arXiv subjects

Kyoung-Min Kim

Publications and source records attributed to Kyoung-Min Kim.

At least 19 recordsLinked to original sources

arXiv2026

Giant orbital Hall effect from cubic Dresselhaus orbital coupling

The orbital Berry curvature (OBC) governs the intrinsic orbital Hall conductivity (OHC), a central quantity in orbitronics. Previous approaches for enhancing the OHC have primarily relied on a linearin-momentum, Rashba-type orbital coupling. Here, we show that cubic Dresselhaus orbital coupling offers a new route to enhancing the OHC. Using an effective two-orbital band model, we show that the cubic coupling generates momentum-space hot spots, absent in the purely linear case, at which the OBC is strongly enhanced. This local enhancement, together with the multiplicity of the hot spots, boosts the OHC by more than an order of magnitude relative to the linear-coupling value. We further find that the OHC diverges inversely with the level splitting in the small-splitting limit, with a divergence coefficient universally seventeen times larger than that of the linear case. These results establish cubic Dresselhaus coupling as a route to giant orbital Hall responses, opening new avenues for orbitronic device applications.

cond-mat.mes-hall↗

arXiv2026

Emergence of moir\'e magnetic chaos in twisted bilayer CrI3

The study of magnetic chaos has traditionally focused on macroscopic variables under external driving. Here we demonstrate a new type of magnetic chaos, termed moir\'e magnetic chaos, associated with mesoscopic magnetic domain variables in twisted bilayer CrI3 without external driving. The domains are stabilized by a characteristic interlayer exchange frustration, which supplies the multiple dynamical degrees of freedom required for autonomous chaos. Through micromagnetic simulations, we show that relaxation toward moir\'e magnetic textures is extremely sensitive to minute local perturbations of the initial state, characterized by substantial finite-time Lyapunov exponents and a final-state sensitivity that persists over five decades of perturbation amplitude. Statistical analysis further reveals that the resulting domain configurations are stochastic and pairwise uncorrelated. Our results identify a form of microscopic, undriven chaos in twisted magnets that extends nonlinear magnetism beyond the conventional driven regime.

cond-mat.mes-hall↗

arXiv2026

Non-Perturbative Renormalization Group for Ising-Nematic Criticality: A Closed-Form Nonlocal Ansatz

The two-dimensional metallic quantum critical problem is a long-standing puzzle that is widely believed to hold the key to resolving ubiquitous non-Fermi liquid behavior in strongly correlated electronic systems. In this study, we present a non-perturbative renormalization group (RG) analysis of the metallic Ising-nematic quantum critical point in two dimensions, formulated directly around an intrinsically nonlocal infrared (IR) boson propagator. Rather than treating the anomalous dynamical critical exponent $a$ as a fixed phenomenological parameter, we regard it as an intrinsic component of the fixed-point data to be determined from the internal consistency of the low-energy patch field theory under highly anisotropic scaling dimensions ($[k_0]=a+1$, $[k_x]=2$, $[k_y]=1$). While the leading two-loop diagrammatics vanish identically due to kinematic pole configurations, our three-loop evaluation reveals a profound structural asymmetry between the sectors: the fermion self-energy and Yukawa vertex receive non-vanishing logarithmic corrections, whereas the corresponding bosonic counter-term remains strictly zero. Consequently, we find that no self-consistent, intersecting fixed-point solution for the exponent $a$ exists within the three-loop truncation, failing to reproduce the physical value of $a \approx 1.85$ observed in quantum Monte Carlo simulations. We conjecture that the cross-linked topology of the four-loop boson self-energy diagrams is exactly marginal and yields the minimal, mandatory bosonic counter-term required to restore multi-sector self-consistency. Our framework establishes a rigid multi-loop matching scheme necessary to uniquely pin down the critical exponent, and uncovers a stable phase space for field anomalous dimensions.

