arXiv subjects

Jaewon Choi

Publications and source records attributed to Jaewon Choi.

At least 19 recordsLinked to original sources

arXiv2026

RIXS study of strain-tuned coupled valence and spin-state transition in epitaxial $(\mathrm{Pr}_{0.85}\mathrm{Y}_{0.15})_{0.7}\mathrm{Ca}_{0.3}\mathrm{CoO}_{3}$ films

Strain-tuned $(\mathrm{Pr}_{0.85}\mathrm{Y}_{0.15})_{0.7}\mathrm{Ca}_{0.3}\mathrm{CoO}_{3}$ thin films provide a model system for controlling coupled valence, spin-state, structural and metal-insulator transitions (VSSTs) in perovskite cobaltite materials. Here, we use Co $L_{2,3}$ and Pr $M_{4,5}$ X-ray Absorption Spectroscopy (XAS), along with Co $L_{3}$ edge Resonant Inelastic X-ray Scattering (RIXS), to probe the local electronic structure across the strain-controlled VSST in epitaxial (Pr,Y,Ca)CoO$_{3}$ thin films. In compressively strained films on YAlO$_{3}$ (YAO) substrates, cooling through the transition produces Pr$^{4+}$-like spectral weight, suppresses Co$^{4+}$-like character, and favors predominantly low-spin Co$^{3+}$. The changes at the Pr and Co edges are quantitatively consistent with a reduction of approximately 0.15 holes per Co in the Co--O network. By contrast, tensile-strained thin films on La$_{0.18}$Sr$_{0.82}$Al$_{0.59}$Ta$_{0.41}$O$_{3}$ (LSAT) substrates have nearly temperature-independent spectra. Multiplet calculations reproduce the principal XAS lineshapes and local $dd$ excitations at high energy loss. Additionally, low-energy RIXS intensity recovered above the transition is consistent with a combination of Co$^{4+}$-like local excitations and a broader particle--hole continuum associated with the metallic phase. These results provide element-resolved evidence for charge compensation between the Pr and Co--O sublattices and support cooperative coupling among charge-transfer, spin-state, and lattice structure at the strain-controlled VSST.

cond-mat.str-el↗

arXiv2026

Mean Accuracy Is the Wrong Estimand for a Release Decision

A release decision about a fine-tuned model is an operational commitment about the \emph{next} rerun of its training recipe. Yet standard benchmark reporting gives only the mean accuracy of past seeds. These target fundamentally different questions: release decisions depend on catastrophic tail risk, whereas standard evaluation reports only central tendency. Because fine-tuning induces bimodal mode collapse, high average accuracy frequently conceals severe failure rates. Auditing $2\,124$ fine-tuned models across eleven language models, we show that shipping models based on mean accuracy violates a $10\%$ deployment failure budget $48.0\%$ of the time. Common heuristics like $\bar b - 2σ$ prove equally dangerous: by underestimating variance on small samples, they become most permissive when evidence is weakest. We introduce \textbf{recipe cards}, an evaluation framework that directly certifies rerun tail failure rates using exact finite-sample binomial bounds. Recipe cards reduce held-out budget violations to $0.6\%$, enforce a mathematical sample floor ($N \ge 22$) that prevents certification on thin evidence, and explicitly refuse unverified claims under domain and task shifts.

cs.LG↗

arXiv2026

Unveiling the nature of collective charge excitations in a cuprate superconductor

Emergent symmetry breakings in condensed matter systems are often intimately linked to collective excitations. For example, the intertwined spin-charge stripe order in cuprate superconductors is associated with spin and charge excitations. While the collective behavior of spin excitations is well established, the nature of charge excitations remains to be understood. Here we present a high-resolution resonant inelastic x-ray scattering (RIXS) study of charge excitations in the stripe-ordered cuprate La$_{1.675}$Eu$_{0.2}$Sr$_{0.125}$CuO$_4$. The RIXS spectra consist of both charge and phonon excitations around the charge ordering wave vector. By modeling the momentum-dependent phonon intensity, the charge-excitation spectral weight is extracted over a wide energy range. As such, we reveal the highly dispersive nature of the charge excitations, with an energy scale comparable to the spin excitations. Since charge order and superconductivity in cuprates are possibly driven by the same electronic correlations, determining the interaction strength underlying charge order is essential to establishing a comprehensive microscopic model of high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2026

