arXiv subjects

Seik Pak

Publications and source records attributed to Seik Pak.

5 recordsLinked to original sources

arXiv2026

p wave Magnetism and Gate-Tunable Edelstein Response in van der Waals Heterostructures

Odd-parity magnetism has attracted significant interest for its unconventional spin splitting. However, a concrete microscopic route for its realization remains elusive. In this work, we propose van der Waals heterostructures of stripe antiferromagnets (sAFMs) as an ideal platform for electrically controllable $p$-wave magnetism. In the sAFM/metal/sAFM structure, the leading RKKY-type exchange interaction is canceled due to the symmetry of the stacking pattern. This exposes a higher-order biquadratic interaction as a dominant contribution that drives a filling-controlled transition from a collinear phase to an orthogonal $p$-wave configuration. The resulting $p$-wave phase exhibits a gate-tunable Edelstein response, which originates from magnetic symmetry breaking rather than conventional relativistic spin-momentum locking and remains robust even under substantial spin-orbit coupling. Finally, we propose material candidates for the realization of our theory. Our results establish van der Waals heterostructures as a practical platform for non-relativistic spintronics with electric control of odd-parity spin textures.

cond-mat.str-el↗

arXiv2025

Strong interlayer coupling and chiral flat-band cascades in twisted bilayer gratings

From atomic crystals to macroscopic material structures, twisted bilayer systems have emerged as a promising route to control wave phenomena. In few-layer van der Waals (vdW) materials, however, the intrinsically weak interlayer coupling typically demands fine control of small twist angles to reach magic-angle conditions. Here, we show that one-dimensional photonic crystal bilayers can overcome this limitation by accessing a regime of strong interlayer coupling -- comparable to intralayer coupling. This strong coupling enables flat-band formation over a broad angular range even at large twist angles. We experimentally realize this regime by stacking WS$_2$ gratings using a two-step lithography method, resulting in ultra-wide chiral flat-band cascades in magic-angle twisted bilayer gratings. Our work not only provides a platform for designing photonic applications with tunable localization but also explores a new regime of physics unattainable in conventional solid-state based moir\'e systems.

physics.optics↗

arXiv2025

Eta-pairing state in flatband lattice: Interband coupling effect on entanglement entropy logarithm

The eta-pairing state is the eigenstate of the hypercubic Hubbard model, which exhibits anomalous logarithmic scaling of entanglement entropy. In multi-band systems, eta-pairing can be exact eigenstate when the band is flat without interband coupling. However, typical flatband systems such as Lieb and Kagome lattices often feature band touchings, where interband coupling effects are non-negligible. Using the Creutz ladder, we investigate the deformation of eta-pairing states under the interband coupling effect. Our results show corrections to entanglement entropy scaling, with modified eta-pairing states displaying broadened doublons, nonuniform energy spacing, and deviations from exact behavior for configurations with more than one eta-pair, even in the large band gap limit, except at t = 0. Through a Schrieffer-Wolff transformation, we quantify corrections to the spectrum generating algebra, offering insights into the interplay between interaction-driven phenomena and band structure effects. These findings illuminate the robustness and limitations of eta-pairing in realistic flatband systems.

cond-mat.str-el↗

arXiv2025

Stacking-dependent topological electronic structures in honeycomb-kagome heterolayers

Heterostructures of stacked two-dimensional lattices have shown great promise for engineering novel material properties. As an archetypal example of such a system, the hexagon-shared honeycomb-kagome lattice has been experimentally synthesized in various material platforms. In this work, we explore three rotationally symmetric variants of the honeycomb-kagome lattice: the hexagonal, triagonal, and biaxial phases. While the triagonal and biaxial phases exhibit trivial insulating and Dirac semimetal band structures, respectively, the hexagonal phase hosts a higher-order topological phase driven by band inversion near the $\Gamma$-point. This highlights a key distinction from the conventional band inversions at the $K$-point observed in hexagonal homobilayer systems. Furthermore, we demonstrate how the distinct topological properties of these phases result in network band structures within moir\'e heterostructures formed by twisted or lattice-mismatched HK systems. These network band structures can be experimentally observed through extrinsic twisting or intrinsic lattice mismatching between the honeycomb and kagome systems.

cond-mat.mtrl-sci↗

arXiv2023

PT-symmetric Non-Hermitian Hopf Metal

Hopf insulator is a representative class of three-dimensional topological insulators beyond the standard topological classification methods based on K-theory. In this letter, we discover the metallic counterpart of the Hopf insulator in the non-Hermitian systems. While the Hopf invariant is not a stable topological index due to the additional non-Hermitian degree of freedom, we show that the PT-symmetry stabilizes the Hopf invariant even in the presence of the non-Hermiticity. In sharp contrast to the Hopf insulator phase in the Hermitian counterpart, we discover an interesting result that the non-Hermitian Hopf bundle exhibits the topologically protected non-Hermitian degeneracy, characterized by the two-dimensional surface of exceptional points. Despite the non-Hermiticity, the Hopf metal has the quantized Zak phase, which results in bulk-boundary correspondence by showing drumhead-like surface states at the boundary. Finally, we show that, by breaking PT-symmetry, the nodal surface deforms into the knotted exceptional lines. Our discovery of the Hopf metal phase firstly confirms the existence of the non-Hermitian topological phase outside the framework of the standard topological classifications.

cond-mat.mes-hall↗