cond-mat.str-el↗

arXiv2026

Topological Ising superconductivity in two-dimensional p-wave magnet

Fermi-surface spin splitting generated by non-relativistic exchange fields provides a new route to topological superconductivity without relying on strong spin-orbit coupling. Here, we study superconducting instabilities of a square-lattice $p$-wave magnet with onsite and nearest-neighbour attractive interactions. The odd-parity exchange field removes inversion symmetry in the spin-split electronic structure, mixing singlet and triplet order parameters within a single symmetry channel. The leading instability is a mixed-parity A$_1$ Ising state, in which singlet components coexist with the $p_x$-wave triplet component, whose $d$-vector is locked along the exchange-field axis. As the nearest-neighbour attraction grows, this Ising state undergoes a transition into a nodal topological superconducting phase with Majorana edge modes protected by momentum-resolved winding numbers. These modes extend over finite momentum intervals bounded by the surface projections of bulk point nodes. We further show that a Zeeman field perpendicular to the exchange field can induce a $\mathbb{Z}_2$ topological superconducting phase. Our results identify $p$-wave magnets as a versatile testbed for topological superconductivity driven by non-relativistic spin splitting.

cond-mat.str-el↗

arXiv2026

Mobility-edge-embedded Hofstadter butterfly from a tilt-induced quasiperiodic potential

The Hofstadter butterfly (HB) and mobility edges (MEs) are hallmark phenomena of quasiperiodic systems, yet their interplay remains elusive. Here, we demonstrate their coexistence within a tilt-induced quasiperiodic potential on a square lattice, giving rise to a ``mobility-edge-embedded Hofstadter butterfly'' (MEE-HB). This potential is generated by aligning a periodic potential at an angle relative to the lattice axes -- a configuration readily accessible in optical lattice experiments. Using a tight-binding model, we show that the MEE-HB manifests as a fractal energy splitting pattern hosting MEs that separate extended and localized states. Our Harper-like equation shows that the fractal pattern originates from one-dimensional quasiperiodic potentials, while MEs stem from effective long-range hopping. Notably, the MEE-HB exhibits a fractal dimension of $0.8$--$1.0$, significantly exceeding the $0.4$--$0.6$ range of the standard butterfly, indicating a denser spectrum. Our findings establish tilt-induced potentials as a versatile platform for exploring the interplay between fractal structures and localization.

cond-mat.dis-nn↗

arXiv2026

Optimized control protocols for stable skyrmion creation using deep reinforcement learning

Generating stable magnetic skyrmions is essential for the practical application of skyrmion-based spintronic devices in thermally agitating environments. Here, we present a deep reinforcement learning (DRL) approach to identify advanced dynamic magnetic-field-temperature paths that create skyrmions with enhanced thermal stability. The trained DRL agent discovers an optimized field-temperature path that achieves a higher success rate for skyrmion formation in Fe3GeTe2 monolayers compared to previous fixed-temperature field sweeps. Additionally, the generated skyrmions exhibit longer lifetimes due to their isotropic shape and equilibrium size, both of which place them near a local energy minimum and thereby hinder annihilation. We demonstrate that these advancements stem from the targeted minimization of the dissipated work, which ensures that the driven skyrmion states remain close to their equilibrium distributions by upper-bounding the Kullback--Leibler divergence. Our findings suggest that a physics-informed DRL framework streamlines the identification of optimized protocols for skyrmion creation.

cond-mat.mes-hall↗

arXiv2026

Emergence of chiral $p$-wave and $d$-wave states in $g$-wave altermagnets

Altermagnets emerge as a novel platform for realizing unconventional superconductivity through their exotic momentum-dependent spin-splitting of electronic band structures. Recent experiments have uncovered a novel form of altermagnetism with distinctive $g$-wave symmetry in CrSb. However, the potential for unconventional superconductivity arising from $g$-wave altermagnetism in such systems remains largely unexplored. In this study, we discover the emergence of chiral superconducting states in three-dimensional $g$-wave altermagnetic metals. Through systematic self-consistent mean-field analysis on the extended attractive Hubbard model combined with $g$-wave altermagnetic exchange fields in a three-dimensional hexagonal lattice, as observed in CrSb, we find that the altermagnetic spin splitting of Fermi surfaces favors chiral $p$-wave states as the dominant pairing channel under strong altermagnetic fields and high electron densities, while chiral $d$-wave states become predominant under weak altermagnetic fields and intermediate electron densities. Conversely, at weak altermagnetic fields and typical electron densities, non-chiral $s$-, extended $s$-, or $f$-wave states become stabilized. We also showcase the possible experimental detection using the quasiparticle energy dispersions and the density of states to distinguish different pairing symmetries. These findings underscore the potential of $g$-wave altermagnets to host sought-after chiral and gapless superconductivity.