PSCT-Net: Geometry-Aware Pediatric Skull CT Reconstruction via Differentiable Back-Projection and Attention-Guided Refinement

Computed Tomography (CT) is essential for diagnosing pediatric craniofacial abnormalities, yet poses radiation risks to developing anatomies. Reconstructing 3D CT from sparse bi-planar X-rays offers a low-dose alternative but is severely ill-posed. Existing methods employ geometry-agnostic feature lifting, naively projecting 2D features into 3D without explicit spatial modeling, causing depth ambiguity and degraded osseous boundaries. We present PSCT-Net, a geometry-aware framework with differentiable back-projection. Differentiable back-projection establishes a spatially faithful volumetric prior, alleviating depth ambiguity. An Attention-Guided Projection (AGP-3D) module then learns non-linear voxel-wise correspondences between 2D regions and 3D locations. A Bidirectional Mamba (BiM-3D) module captures long-range volumetric dependencies with linear complexity. We further curate a private institutional pediatric skull CT cohort, PedSkull-CT, comprising normal and pathological cases for internal evaluation, addressing the gap in adult-centric, trunk-focused datasets. Project page and code are available at https://dydevelop.github.io/PSCT-Net/.

cs.CV↗

arXiv2026

Selective coupling of high-order phonons in La2-xSrxCuO4

In crystals and molecules with strong coupling between the charge carriers and atomic vibrations, high order harmonics of the vibrational excitations can be observed with intensity following the Franck-Condon envelope. Here, we uncover a new regime of electron-phonon coupling in La2-xSrxCuO4 using resonant inelastic X-ray scattering. In the undoped compound, we find sharp peaks at approximately 85$\pm$5 meV, 180$\pm$7 meV, and 330$\pm$5 meV with no observable dispersion, and an absence of appreciable intensity from other modes observed in Raman spectroscopy in this energy range. The 180 meV and 330 meV excitations have energies approximately consistent with two-phonon and four-phonon in-plane Cu-O bond stretching mode, suggesting that the resonantly excited valence electrons strongly couple to this mode. These excitations exhibit significant doping dependence, eventually becoming unresolvable at 10% hole doping, where the low energy charge excitations are dominated by dispersive plasmons. The observation of selective coupling to even-ordered phonons and stark contrast with the Raman spectra indicate anomalous electron-phonon coupling beyond the Franck-Condon picture. One intriguing possibility is the existence of locally paired quasiparticles in insulating cuprates.

cond-mat.str-el↗

arXiv2026

$g$-wave altermagnetic order parameter in hematite

Altermagnets combine the vanishing net magnetization of antiferromagnets with momentum-dependent spin splitting. Magnon band splitting provides a direct probe of altermagnetic order and may enable chirality-selective magnon transport, yet the momentum-space symmetry of this splitting has not been determined quantitatively. Here we use inelastic neutron scattering to map the momentum dependence of altermagnetic magnon splitting in hematite ($α$-Fe$_2$O$_3$). The splitting vanishes along nodal directions and reaches maxima off the nodes, revealing the $g$-wave symmetry of the altermagnetic order parameter. These results agree with linear spin-wave theory calculations based on the altermagnetic model, which further identify the nondegenerate branches as magnons of opposite chirality and trace the splitting to symmetry-inequivalent long-range exchange interactions. Our results provide the first quantitative determination of the momentum-space symmetry of altermagnetic chiral magnons. These findings, together with hematite's high magnetic ordering temperature and low magnon damping, establish it as a promising platform for low-dissipation, symmetry-selective magnonic applications.

cond-mat.str-el↗

arXiv2026

Fund2Persona: A Framework for Building and Refining Financial Advisor Personas from Fund Disclosure Data

