arXiv subjects

Jaewook Kim

Publications and source records attributed to Jaewook Kim.

At least 19 recordsLinked to original sources

arXiv2026

Automated Outlier-Robust Bayesian Profile Fitting for Magnetically Confined Plasmas with Modified Tanh Profiles and Good-and-Bad Gaussian Mixture Likelihoods

We present an outlier-robust Bayesian approach for automated kinetic profile fitting in magnetically confined plasmas with the modified tanh (mtanh) parametrisation and demonstrate its implementation on KSTAR. The method addresses two systematic obstacles: anomalous diagnostic channels can bias least-squares fits, and multimodality of the mtanh cost surface can trap deterministic optimisers in secondary minima. The deployed workflow uses a good-and-bad Gaussian mixture likelihood based on the Box--Tiao formulation as the default outlier-robust likelihood for fitted diagnostic channels, with posterior outlier probabilities retained as channel-level quality indicators. The posterior is sampled with an affine-invariant ensemble MCMC sampler initialised near the result of deterministic maximum a posteriori (MAP)-seeking optimisation, reducing sensitivity to secondary minima on the multimodal mtanh surface. A batch automation layer retrieves diagnostic data from MDSplus and fits arbitrary time slices in parallel for the quantities $n_e$, $T_e$, $T_i$, and $v_T$ for which the relevant diagnostics are available. Results are written in formats suitable for MDSplus upload and downstream analysis. Representative KSTAR H-mode cases show that the mixture likelihood downweights contaminated measurements while preserving plausible pedestal profiles. The workflow provides a practical basis for future large-scale kinetic profile production for kinetic-EFIT, TRANSP, FASTRAN, and data-driven analysis workflows.

physics.plasm-ph↗

arXiv2026

A2SG:Adaptive and Asymmetric Surrogate Gradients for Training Deep Spiking Neural Networks

Training deep spiking neural networks (SNNs) remains challenging due to sharp loss landscapes and temporal inconsistency caused by surrogate gradients. To address these challenges, we propose a unified framework: adaptive and asymmetric surrogate gradients A2SG. The adaptive gradients adjust an effective window for spatio-temporal adaptation, reducing spatial gradient variation and maintaining directional consistency of gradients over time. The asymmetric gradients reflect neuronal dynamics by assigning larger gradients to neurons with higher membrane potentials, and we prove that they yield lower variation than symmetric surrogates. Our analysis further establishes a direct connection between local gradient variation and the curvature of the loss landscape, providing a principled explanation for how A2SG promotes convergence to flatter minima and improves generalization. We conduct extensive experiments on diverse models, including CNN-based and Transformer-based SNNs, across various tasks such as image classification using both static and neuromorphic datasets, as well as segmentation. The results demonstrate that A2SG consistently improves accuracy and energy efficiency, establishing it as a general and reliable solution for training deep SNNs. Our code is available at https://github.com/KIST-NCL/A2SG.git.

cs.NE↗

arXiv2026

Ensuring Reliability in Programming Knowledge Tracing: A Re-evaluation of Attention-augmented Models and Experimental Protocols

Programming Knowledge Tracing (PKT) has recently advanced through hybrid approaches that integrate attention-based feature modeling for code representation with RNN-based sequential prediction. While these models report strong empirical performance, their reliability can be sensitive to subtle implementation and experimental design choices. This study revisits representative PKT models and shows that reported gains can be substantially influenced by model configuration and sequence construction practices. We identify issues in attention dimension settings that affect performance estimates, and demonstrate that improper ordering of student attempts, such as ignoring ServerTimestamp, can violate temporal causality and lead to overly optimistic results. To ensure consistent evaluation, hyperparameters are selected via grid search guided by a single designated fold and then fixed uniformly across all folds during cross-validation. We further analyze the role of assignment-wise characteristics and systematically explore the impact of maximum sequence length. Using this protocol, we re-evaluate PKT models on the CodeWorkout dataset. Our results show that, under controlled and consistent settings, the performance gap between attention-enhanced models and standard DKT is significantly reduced, and increased architectural complexity does not consistently translate into superior performance. Beyond individual model comparisons, this work provides practical guidance for reliable and comparable evaluation in programming knowledge tracing.

cs.LG↗

arXiv2026

Study of Low-Frequency Core-Edge Coupling in a Tokamak: I. Experimental Observation in KSTAR