cond-mat.supr-con↗

arXiv2026

Structural constraints on mobility edges in one-dimensional quasiperiodic systems

Mobility edges commonly arise in one-dimensional quasiperiodic systems once exact self-duality is broken, yet their origin is typically understood only at the level of individual Hamiltonians. Here we show that mobility edge positions are not independent spectral features of individual Hamiltonians, but are structurally constrained across quasiperiodic Hamiltonians related by an isospectral duality. Using a bichromatic Aubry--Andr\'e model as a minimal setting, we demonstrate that this constraint is encoded in an exact identity for Lyapunov exponents derived from the Thouless formula. As a consequence, the mobility edge positions are restricted to a reduced set of energies. In the self-dual limit, these mobility edge positions coincide at a single localization--delocalization transition. This structural constraint enforces a linear critical scaling of the physical Lyapunov spectrum near the self-dual point. Numerical results confirm a critical exponent consistent with the standard Aubry--Andr\'e value of $\nu = 1$, while simultaneously revealing a novel, non-universal energy-dependent prefactor.

cond-mat.dis-nn↗

arXiv2025

Neural Scaling Laws for Deep Regression

Neural scaling laws--power-law relationships between generalization errors and characteristics of deep learning models--are vital tools for developing reliable models while managing limited resources. Although the success of large language models highlights the importance of these laws, their application to deep regression models remains largely unexplored. Here, we empirically investigate neural scaling laws in deep regression using a parameter estimation model for twisted van der Waals magnets. We observe power-law relationships between the loss and both training dataset size and model capacity across a wide range of values, employing various architectures--including fully connected networks, residual networks, and vision transformers. Furthermore, the scaling exponents governing these relationships range from 1 to 2, with specific values depending on the regressed parameters and model details. The consistent scaling behaviors and their large scaling exponents suggest that the performance of deep regression models can improve substantially with increasing data size.

cs.LG↗

arXiv2025

Emergence of Unpinned Dirac Points in Defected Photonic Crystals

Unpinned Dirac points (DPs) are nodal point degeneracies that occur at generic momentum points lacking high symmetry, often exhibiting characteristics typically forbidden by symmetry. While this phenomenon has been observed in solid-state materials, the formation of unpinned DPs in photonic crystals has not been extensively studied. This study presents a novel approach to achieving unpinned DPs in two-dimensional ``defected photonic crystals"-photonic superlattices created by modifying the size or refractive index of dielectric disks at specific sites. The defected photonic crystal hosts multiple unpinned DPs through accidental band crossings induced by rotational symmetry breaking across the nodal line degeneracies prevalent in the superlattice. Furthermore, the number of unpinned DPs, along with their positions and anisotropic dispersions, can be manipulated by adjusting the size or refractive index of the dielectric disks. This study pioneers the exploration of defected photonic crystals, proposing them as effective means of achieving unpinned DPs and facilitating the investigation of novel relativistic photonic wave phenomena with significant flexibility and control.

physics.optics↗

arXiv2025

Unconventional p-wave and finite-momentum superconductivity induced by altermagnetism through the formation of Bogoliubov Fermi surface

Altermagnet is an exotic class of magnetic materials wherein the Fermi surface exhibits a momentum-dependent spin-splitting while maintaining a net zero magnetization. Previous studies have shown that this distinctive spin-splitting can induce chiral p-wave superconductors or Fulde-Ferrell (FF) superconducting states carrying finite momentum. However, the underlying mechanisms of such unconventional superconductivities remain elusive. Here, we propose that the formation of the Bogoliubov Fermi surface (BFS) through the exchange field can play a significant role in such phenomena. Through a systematic self-consistent mean-field analysis on the extended attractive Hubbard model combined with the d-wave spin-splitting induced by the exchange field, as observed in RuO2, we demonstrate that the formation of the BFS suppresses conventional spin-singlet superconducting states with s-wave characteristics. In contrast, the chiral p-wave state maintains a fully gapped spectrum without the Fermi surface, thereby becoming the ground state in the strong field regime. In the intermediate regime, we find that the FF state becomes the predominant state through the optimization of available channels for Cooper pairing. Moreover, we illustrate how the prevalence of the chiral p-wave and FF states over the s-wave state changes under the variation of the field strength or chemical potential. Our findings provide valuable insights into potential pathways for realizing sought-after topological p-wave superconductivity and finite momentum pairing facilitated by altermagnetism.

