - Artificial Intelligence (2)
Physics - Optics (2)
Statistics - Machine Learning (2)
Statistics - Theory (1)
Mathematics - Statistics (1)
Computer Science - Discrete Mathematics (1)
Physics - Accelerator Physics (1)
Statistics - Methodology (1)
Mathematics - Probability (1)
Quantitative Biology - Quantitative Methods (1)
Quantitative Biology - Neurons and Cognition (1)
Physics - Other (1)
Mathematics - Geometric Topology (1)
Computer Science - Computer Vision and Pattern Recognition (1)
Computer Science - Graphics (1)
Mathematics - Commutative Algebra (1)
High Energy Physics - Experiment (1)
Mathematics - Optimization and Control (1)
Nuclear Experiment (1)

Publications Authored By H. Choi

We propose an on-chip scalable cluster-state quantum computing (QC) architecture comprising a two-dimensional array of atomic qubits and detectors, networked by photonic switches and waveguides. A major barrier to scaling up such systems lies in efficiently entangling neighboring atomic memories by conversion of the qubits to photons and establishing entanglement via Bell measurements in the optical domain, all within the coherence time. Our architecture leverages percolation theory to significantly reduce the time required to create a universal-QC-capable cluster of atomic memories, compared with recently-studied architectures that rely on repeat-until-success entanglement connections. Read More

To watch 360{\deg} videos on normal 2D displays, we need to project the selected part of the 360{\deg} image onto the 2D display plane. In this paper, we propose a fully-automated framework for generating content-aware 2D normal-view perspective videos from 360{\deg} videos. Especially, we focus on the projection step preserving important image contents and reducing image distortion. Read More

For effective treatment of Alzheimer disease (AD), it is important to identify subjects who are most likely to exhibit rapid cognitive decline. Herein, we developed a novel framework based on a deep convolutional neural network which can predict future cognitive decline in mild cognitive impairment (MCI) patients using flurodeoxyglucose and florbetapir positron emission tomography (PET). The architecture of the network only relies on baseline PET studies of AD and normal subjects as the training dataset. Read More

Using the latest cosmological hydrodynamic N-body simulations of groups and clusters, we study how location in phase-space coordinates at $z$$=$$0$ can provide information on environmental effects acting in clusters. We confirm the results of previous authors showing that galaxies tend to follow a typical path in phase-space as they settle into the cluster potential. As such, different regions of phase-space can be associated with different times since first infalling into the cluster. Read More

Topological superconductors as characterized by Majorana surface states has been actively searched for their significance in fundamental science and technological implication. The large spin-orbit coupling in Bi-Pd binaries has stimulated extensive investigations on the topological surface states in these superconducting compounds. Here we report a study of normal-state electronic structure in a centrosymmetric $\alpha$-PdBi2 within density functional theory calculations. Read More

In this article we prove that, if $X$ is a smooth $4$-manifold containing an embedded double node neighborhood, all knot surgery $4$-manifolds $X_K$ are mutually diffeomorphic to each other after a connected sum with $\mathbb{CP}^2$. Hence, by applying to the simply connected elliptic surface $E(n)$, we also show that every knot surgery $4$-manifold $E(n)_K$ is almost completely decomposable. Read More

In search of novel, improved materials for magnetic data storage and spintronic devices, compounds that allow a tailoring of magnetic domain shapes and sizes are essential. Good candidates are materials with intrinsic anisotropies or competing interactions, as they are prone to host various domain phases that can be easily and precisely selected by external tuning parameters such as temperature and magnetic field. Here, we utilize vector magnetic fields to visualize directly the magnetic anisotropy in the uniaxial ferromagnet CeRu$_2$Ga$_2$B. Read More

A class of graphs is $\chi$-bounded if there exists a function $f:\mathbb N\rightarrow \mathbb N$ such that for every graph $G$ in the class and an induced subgraph $H$ of $G$, if $H$ has no clique of size $q+1$, then the chromatic number of $H$ is less than or equal to $f(q)$. We denote by $W_n$ the wheel graph on $n+1$ vertices. We show that the class of graphs having no vertex-minor isomorphic to $W_n$ is $\chi$-bounded. Read More

We propose a general protocol for low-control refrigeration and thermometry of thermal qubits, which can be implemented using electronic spins in diamond. The refrigeration is implemented by a probe, consisting of a network of spins with two-body XXZ interactions. The protocol involves two operations: (i) free evolution of the probe; and (ii) a swap gate between one spin in the probe and the thermal qubit we wish to cool. Read More