Double-peaked fishbone events across multiple KSTAR discharges are investigated. The normalized beta $\beta_{\mathrm{N}}$ and the edge safety factor $q_{\mathrm{95}}$ under which the fishbones appear vary depending on the presence and form of external magnetic perturbations. The fishbone strength is closely related to $\beta_{\mathrm{N}}$ and $q_{\mathrm{95}}$: as $\beta_{\mathrm{N}}$ increases and $q_{\mathrm{95}}$ decreases, the fishbone strength increases. Measured fishbone-relevant signals are decomposed into amplitude envelope and phase components in the temporal domain, which are analyzed separately. In terms of the amplitude envelope component, the edge electron temperature fluctuation $\tilde T_\mathrm{e}^{\mathrm{Edge}}$ becomes more correlated with the poloidal magnetic fluctuation $\dot{B}_\mathrm{\theta}$ compared to the core electron temperature fluctuation $\tilde T_\mathrm{e}^{\mathrm{Core}}$ as fishbone strength increases. In terms of the phase component, the phase of $\tilde T_\mathrm{e}^{\mathrm{Edge}}$ precedes the phase of $\tilde T_\mathrm{e}^{\mathrm{Core}}$ except in the case of very weak fishbones where the phase relations are inconclusive due to weak fishbone activity at the edge plasma, which is comparable to background fluctuations. The investigation suggests the possibility that the edge activity is not a mere side effect of the core activity, but could play an active role.

physics.plasm-ph↗

arXiv2025

Ovonic switches enable energy-efficient dendrite-like computing

Over the last decade, dendrites within individual biological neurons, which were previously thought to generally perform information pooling and networking, have now been shown to express complex temporal dynamics, Boolean-like logic, arithmetic, signal discrimination, and edge detection for image and sound recognition. Mimicking this rich functional density could offer a powerful primitive for neuromorphic computing, which has sought to replace the aging digital computing paradigms using biological inspirations. Here, using electrically driven Ovonic threshold switching in Sb-Te-doped GeSe, we demonstrate a single two-terminal component capable of self-sustained dynamics and universal Boolean logic, in addition to XOR operations (which is traditionally thought to require a network of active components). We then employ logic-driven dynamics in a single component to detect and estimate the gradients of edges in images, a task that otherwise requires elaborate circuits. A network of Ovonic switches exhibits properties of a half adder and a full adder, in addition to discriminative logic accommodating inhibitory and excitatory signals. We show that this computational primitive is not only seemingly simpler, but also offers many orders of magnitude improved energy efficiency compared to prevailing digital solutions. As such, this work paves the path for potentially emulating dendrites for efficient post-digital neuromorphic computing.

cs.ET↗

arXiv2024

A More Accurate Approximation of Activation Function with Few Spikes Neurons

Recent deep neural networks (DNNs), such as diffusion models [1], have faced high computational demands. Thus, spiking neural networks (SNNs) have attracted lots of attention as energy-efficient neural networks. However, conventional spiking neurons, such as leaky integrate-and-fire neurons, cannot accurately represent complex non-linear activation functions, such as Swish [2]. To approximate activation functions with spiking neurons, few spikes (FS) neurons were proposed [3], but the approximation performance was limited due to the lack of training methods considering the neurons. Thus, we propose tendency-based parameter initialization (TBPI) to enhance the approximation of activation function with FS neurons, exploiting temporal dependencies initializing the training parameters.

cs.NE↗

arXiv2024

Atomic-scale tracking of topological defect motion and incommensurate charge order melting

Charge order pervades the phase diagrams of quantum materials where it competes with superconducting and magnetic phases, hosts electronic phase transitions and topological defects, and couples to the lattice generating intricate structural distortions. Incommensurate charge order is readily stabilized in manganese oxides where it is associated with anomalous electronic and magnetic properties, but its nanoscale structural inhomogeneity complicates precise characterization and understanding of its relationship with competing phases. Leveraging atomic-resolution variable temperature cryogenic scanning transmission electron microscopy, we characterize the thermal evolution of charge order as it transforms from its ground state in a model manganite system. We find that mobile networks of discommensurations and dislocations generate phase inhomogeneity and induce global incommensurability in an otherwise lattice-locked modulation. Driving the order to melt at high temperatures, the discommensuration density grows and regions of order locally decouple from the lattice periodicity.