Demand for personalized financial advice is growing, yet current LLM-based advisors often fail to provide consistent and specialized guidance. Simple persona prompts rarely specify how a financial advisor should reason and often drift toward generic recommendations. We propose Fund2Persona, a framework that builds financial-advisor personas from real-world fund disclosures and refines them through an actor-scorer-patcher loop. We test whether the resulting personas can predict held-out portfolio changes and produce commentary consistent with fund managers' own explanations. They outperform generic baselines on both tasks. We further study two downstream diagnostics: market-scenario generation, where persona retrieval broadens plausible investment views, and multi-turn investor-advisor conversations, where matched personas give more specific and useful advice than a generic advisor. These results suggest that real fund data can bring manager-specific investment expertise to LLM advisors rather than merely changing an LLM's surface style.

cs.CL↗

arXiv2026

FocusGen: Expanding Visual Design Exploration with a Simulated Focus Group of Persona Agents

Creative professionals rarely design for themselves--they design for audiences whose preferences they must anticipate. Yet current text-to-image exploration tools derive diversity entirely from the designer's own input--their prompts, their chosen dimensions, their search queries--confining exploration to what the designer already knows to look for. We present FocusGen, an interactive system that introduces external perspectives into visual design exploration through a "virtual focus group" of simulated persona agents. In contrast to prior persona systems in which multiple agents converge as critics on a single evolving artifact, FocusGen uses personas as parallel generators: each agent--constructed from demographic data, a procedurally generated backstory, and aesthetic preferences elicited through interviews--independently drives an iterative generation loop that produces its own visual concept, transforming one design brief into a spectrum of audience-conditioned directions. With real human participants, we confirm that the iterative refinement loop produces outputs people prefer over zero-shot generation. With synthetic agents at scale, we show that persona conditioning yields higher visual diversity than a generic-assistant baseline--measured by CLIP distance and corroborated by human perceptual judgments--and that open-ended preference interviews yield more diverse outputs than structured ones for both human and synthetic cohorts, while also revealing that agent cohorts recover only part of the diversity of comparable human cohorts. A qualitative study with 16 creative professionals suggests FocusGen helps designers discover unanticipated directions, overcome fixation, and probe audience contexts--while surfacing stereotyping risks that we analyze. We position FocusGen as a divergence scaffold for early-stage ideation rather than a substitute for audience research.

cs.HC↗

arXiv2026

Narrow-Shell Stochasticity in Source-Sink Models of the Low Earth Orbit Environment

Deterministic source-sink models are widely used to assess the long-term evolution, capacity, and sustainability of the low Earth orbit (LEO) environment. These models propagate shell-averaged populations through ordinary differential equations (ODEs), relying on individual collision, disposal, and decay events to average out within sufficiently large altitude shells. As constellation traffic is increasingly organized into kilometer- and sub-kilometer-scale shells, this averaging assumption becomes strained. We formulate the multi-shell, multi-species LEO environment as a Markov jump process and, from this formulation, recover the conventional source-sink ODE as a large-volume limit and derive a stochastic differential equation (SDE) approximation whose fluctuations scale as V^{-1/2} in the per-shell volume V. The diffusion approximation is validated against an exact discrete-event simulation of the underlying jump process. Sweeping V at fixed spatial density over the 450-800 km band, we find that the two descriptions agree at shell volumes comparable to those used in established source-sink models but diverge as shells narrow. At the finest shell volume considered, the mean debris population reaches roughly 4.5 times the ODE prediction, with several realizations undergoing runaway growth absent from the ODE trajectory. This departure is driven by nonlinear collision terms, through which population variance and covariance raise expected collision rates, generating further debris and reinforcing the collision-debris feedback. Narrow-shell stochasticity thus both widens the outcome distribution and shifts the expected trajectory. Because the stochastic model shares its parameterization with the deterministic one, it provides a scale-consistent extension of existing source-sink models for evaluating shell configuration, collision risk, and long-term LEO sustainability.