cond-mat.supr-con↗

arXiv2024

Emergence of Meron Kekulé lattices in twisted Néel antiferromagnets

A Kekulé lattice is an exotic, distorted lattice structure exhibiting alternating bond lengths, distinguished from naturally formed atomic crystals. Despite its evident applicability, the formation of a Kekulé lattice from topological solitons in magnetic systems has remained elusive. Here, we propose twisted bilayer easy-plane Néel antiferromagnets as a promising platform for achieving a "Meron Kekulé lattice"--a distorted topological soliton lattice comprised of antiferromagnetic merons as its lattice elements. We demonstrate that the cores of these merons are stabilized into the Kekulé-O pattern with different intracell and intercell bond lengths across moiré supercells, thereby forming a Meron Kekulé lattice. Moreover, the two bond lengths of the Meron Kekulé lattice can be fine-tuned by adjusting the twist angle and specifics of the interlayer exchange coupling, suggesting extensive control over the meron lattice configuration in contrast to conventional magnetic systems. These discoveries pave the way for exploring topological solitons with distinctive Kekulé attributes.

cond-mat.str-el↗

arXiv2024

Phonon-mediated spin transport in quantum paraelectric metals

The concept of ferroelectricity is now often extended to include continuous inversion symmetry-breaking transitions in various metals and doped semiconductors. Paraelectric metals near ferroelectric quantum criticality, which we term `quantum paraelectric metals,' typically possess soft transverse optical phonons that have Rashba-type coupling to itinerant electrons in the presence of spin-orbit coupling. We find through the Kubo formula calculation that such Rashba electron-phonon coupling has a profound impact on electron spin transport. While the spin Hall effect arising from non-trivial electronic band structures has been studied extensively, we find here the presence of the Rashba electron-phonon coupling can give rise to spin current, including spin Hall current, in response to an inhomogeneous electric field even with a completely trivial band structure. Furthermore, this spin conductivity displays unconventional characteristics, such as quadrupolar symmetry associated with the wave vector of the electric field and a thermal activation behavior characterized by scaling laws dependent on the phonon frequency to temperature ratio. These findings shed light on exotic electronic transport phenomena originating from ferroelectric quantum criticality, highlighting the intricate interplay of charge and spin degrees of freedom.

cond-mat.str-el↗

arXiv2024

Deep learning methods for Hamiltonian parameter estimation and magnetic domain image generation in twisted van der Waals magnets

The application of twist engineering in van der Waals magnets has opened new frontiers in the field of two-dimensional magnetism, yielding distinctive magnetic domain structures. Despite the introduction of numerous theoretical methods, limitations persist in terms of accuracy or efficiency due to the complex nature of the magnetic Hamiltonians pertinent to these systems. In this study, we introduce a deep-learning approach to tackle these challenges. Utilizing customized, fully connected networks, we develop two deep-neural-network kernels that facilitate efficient and reliable analysis of twisted van der Waals magnets. Our regression model is adept at estimating the magnetic Hamiltonian parameters of twisted bilayer CrI3 from its magnetic domain images generated through atomistic spin simulations. The generative model excels in producing precise magnetic domain images from the provided magnetic parameters. The trained networks for these models undergo thorough validation, including statistical error analysis and assessment of robustness against noisy injections. These advancements not only extend the applicability of deep-learning methods to twisted van der Waals magnets but also streamline future investigations into these captivating yet poorly understood systems.

cond-mat.str-el↗

arXiv2024

Disordered non-Fermi liquid fixed point for two-dimensional metals at Ising-nematic quantum critical points