cond-mat.str-el↗

arXiv2024

Magnetochiral Tunneling in Paramagnetic Co$_{1/3}$NbS$_2$

Electric currents have the intriguing ability to induce magnetization in nonmagnetic crystals with sufficiently low crystallographic symmetry. Some associated phenomena include the non-linear anomalous Hall effect in polar crystals and the nonreciprocal directional dichroism in chiral crystals when magnetic fields are applied. In this work, we demonstrate that the same underlying physics is also manifested in the electronic tunneling process between the surface of a nonmagnetic chiral material and a magnetized scanning probe. In the paramagnetic but chiral metallic compound Co$_{1/3}$NbS$_2$, the magnetization induced by the tunneling current is shown to become detectable by its coupling to the magnetization of the tip itself. This results in a contrast across different chiral domains, achieving atomic-scale spatial resolution of structural chirality. To support the proposed mechanism, we used first-principles theory to compute the chirality-dependent current-induced magnetization and Berry curvature in the bulk of the material. Our demonstration of this magnetochiral tunneling effect opens up a new avenue for investigating atomic-scale variations in the local crystallographic symmetry and electronic structure across the structural domain boundaries of low-symmetry nonmagnetic crystals.

cond-mat.mtrl-sci↗

arXiv2023

Gradient Scaling on Deep Spiking Neural Networks with Spike-Dependent Local Information

Deep spiking neural networks (SNNs) are promising neural networks for their model capacity from deep neural network architecture and energy efficiency from SNNs' operations. To train deep SNNs, recently, spatio-temporal backpropagation (STBP) with surrogate gradient was proposed. Although deep SNNs have been successfully trained with STBP, they cannot fully utilize spike information. In this work, we proposed gradient scaling with local spike information, which is the relation between pre- and post-synaptic spikes. Considering the causality between spikes, we could enhance the training performance of deep SNNs. According to our experiments, we could achieve higher accuracy with lower spikes by adopting the gradient scaling on image classification tasks, such as CIFAR10 and CIFAR100.

cs.NE↗

arXiv2023

Noncollinear magnetic order, in-plane anisotropy, and magnetoelectric coupling in a pyroelectric honeycomb antiferromagnet Ni$_{2}$Mo$_{3}$O$_{8}$

Ni$_{2}$Mo$_{3}$O$_{8}$ is a pyroelectric honeycomb antiferromagnet exhibiting peculiar changes of its electric polarization at magnetic transitions. Ni$_{2}$Mo$_{3}$O$_{8}$ stands out from the isostructural magnetic compounds, showing an anomalously low magnetic transition temperature and unique magnetic anisotropy. We determine the magnetic structure of Ni$_{2}$Mo$_{3}$O$_{8}$ utilizing high-resolution powder and single-crystal neutron diffraction. A noncollinear stripy antiferromagnetic order is found in the honeycomb planes. The magnetic space group is \textit{P$_C$na}2$_1$. The in-plane magnetic connection is of the stripy type both for the $ab$-plane and the $c$-axis spin components. This is a simpler connection than the one proposed previously. The ferromagnetic interlayer order of the $c$-axis spin components in our model is also distinct. The magnetic anisotropy of Ni$_{2}$Mo$_{3}$O$_{8}$ is characterized by orientation-dependent magnetic susceptibility measurements on a single crystal, consistent with neutron diffraction analysis. The local magnetoelectric tensor analysis using our magnetic models provides new insights into its magnetoelectric coupling and polarization. Thus, our results deliver essential information for understanding both the unusual magnetoelectric properties of Ni$_{2}$Mo$_{3}$O$_{8}$ and the prospects for observation of exotic nonreciprocal, Hall, and magnonic effects characteristic to this compound family.