astro-ph.EP↗

arXiv2026

Your AI, On a Dial: Controlling Investment Bias in LLMs with a Single Neuron

Large language models (LLMs) are increasingly used in investment decision-making, yet prior work shows that they exhibit systematic, model-specific investment preferences. We study whether a model's overall investment stance can be calibrated to a specified direction and strength. We introduce an investment-bias dial, an inference-time intervention on a single neuron that continuously adjusts a model-level decision prior---its overall tendency toward buying or selling---without targeting specific firms or investment attributes. Using matched positive and negative evidence, we evaluate five open-weight LLMs and find that the dial produces monotonic changes in investment stance without modifying prompts or model parameters. At the response level, the dial shifts both investment decisions and the evidential emphasis of generated rationales under identical inputs. In an agentic retrieval setting, the dial also changes what information the model searches for, which evidence it selects, and which evidence is reflected in its final analysis. In a long-context evaluation, the dial maintains stable stance control as context length increases, whereas a matched system-prompt instruction progressively attenuates. We further show that changes in the dial propagate to security rankings and downstream portfolio composition in an exploratory backtest. Overall, our results show that an LLM's aggregate investment stance can be calibrated toward a specified target at inference time.

cs.AI↗

arXiv2026

Distilling Temporal Coherence into 2D Networks for Transrectal Ultrasound Prostate Video Segmentation

Real-time video segmentation of the prostate in Transrectal Ultrasound (TRUS) is essential for image-guided interventions. While conventional 2D methods suffer from inter-frame inconsistencies by disregarding temporal context, 3D architectures incur prohibitive latency. To resolve this dilemma, we present a Temporally Consistent Learning Framework that distills temporal coherence into a 2D network during training, preserving single-frame inference efficiency. Our design is driven by a key clinical observation: the prostate exhibits geometric stability, whereas the surrounding acoustic environment fluctuates due to physiological motion and transducer pressure. Because conventional temporal constraints propagate erroneous gradients from these unstable regions, we introduce a Confidence-Weighted Temporal Consistency objective derived from optical flow warping residuals, selectively attenuating contributions from unreliable regions. Complementing this pixel-wise constraint, a Dual-scale Prototype Alignment Module enforces semantic coherence through contrastive optimization of local boundary and global semantic features. Furthermore, to eliminate the need for dense per-frame video annotations, we employ geometric equivariance-based pseudo-labeling with knowledge distillation from a pretrained teacher. Extensive experiments on SUN-SEG and our newly introduced TRUS-V benchmark (2,679 frames) demonstrate state-of-the-art accuracy and temporal consistency at real-time speed. Code and dataset are available at https://github.com/DYDevelop/DTC-TRUS.

cs.CV↗

arXiv2026

$d$-wave altermagnetism revealed by resonant inelastic X-ray scattering

Altermagnetism defines a third fundamental class of collinear magnetic order, featuring compensated magnetic moments with antiparallel spin alignment, yet lifted Kramers degeneracy without the need for relativistic spin-orbit coupling. Its ability to host spin-polarized electronic bands and unconventional chiral magnons makes it a promising platform for functional materials. However, experimental verification has proven challenging; while circular dichroism in resonant inelastic X-ray scattering (RIXS) has been suggested as a signature of chiral magnons, it remains controversial whether this effect is an intrinsic property of altermagnetism or an artifact of experimental geometry. In this work, we resolve this debate and provide unambiguous experimental evidence of $d$-wave altermagnetism in the strongly correlated Lieb-lattice magnet La$_2$O$_3$Mn$_2$Se$_2$. The RIXS spectra exhibit a $d$-wave-symmetry circular dichroism in the magnetic excitations that vanishes in the paramagnetic phase. Through RIXS-operator symmetry analysis and exact-diagonalization calculations, we prove that the observed dichroism is a direct consequence of altermagnetic symmetry constraints, independent of magnon branch splitting. Our results provide definitive evidence for the experimental realization of $d$-wave altermagnetism in La$_2$O$_3$Mn$_2$Se$_2$ and establish circularly polarized RIXS as a highly symmetry-sensitive spectroscopic framework for detecting magnetic phases that evade conventional probes.