The traditional view of the morphology-spin connection is being challenged by recent integral-field-unit observations, as the majority of early-type galaxies are found to have a rotational component that is often as large as a dispersion component. Mergers are often suspected to be critical in galaxy spin evolution, yet the details of their roles are still unclear. We present the first results on the spin evolution of galaxies in cluster environments through a cosmological hydrodynamic simulation. Read More

Using dynamic cantilever magnetometry and experimentally determining the cantilever's vibrational mode shape, we precisely measured the magnetic moment of a lithographically defined micron-sized superconducting Nb ring, a key element for the previously proposed subpiconewton force standard. The magnetic moments due to individual magnetic fluxoids and a diamagnetic response were independently determined at T = 4.3 K, with a subfemtoampere-square-meter resolution. Read More

The irrotational nature of superfluid helium was discovered through its decoupling from the container under rotation. Similarly, the resonant period drop of a torsional oscillator (TO) containing solid helium was first interpreted as the decoupling of solid from the TO and appearance of supersolid. However, the resonant period can be changed by mechanisms other than supersolid, such as the elastic stiffening of solid helium that is widely accepted as the reason for the TO response. Read More

In this paper, cyber attack detection and isolation is studied on a network of UAVs in a formation flying setup. As the UAVs communicate to reach consensus on their states while making the formation, the communication network among the UAVs makes them vulnerable to a potential attack from malicious adversaries. Two types of attacks pertinent to a network of UAVs have been considered: a node attack on the UAVs and a deception attack on the communication between the UAVs. Read More

We investigate the two-particle twist-3 distribution amplitudes (DAs) of the pseudoscalar mesons, in particular pseudoscalar ($\phi^P_{3;M}(x)$) and pseudotensor ($\phi^\sigma_{3;M}(x)$) DAs of the pion and kaon, in the light-front quark model based on the variational principle. We find that the behavior of the conformal symmetry in each meson distribution amplitude depends on the chiral limit characteristics of the light-front trial wave function taken in the variational principle. We specifically take the two different light-front trial wave functions, Gaussian vs. Read More

This paper addresses path planning of an unmanned aerial vehicle (UAV) with remote sensing capabilities (or wireless communication capabilities). The goal of the path planning is to find a minimum-flight-time closed tour of the UAV visiting all executable areas of given remote sensing and communication tasks; in order to incorporate the nonlinear vehicle dynamics, this problem is regarded as a dynamically-constrained traveling salesman problem with neighborhoods. To obtain a close-to-optimal solution for the path planning in a tractable manner, a sampling-based roadmap algorithm that embeds an optimal control-based path generation process is proposed. Read More

The primate visual system has an exquisite ability to discriminate partially occluded shapes. Recent electrophysiological recordings suggest that response dynamics in intermediate visual cortical area V4, shaped by feedback from prefrontal cortex (PFC), may play a key role. To probe the algorithms that may underlie these findings, we build and test a model of V4 and PFC interactions based on a hierarchical predictive coding framework. Read More

Twisted bilayer graphene offers a unique bilayer two-dimensional-electron system where the layer separation is only in sub-nanometer scale. Unlike Bernal-stacked bilayer, the layer degree of freedom is disentangled from spin and valley, providing eight-fold degeneracy in the low energy states. We have investigated broken-symmetry quantum Hall (QH) states and their transitions due to the interplay of the relative strength of valley, spin and layer polarizations in twisted bilayer graphene. Read More

In VO$_2$, the explicit origin of the insulator-to-metal transition is still disputable between Peierls and Mott insulators. Along with the controversy, its second monoclinic (M2) phase has received considerable attention due to the presence of electron correlation in undimerized vanadium ions. However, the origin of the M2 phase is still obscure. Read More

This work presents a multiscale framework to solve an inverse reinforcement learning (IRL) problem for continuous-time/state stochastic systems. We take advantage of a diffusion wavelet representation of the associated Markov chain to abstract the state space. This not only allows for effectively handling the large (and geometrically complex) decision space but also provides more interpretable representations of the demonstrated state trajectories and also of the resulting policy of IRL. Read More