cond-mat.str-el↗

arXiv2022

Mesoscopic transport in KSTAR plasmas: avalanches and the $E \times B$ staircase

The self-organization is one of the most interesting phenomena in the non-equilibrium complex system, generating ordered structures of different sizes and durations. In tokamak plasmas, various self-organized phenomena have been reported, and two of them, coexisting in the near-marginal (interaction dominant) regime, are avalanches and the $E \times B$ staircase. Avalanches mean the ballistic flux propagation event through successive interactions as it propagates, and the $E \times B$ staircase means a globally ordered pattern of self-organized zonal flow layers. Various models have been suggested to understand their characteristics and relation, but experimental researches have been mostly limited to the demonstration of their existence. Here we report detailed analyses of their dynamics and statistics and explain their relation. Avalanches influence the formation and the width distribution of the $E \times B$ staircase, while the $E \times B$ staircase confines avalanches within its mesoscopic width until dissipated or penetrated. Our perspective to consider them the self-organization phenomena enhances our fundamental understanding of them as well as links our findings with the self-organization of mesoscopic structures in various complex systems.

physics.plasm-ph↗

arXiv2022

Magnetic phase diagram of a 2-dimensional triangular lattice antiferromagnet Na$_2$BaMn(PO$_4$)$_2$

We report the magnetic phase transitions of a spin-5/2, 2-dimensional triangular lattice antiferromagnet (AFM) Na$_2$BaMn(PO$_4$)$_2$. From specific heat measurements, we observe two magnetic transitions at temperatures 1.15 K and 1.30 K at zero magnetic field. Detailed AC magnetic susceptibility measurements reveal multiple phases including the $\uparrow$$\uparrow$$\downarrow$ (up-up-down)-phase between 1.9 T and 2.9 T at 47 mK when magnetic field is applied along the $c$ axis, implying that Na$_2$BaMn(PO$_4$)$_2$ is a classical 2$d$ TL Heisenberg AFM with easy-axis anisotropy. However, it deviates from an ideal model as evidenced by a hump region with hysteresis between the $\uparrow$$\uparrow$$\downarrow$ and $V$-phases and weak phase transitions. Our work provides another experimental example to study frustrated magnetism in 2$d$ TL AFM which also serves as a reference to understand the possible quantum spin liquid behavior and anomalous phase diagrams observed in sibling systems Na$_2$Ba$M$(PO$_4$)$_2$ ($M$ = Co, Ni).

cond-mat.str-el↗

arXiv2021

Selective observation of surface and bulk bands in polar WTe2 by laser-based spin- and angle-resolved photoemission spectroscopy

The electronic state of WTe2, a candidate of type-II Weyl semimetal, is investigated by using laser-based spin- and angle-resolved photoemission spectroscopy (SARPES). We prepare the pair of WTe2 samples, one with (001) surface and the other with (00-1) surface, by "sandwich method", and measure the band structures of each surface separately. The Fermi arcs are observed on both surfaces. We identify that the Fermi arcs on the two surfaces are both originating from surface states. We further find a surface resonance band, which connects with the Fermi-arc band, forming a Dirac-cone-like band dispersion. Our results indicate that the bulk electron and hole bands are much closer in momentum space than band calculations.

cond-mat.mtrl-sci↗

arXiv2021

Crystal-field excitations and vibronic modes in triangular-lattice spin-liquid candidate TbInO$_3$

We study the ground state properties, the electronic excitations and lattice dynamics in spin-liquid candidate TbInO$_3$. By employing polarization resolved Raman spectroscopy we define the inter- and intra-multiplet excitations, and establish the low-energy crystal-field (CF) level scheme. In particular, we demonstrate that the ground state of the Tb$^{3+}$ ions is a non-Kramers doublet, and relate the enhanced linewidth of the CF modes to the magnetic fluctuations near the spin-liquid ground state. We identify all 38 allowed Raman-active phonon modes at low temperature. Moreover, we observe hybrid vibronic excitations involving coupled CF and low-lying phonon modes, suggesting strong spin-lattice dynamics. We develop a model for vibronic states and obtain the parameters of the bare responses and coupling strength. We further demonstrate that the obtained CF level scheme is consistent with specific heat data.

cond-mat.str-el↗

arXiv2021

Large Orbital Magnetic Moment and Strong Perpendicular Magnetic Anisotropy in Heavily Intercalated Fe$_{x}$TiS$_2$