cond-mat.str-el↗

arXiv2026

Direct High-Magnetic-Field Coupling to Stripe Order in a Cuprate Superconductor

Superconductivity in cuprates emerges out of a complex normal state that hosts density waves, pseudogap physics, and strange metal properties. Here, we access this normal state by synchronizing free-electron laser x-rays with high-magnetic-field pulses up to 44 T. We observe a linear increase in charge order amplitude and correlation length that persists far above the vortex melting transition. This behavior is incompatible with standard phase competition between charge order and superconductivity. By means of conventional hard x-ray diffraction and magnetostriction, we show that applied fields also enhance monoclinic lattice distortions. However, this magnetoelastic response is weaker and an epiphenomenon of the stripe order enhancement. Combined with recent observations of field-linear spin freezing, our results point to a direct coupling between magnetic field and the spin component of stripe order in the high-field normal state -- a mechanism independent of superconductivity suppression that has so far remained hidden from scattering probes.

cond-mat.str-el↗

arXiv2026

Strongly-coupled hybrid lattice-plasmons in layered cuprates

Metallic systems with delocalized valence electrons host collective charge density oscillations known as plasmons. On the other hand, conventional insulators do not have free electrons and the low energy charge degrees of freedom are pinned to the ions. The fate of the collective charge excitations in the intermediate regime is an outstanding question. This problem is especially important for strongly correlated systems such as the layered cuprates, where unconventional superconductivity and other emergent phenomena arise from valence electrons on the border between Mott localization and itinerancy. Using resonant inelastic X-ray scattering, we track this evolution in the prototypical electron-doped cuprate Nd2-xCexCuO4. We find a continuous transformation of the low-energy charge response: from an acoustic plasmon in the metallic regime, to a gapped hybrid mode at intermediate doping, and finally to a nearly dispersionless 139 meV excitation at half filling. Remarkably, the 139 meV excitation has approximately twice the energy of the oxygen breathing phonon responsible for the dispersion kink observed in angle-resolved photoemission spectroscopy, and is consistent with a putative 2-phonon excitation observed in Raman spectroscopy. These results establish a unified picture of collective charge excitations across the phase diagram of electron-doped cuprates, showing that such modes persist across the Mott transition via strong coupling to lattice degrees of freedom and revealing a missing link in the charge dynamics of carrier doped Mott insulators.

cond-mat.str-el↗

arXiv2026

$3d_{z^2}$ orbital delocalization and magnetic collapse in superconducting (La,Pr)$_3$Ni$_2$O$_{7-δ}$ films

The recent discovery of Ruddlesden--Popper (RP) nickelate thin-film superconductors has opened a new frontier in unconventional superconductivity. Its realization requires both compressive epitaxial strain and highly oxidative growth conditions, yet the microscopic pathway from the parent phase to the superconducting phase remains elusive. Here, X-ray absorption spectra and resonant inelastic X-ray scattering are employed to track this evolution by independently tuning strain and oxygen content in (La,Pr)$_3$Ni$_2$O$_{7-δ}$ thin films. We uncover a remarkable two-step narrative. First, signatures of delocalization emerge in the same way upon two independent tunings: Spectral weight transfers from a ''Upper Hubbard''-like peak to the hole-like peak associated with O $2p_z$ state, and in parallel, the initially localized Ni $3d_{z^2}$ orbital becomes more itinerant followed by the broadening and weakening of $dd$ orbital excitations. Second, as itinerancy increases, long-range spin-density-wave (SDW) order is suppressed in both intensity and correlation length, indicating direct competition with superconductivity. Yet, short-range magnons persist: they become damped but their bandwidth stays unchanged. Our results paint a coherent picture that both strain and oxygenation drive the RP bilayer nickelates towards the superconducting instability, where the O $2p_z$ and Ni $3d_{z^2}$ orbitals become delocalized. Concomitantly, the long-range magnetic order loses coherence and gets suppressed. These findings establish an orbital-selective route to RP nickelate superconductivity, in which the delocalization of the $2p_z$ and $3d_{z^2}$ orbitals and the robust short-range magnons upon the melting of SDW order are prerequisites, providing strong constraints for theory and the roadmap for designing nickelate superconductors.