We analyzed two twist-3 distribution amplitudes of pion, i.e. pseudoscalar $\phi^P_{3;\pi}(x)$ and pseudotensor $\phi^\sigma_{3;\pi}(x)$, within the LFQM. Read More

In this note we show that in a commutative ring $R$ with unity, for any $n > 0$, if $I$ is an $n$-absorbing ideal of $R$, then $(\sqrt{I})^{n} \subseteq I$. Read More

This work presents a multiscale framework to solve a class of stochastic optimal control problems in the context of robot motion planning and control in a complex environment. In order to handle complications resulting from a large decision space and complex environmental geometry, two key concepts are adopted: (a) a diffusion wavelet representation of the Markov chain for hierarchical abstraction of the state space; and (b) a desirability function-based representation of the Markov decision process (MDP) to efficiently calculate the optimal policy. In the proposed framework, a global plan that compressively takes into account the long time/length-scale state transition is first obtained by approximately solving an MDP whose desirability function is represented by coarse scale bases in the hierarchical abstraction. Read More

Using high resolution hydrodynamical cosmological simulations, we conduct a comprehensive study of how tidal stripping removes dark matter and stars from galaxies. We find that dark matter is always stripped far more significantly than the stars -- galaxies that lose $\sim$80$\%$ of their dark matter, typically lose only 10$\%$ of their stars. This is because the dark matter halo is initially much more extended than the stars. Read More

Informative path planning (IPP) is used to design paths for robotic sensor platforms to extract the best/maximum possible information about a quantity of interest while operating under a set of constraints, such as the dynamic feasibility of vehicles. The key challenges of IPP are the strong coupling in multiple layers of decisions: the selection of locations to visit, the allocation of sensor platforms to those locations; and the processing of the gathered information along the paths. This paper presents a systematic procedure for IPP and environmental mapping using multiple UAV sensor platforms. Read More

Topological superconductors are one of the most actively studied materials these days. They are a promising candidate for hosting Majorana fermions either on their boundaries or in vortex cores. Detecting 1D edge current around the periphery of a 2D $p_x + ip_y$ superconductor would be a hallmark signature of topological superconductivity, but Majorana fermions are not amenable to electronic current measurements due to their charge neutral nature. Read More

We propose to measure the photo-production cross section of $J/{\psi}$ near threshold, in search of the recently observed LHCb hidden-charm resonances $P_c$(4380) and $P_c$(4450) consistent with 'pentaquarks'. The observation of these resonances in photo-production will provide strong evidence of the true resonance nature of the LHCb states, distinguishing them from kinematic enhancements. A bremsstrahlung photon beam produced with an 11 GeV electron beam at CEBAF covers the energy range of $J/{\psi}$ production from the threshold photo-production energy of 8. Read More

The symmetry requirement and the origin of magnetic orders coexisting with superconductivity have been strongly debated issues of iron-based superconductors (FeSCs). Observation of C$_4$-symmetric antiferromagnetism in violation of the inter-band nesting condition of spin-density waves in superconducting ground state will require significant change in our understanding of the mechanism of FeSC. The superconducting material Sr$_2$VO$_3$FeAs, a bulk version of monolayer FeSC in contact with a perovskite layer with its magnetism ($T_N$ ~ 50 K) and superconductivity ($T_c$ ~ 37 K) coexisting at parent state, has no reported structural orthorhombic distortion and thus makes a perfect system to look for theoretically expected C$_4$ magnetisms. Read More

The physics at interfaces between monolayer iron-based superconductors (FeSC) and perovskite substrates has received considerable attention due to the unusually high $T_c$ of ~100 K found recently in monolayer FeSe on SrTiO$_3$. It has been suggested that forward-scattering interfacial phonons coupled with the Fe-layer electrons can enhance superconductivity from almost any kind of pre-existing electron-based pairing, initiating a quest for perovskite-clad FeSC monolayers and bulk heterostructures with higher coupling efficiency with interfacial phonons. Here we report a spectroscopic imaging scanning tunneling microscopy (SI-STM) study on a parent-compound superconductor Sr$_2$VO$_3$FeAs, the only currently known self-assembled bulk example of FeSC monolayers on perovskite layers with substantially high $T_c$ = 33 - 37 K. Read More

Current language models have a significant limitation in the ability to encode and decode factual knowledge. This is mainly because they acquire such knowledge from statistical co-occurrences although most of the knowledge words are rarely observed. In this paper, we propose a Neural Knowledge Language Model (NKLM) which combines symbolic knowledge provided by the knowledge graph with the RNN language model. Read More