Titanium disulfide TiS$_2$, which is a member of the layered transition-metal dichalcogenides with the 1T-CdI$_2$-type crystal structure, is known to exhibit a wide variety of magnetism through intercalating various kinds of transition-metal atoms of different concentrations. Among them, Fe-intercalated titanium disulfide Fe$_x$TiS$_2$ is known to be ferromagnetic with strong perpendicular magnetic anisotropy (PMA) and large coercive fields ($H_\text{c}$). In order to study the microscopic origin of the magnetism of this compound, we have performed X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements on single crystals of heavily intercalated Fe$_x$TiS$_2$ ($x\sim0.5$). The grown single crystals showed a strong PMA with a large $H_\text{c}$ of $\mu_0H_\text{c} \simeq 1.0\ \text{T}$. XAS and XMCD spectra showed that Fe is fully in the valence states of 2+ and that Ti is in an itinerant electronic state, indicating electron transfer from the intercalated Fe atoms to the host TiS$_2$ bands. The Fe$^{2+}$ ions were shown to have a large orbital magnetic moment of $\simeq 0.59\ \mu_\text{B}\text{/Fe}$, to which, combined with the spin-orbit interaction and the trigonal crystal field, we attribute the strong magnetic anisotropy of Fe$_x$TiS$_2$.

cond-mat.mtrl-sci↗

arXiv2021

Field-tunable toroidal moment in a chiral-lattice magnet

A toroidal dipole moment appears independent of the electric and magnetic dipole moment in the multipole expansion of electrodynamics. It arises naturally from vortex-like arrangements of spins. Observing and controlling spontaneous long-range orders of toroidal moments are highly promising for spintronics but remain challenging. Here we demonstrate that a vortex-like spin configuration with a staggered arrangement of toroidal moments, a ferritoroidal state, is realized in a chiral triangular-lattice magnet BaCoSiO4. Upon applying a magnetic field, we observe multi-stair toroidal transitions correlating directly with metamagnetic transitions. We establish a first-principles microscopic Hamiltonian that explains both the formation of toroidal states and the metamagnetic toroidal transition as a combined effect of the magnetic frustration and the Dzyaloshinskii-Moriya interactions allowed by the crystallographic chirality in BaCoSiO4.

cond-mat.str-el↗

arXiv2021

Noncollinear antiferromagnetic order in the buckled honeycomb lattice of magnetoelectric Co4Ta2O9 determined by single-crystal neutron diffraction

Co4Ta2O9 exhibits a three-dimensional magnetic lattice based on the buckled honeycomb motif. It shows unusual magnetoelectric effects, including the sign change and non-linearity. These effects cannot be understood without the detailed knowledge of the magnetic structure. Herein, we report neutron diffraction and direction-dependent magnetic susceptibility measurements on Co4Ta2O9 single crystals. Below 20.3 K, we find a long-range antiferromagnetic order in the alternating buckled and flat honeycomb layers of Co2+ ions stacked along the c axis. Within experimental accuracy, the magnetic moments lie in the ab plane. They form a canted antiferromagnetic structure with a tilt angle of ~ 14 degrees at 15 K in the buckled layers, while the magnetic moments in each flat layer are collinear. This is directly evidenced by a finite (0, 0, 3) magnetic Bragg peak intensity, which would be absent in the collinear magnetic order. The magnetic space group is C2'/c. It is different from the previously reported C2/c' group, also found in the isostructural Co4Nb2O9. The revised magnetic structure successfully explains the major features of the magnetoelectric tensor of Co4Ta2O9 within the framework of the spin-flop model.

cond-mat.str-el↗

arXiv2020

Frequency-tunable nano-oscillator based on Ovonic Threshold Switch (OTS)

Nano-oscillator devices are gaining more and more attention as a prerequisite for developing novel energy-efficient computing systems based on coupled oscillators. Here, we introduce a highly scalable, frequency-tunable nano-oscillator consisting of one Ovonic threshold switch (OTS) and a field-effect transistor (FET). It is presented that the proposed device shows an oscillating behavior with a natural frequency (f_{nat}) adjustable from 0.5 to 2 MHz depending on the gate voltage applied to the FET. In addition, under a small periodic input, it is observed that the oscillating frequency (f_{osc}) of the device is locked to the frequency (f_{in}) of the input when f_{in} ~ f_{nat}, demonstrating the so-called synchronization phenomenon. It also shows the phase lock of the combined oscillator network using circuit simulation, where the phase relation between the oscillators can be controlled by the coupling strength. These results imply that the proposed device is promising for applications in oscillator-based computing systems.

cond-mat.mtrl-sci↗