cond-mat.supr-con↗

arXiv2025

Attention Factors for Statistical Arbitrage

Statistical arbitrage exploits temporal price differences between similar assets. We develop a framework to jointly identify similar assets through factors, identify mispricing and form a trading policy that maximizes risk-adjusted performance after trading costs. Our Attention Factors are conditional latent factors that are the most useful for arbitrage trading. They are learned from firm characteristic embeddings that allow for complex interactions. We identify time-series signals from the residual portfolios of our factors with a general sequence model. Estimating factors and the arbitrage trading strategy jointly is crucial to maximize profitability after trading costs. In a comprehensive empirical study we show that our Attention Factor model achieves an out-of-sample Sharpe ratio above 4 on the largest U.S. equities over a 24-year period. Our one-step solution yields an unprecedented Sharpe ratio of 2.3 net of transaction costs. We show that weak factors are important for arbitrage trading.

cs.LG↗

arXiv2025

The doping evolution of the charge density wave and charge density fluctuations in La$_{2-x}$Sr$_x$CuO$_4$

Cuprate superconductors show various collective charge correlations that are intimately connected with their electronic properties. In particular, charge order in the form of an incommensurate charge density wave (CDW) order with an in-plane wavevector $δ_{\text{CDW}} \approx $ 0.23--0.35~r.l.u. appears to be universally present. In addition to CDW, dynamic charge density fluctuations (CDF) are also present with wavevectors comparable to $δ_{\text{CDW}}$. CDFs are present up to $\sim300\;$K and have relatively short correlation lengths of $ξ\sim 20$\;Å. Here we use Cu-$L_3$ and O-$K$ resonant inelastic X-ray scattering (RIXS) to study the doping dependence of CDW and CDFs in La$_{2-x}$Sr$_x$CuO$_4$. We fit our data with (quasi)elastic peaks resulting from the CDW and up to four inelastic modes associated with oxygen phonons that can be strongly coupled to the CDFs. Our analysis allows us to separate the charge correlations into three components: the CDW with wavevector $δ_{4a-\text{CDW}} \approx 0.24$ and two CDF components with $δ_{4a-\text{CDF}} \approx 0.24$ and $δ_{3a-\text{CDF}} \approx 0.30$. We find that for $T \approx T_c$ the CDW coexists with the CDFs for dopings near $x=p \sim 1/8$. The $4a$-CDW disappears beyond $x=0.16$ and the $4a$-CDF beyond $x=0.19$, leaving only a weak $3a$-CDF at the highest doping studied, $x=0.22$. Our data suggest that low-energy charge fluctuations exist up to doping $x=0.19=p^{\star}$, where the pseudogap disappears, however, we find no evidence that they are associated with a quantum critical point.

cond-mat.supr-con↗

arXiv2025

Persistent paramagnons in high-temperature infinite-layer nickelate superconductors

The recent discovery of high-temperature superconductivity in hole-doped SmNiO$_2$, exhibiting the record-high transition temperature $T_c$ among infinite-layer (IL) nickelates, has opened a new avenue for exploring design principles of superconductivity. Experimentally determining the electronic structure and magnetic interactions in this new system is crucial to elucidating the mechanism behind the enhanced superconductivity. Here, we report a Ni $L$-edge resonant inelastic x-ray scattering (RIXS) study of superconducting Sm-based IL nickelate thin films Sm$_{1-x-y-z}$Eu$_x$Ca$_y$Sr$_z$NiO$_2$ (SECS). Dispersive paramagnonic excitations are observed in both optimally and overdoped SECS samples, supporting a spin-fluctuation-mediated pairing scenario. However, despite the two-fold enhancement of $T_c$ in the Sm-based nickelates compared to their Pr-based counterparts, the effective exchange coupling strength is reduced by approximately $20\%$. This behavior contrasts with hole-doped cuprates, where magnetic interactions correlate positively with $T_c$, highlighting essential differences in their superconducting mechanisms.

cond-mat.supr-con↗