Functional data analysis, FDA, is now a well established discipline of statistics, with its core concepts and perspectives in place. Despite this, there are still fundamental statistical questions which have received relatively little attention. One of these is the systematic construction of confidence regions for functional parameters. Read More

A homogeneous set of a graph $G$ is a set $X$ of vertices such that $2\le \lvert X\rvert <\lvert V(G)\rvert$ and no vertex in $V(G)-X$ has both a neighbor and a non-neighbor in $X$. A graph is prime if it has no homogeneous set. We present an algorithm to decide whether a class of graphs given by a finite set of forbidden induced subgraphs contains infinitely many non-isomorphic prime graphs. Read More

We first observe a potential weakness of continuous vector representations of symbols in neural machine translation. That is, the continuous vector representation, or a word embedding vector, of a symbol encodes multiple dimensions of similarity, equivalent to encoding more than one meaning of the word. This has the consequence that the encoder and decoder recurrent networks in neural machine translation need to spend substantial amount of their capacity in disambiguating source and target words based on the context which is defined by a source sentence. Read More

PAL-XFEL, a 4th generation light source, is currently being installed and will be completed by December of 2015 so that users can be supported beginning in 2016. PAL-XFEL equipment should continuously maintain the bunch-to-bunch beam parameter (60Hz, Energy 10GeV, Charge 200pC, Bunch Length 60fs, Emittance X/Y 0.481mm/0. Read More

Black phosphorus (BP), a layered van der Waals material, reportedly has a band gap sensitive to external perturbations and manifests a Dirac-semimetal phase when its band gap is closed. Previous studies were focused on effects of each perturbation, lacking a unified picture for the band-gap closing and the Dirac-semimetal phase. Here, using pseudospins from the glide-reflection symmetry, we study the electronic structures of mono- and bilayer BP and construct the phase diagram of the Dirac-semimetal phase in the parameter space related to pressure, strain, and electric field. Read More

We report on fabricating high-fill-factor plano-convex spherical and square micro-lens arrays on fused silica glass surface using CO2 laser assisted reshaping technique. Initially, periodic micro-pillars have been encoded on the glass surface by means of a femtosecond laser beam. Afterwards, the micro-pillars are polished several times by irradiating a CO2 laser beam on top of the micro-pillars. Read More

This paper studies the problem of parameter learning in probabilistic graphical models having latent variables, where the standard approach is the expectation maximization algorithm alternating expectation (E) and maximization (M) steps. However, both E and M steps are computationally intractable for high dimensional data, while the substitution of one step to a faster surrogate for combating against intractability can often cause failure in convergence. We propose a new learning algorithm which is computationally efficient and provably ensures convergence to a correct optimum. Read More

We report the nanodiamond-derived onion-like carbon successfully applied as an electrocatalyst for [VO]2+/[VO2]+ redox flow battery, as drop-coated (in the as-synthesized state) on glassy carbon or carbon felt electrodes. We show that its reversibility and catalytic activity in its as-synthesized state was comparable to some of the best data in the literature which employed surface modifications. We clarified the origin of such excellent performances by physical/electrochemical analyses. Read More

We consider a Markov chain that iteratively generates a sequence of random finite words in such a way that the $n^{\mathrm{th}}$ word is uniformly distributed over the set of words of length $2n$ in which $n$ letters are $a$ and $n$ letters are $b$: at each step an $a$ and a $b$ are shuffled in uniformly at random among the letters of the current word. We obtain a concrete characterization of the Doob-Martin boundary of this Markov chain. Writing $N(u)$ for the number of letters $a$ (equivalently, $b$) in the finite word $u$, we show that a sequence $(u_n)_{n \in \mathbb{N}}$ of finite words converges to a point in the boundary if, for an arbitrary word $v$, there is convergence as $n$ tends to infinity of the probability that the selection of $N(v)$ letters $a$ and $N(v)$ letters $b$ uniformly at random from $u_n$ and maintaining their relative order results in $v$. Read More

Since microRNAs (miRNAs) play a crucial role in post-transcriptional gene regulation, miRNA identification is one of the most essential problems in computational biology. miRNAs are usually short in length ranging between 20 and 23 base pairs. It is thus often difficult to distinguish miRNA-encoding sequences from other non-coding RNAs and pseudo miRNAs that have a similar length, and most previous studies have recommended using precursor miRNAs instead of mature miRNAs for robust detection. Read More