Understanding the influence of quenched random potential is crucial for comprehending the exotic electronic transport of non-Fermi liquid metals near metallic quantum critical points. In this study, we identify a stable fixed point governing the quantum critical behavior of two-dimensional non-Fermi liquid metals in the presence of a random potential disorder. By performing renormalization group analysis on a dimensional-regularized field theory for Ising-nematic quantum critical points, we systematically investigate the interplay between random potential disorder for electrons and Yukawa-type interactions between electrons and bosonic order-parameter fluctuations in a perturbative epsilon expansion. At the one-loop order, the effective field theory lacks stable fixed points, instead exhibiting a runaway flow toward infinite disorder strength. However, at the two-loop order, the effective field theory converges to a stable fixed point characterized by finite disorder strength, termed the "disordered non-Fermi liquid (DNFL) fixed point." Our investigation reveals that two-loop vertex corrections induced by Yukawa couplings are pivotal in the emergence of the DNFL fixed point, primarily through screening disorder scattering. Additionally, the DNFL fixed point is distinguished by a substantial anomalous scaling dimension of fermion fields, resulting in pseudogap-like behavior in the electron's density of states. These findings shed light on the quantum critical behavior of disordered non-Fermi liquid metals, emphasizing the indispensable role of higher-order loop corrections in such comprehension.

cond-mat.str-el↗

arXiv2024

Existence of a weak-disorder non-Fermi liquid fixed point in the hydrodynamic regime of two-dimensional nematic quantum criticality

Role of quenched randomness in metallic quantum criticality is one of the long standing problems in condensed matter physics. An aspect of the fundamental difficulties lies in the fact that such nonmagnetic disorders lead effective interactions between abundant soft modes near the Fermi surface to be drastically enhanced particularly in the diffusive regime, where the perturbative framework does not work. Here, we revisit the problem of dirty quantum critical metals in a different angle, focusing on the hydrodynamic regime instead of the diffusive regime near the non-Fermi liquid quantum critical point. More concretely, we study effects of mutual correlations between quantum critical nematic fluctuations and weak localization corrections, and show the existence of a weak-disorder non-Fermi liquid fixed point, based on the renormalization group (RG) analysis up to the two-loop order. The two-loop order RG analysis suggests that the absence of quantum coherence in two-particle composite excitations weakens the role of weak localization corrections and allows a weakly disordered non-Fermi liquid metallic state in the hydrodynamic regime of the nematic quantum critical point. Although this dirty non-Fermi liquid metallic state may not be the true infrared stable fixed point at zero temperature, expected to be characterized by the diffusive Ohmic regime, we argue that this weak-disorder non-Fermi liquid metallic fixed point would govern the region of an intermediate energy scale, described by effective hydrodynamics of nematic quantum criticality. In this respect we believe that this research would be an important step in understanding the $T-$linear electrical resistivity as a characteristic feature of non-Fermi liquids and the origin of unconventional superconductivity from effective hydrodynamics of quantum criticality.

cond-mat.str-el↗

arXiv2023

Emergence of stable meron quartets in twisted magnets

The investigation of twist engineering in easy-axis magnetic systems has revealed the remarkable potential for generating topological spin textures, such as magnetic skyrmions. Here, by implementing twist engineering in easy-plane magnets, we introduce a novel approach to achieve fractional topological spin textures such as merons. Through atomistic spin simulations on twisted bilayer magnets, we demonstrate the formation of a stable double meron pair in two magnetic layers, which we refer to as the "Meron Quartet" (MQ). Unlike merons in a single pair, which is unstable against pair annihilation, the merons within the MQ exhibit exceptional stability against pair annihilation due to the protective localization mechanism induced by the twist that prevents the collision of the meron cores. Furthermore, we showcase that the stability of the MQ can be enhanced by adjusting the twist angle, resulting in increased resistance to external perturbations such as external magnetic fields. Our findings highlight the twisted magnet as a promising platform for investigating the intriguing properties of merons, enabling their realization as stable magnetic quasiparticles in van der Waals magnets.

cond-mat.mes-hall↗

arXiv2023

Controllable magnetic domains in twisted trilayer magnets

The use of moiré patterns to manipulate two-dimensional materials has facilitated new possibilities for controlling material properties. The moiré patterns in the two-dimensional magnets can cause peculiar spin texture, as shown by previous studies focused on twisted bilayer systems. In our study, we develop a theoretical model to investigate the magnetic structure of twisted trilayer magnets. Unlike the twisted bilayer, the twisted trilayer magnet has four different local stacking structures distinguished by the interlayer couplings between the three layers. Our results show that the complex interlayer coupling effects in the moiré superlattice can lead to the stabilization of rich magnetic domain structures; these structures can be significantly manipulated by adjusting the twist angle. Additionally, external magnetic fields can easily manipulate these domain structures, indicating potential applications in spintronics devices.

cond-mat.mes-hall↗