We report electric polarization and magnetization measurements in single crystals of double perovskite Lu2MnCoO6 using pulsed magnetic fields and optical second harmonic generation (SHG) in DC magnetic fields. we observe well-resolved magnetic field-induced changes in the electric polarization in single crystals and thereby resolve the question about whether multiferroic behavior is intrinsic to these materials or an extrinsic feature of polycrystals. We find electric polarization along the crystalline b-axis, that is suppressed by applying a magnetic fields along c-axis and advance a model for the origin of magnetoelectric coupling. Read More

Putinar and Vasilescu have given an algebraic characterization of Hamburger moment sequences in several variables. In this paper we give a characterization of sparse moment subsequences of Hamburger moment sequences and consider the problem of completion of these moment subsequences. Read More

In this paper a connection between Hamburger moment sequences and their moment subsequences is given and the determinacy of these problems are related. Read More

Topological motion planning is a planning problem embedding topological concept of trajectories. In this work, we propose two asymptotically optimal sampling-based algorithms for topological motion planning: (a) a batch processing-based planner, termed Fast Marching Homology-embedded Tree star (FMHT*); and (b) an incremental anytime algorithm, termed Rapidly-exploring Random Homology-embedded Tree star (RRHT*). The methods commonly expand a graph directly in the configuration space and project the associated tree onto the topological concept-augmented space; the computational cost in edge computation and collision checking is significantly reduced by allowing trajectories with different topology to share the edge and collision information. Read More

This paper extends the RRT* algorithm, a recently developed but widely-used sampling-based optimal motion planner, in order to effectively handle nonlinear kinodynamic constraints. Nonlinearity in kinodynamic differential constraints often leads to difficulties in choosing appropriate distance metric and in computing optimized trajectory segments in tree construction. To tackle these two difficulties, this work adopts the affine quadratic regulator-based pseudo metric as the distance measure and utilizes iterative two-point boundary value problem solvers for computing the optimized segments. Read More

Studies of electronic interferometers, based on edge channel transport in the quantum Hall effect regime, have been stimulated by the search for evidence of abelian and non-abelian anyonic statistics of fractional charges.These studies found the ubiquitous electronic Fabry-Perot interferometer to be Coulomb dominated, thus masking coherent Aharonov-Bohm interference. Typically, the main signature of the Coulomb dominated regime is the lack of interference of the outer most edge channel as the magnetic field is varied. Read More

Affiliations: 1Kyungpook National University, 2North Carolina State University

We discuss $(\pi^0,\eta,\eta')\to\gamma^*\gamma$ transition form factors using the light-front quark model. Our discussion includes the analysis of the mixing angles for $\eta-\eta'$. Our results for $Q^2 F_{(\pi^0,\eta,\eta')\to\gamma^*\gamma}(Q^2)$ show scaling behavior for high $Q^2$ consistent with pQCD predictions. Read More

We report quantitative experimental measurements of the Abraham force associated with a propagating optical wave. We isolate this force using a guided light wave undergoing an adiabatic mode transformation (AMT) along a liquid-filled hollow optical fiber (HOF). Utilizing this light intensity distribution within the liquid, we were able to generate a time-averaged non-vanishing Abraham force density, while simultaneously suppressing the Abraham-Minkowski force density. Read More

A profound problem in modern condensed matter physics is discovering and understanding the nature of the fluctuations and their coupling to fermions in cuprates which lead to high temperature superconductivity and the invariably associated strange metal state. Here we report the quantitative determination of the normal and pairing self-energies, made possible by laser-based angle-resolved photoemission measurements with unprecedented accuracy and stability. Through a precise inversion procedure, both the effective interactions in the attractive d-wave symmetry and the repulsive part in the full symmetry are determined. Read More

A magnetic skyrmion observed experimentally in chiral magnets is a topologically protected spin texture. For their unique properties, such as high mobility under current drive, skyrmions have huge potential for applications in next-generation spintronic devices. Defects naturally occurring in magnets have profound effects on the static and dynamical properties of skyrmions. Read More

A modified Radon transform for noisy data is introduced and its inversion formula is established. The problem of recovering the multivariate probability density function $f$ from the moments of its modified Radon transform $\widehat{R}f$ is considered. Read More