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Yi-Chun Hung

Publications and source records attributed to Yi-Chun Hung.

At least 19 recordsLinked to original sources

Helical Domain-Wall-Ring Networks Reshape Superconducting Correlations

Extended domain-wall networks that emerge in moir\'e materials provide a distinct platform for quasi-one-dimensional electronic states. However, the interaction-driven orders in confined networks remain largely unexplored. Here, we discuss superconducting (SC) correlations in interacting helical domain-wall-ring networks in the closed topological domains formed within the moir\'e patterns of an underlying twisted bilayer honeycomb lattice. We first analyze the system within the framework of an infinite-size theory and show that inter-ring SC-pair tunneling is renormalization-group relevant and thus enhances SC correlations through inter-ring phase locking. To address finite-size effects resulting from the ring-network geometry, we present a self-consistent variational approach. Our analysis shows that even in the regime where the infinite-size theory predicts strongly coupled pair tunneling, the induced phase-locking scale remains strongly suppressed. In contrast, the SC scaling dimension continues to decrease with stronger inter-ring density-density interaction and a decreasing twist angle, while remaining insensitive to the pair-tunneling strength. This discrepancy demonstrates that ring networks do not simply approach their infinite-size counterparts but can exhibit qualitatively distinct collective behavior. Our study highlights how the interplay of confinement effects and ring-network geometry can reshape SC correlations.

cond-mat.mes-hall

Quantum Geometry Driven Finite-Momentum Exciton Fluctuations in Flat-Band Systems

Quantum geometry is instrumental in stabilizing exotic phenomena in systems ranging from topological insulators to superconductors. In dispersionless flat bands, where the kinetic energy is quenched, the quantum metric emerges as the fundamental driver of macroscopic collective phenomena. Here, we theoretically demonstrate that lattice-geometry-induced flat bands, such as those in kagome and Lieb lattices, provide a fertile platform for realizing a purely quantum-geometry-driven excitonic insulator (EI) phase. By applying an out-of-plane Zeeman field to lift spin degeneracy without spin-orbit coupling, we establish a Ginzburg-Landau framework in which the electron-hole wavefunction-overlap directly maps the flat-band quantum metric onto the macroscopic free energy. This mapping plays a key role in both the EI and the associated superfluid phases, with the coherence length and phase stiffness emerging directly from the quantum metric. Our analysis reveals that under strong interactions, the quantum metric induces a negative effective kinetic coefficient for the amplitude mode. Rather than destabilizing the uniform condensate, this softens the amplitude fluctuations at a finite momentum, giving rise to a finite-momentum superfluid density fluctuation (FMSDF) state. This state is observable as a periodically modulated magnitude of in-plane magnetization fluctuations. Our findings establish a rigorous link between flat-band quantum geometry and dynamic collective excitonic states, with promising pathways for realization in covalent-organic frameworks (COFs).

cond-mat.mes-hall

Inter-Dirac-Cone Coherence Beyond the Valley Framework

Although interactions are known to generate exotic phases in pseudospin-1/2 flat-band Dirac materials, it remains an open question whether higher-pseudospin Dirac cones continue to govern low-energy interaction-driven orders even without a well-defined valley degree of freedom. Here, we demonstrate a new form of inter-Dirac-cone coherence beyond the valley framework, which is realized through a staggered virtual-loop-current (SVLC) order in a partially filled pseudospin-1 Dirac flat band. Remarkably, the SVLC order continues to be driven by coherence between the pseudospin-1 Dirac cones even after the conventional valley-based description ceases to apply. The virtual loop currents are shown to originate from interaction-driven quantum fluctuations of charge densities and are found to exhibit an alternating circulation between the neighboring triangular plaquettes. The resulting spontaneous time-reversal symmetry breaking induces finite intrinsic anomalous Hall conductivity and orbital magnetization. The SVLC order is shown to be the lowest-energy solution in restricted real-space Hartree-Fock calculations in the weakly interacting regime. Our study establishes inter-Dirac-cone coherence as the organizing principle for interaction-driven orders in higher-pseudospin Dirac systems and identifies these systems as a new platform for driving exotic emergent phases beyond the valley framework.

cond-mat.str-el

Hyperbolic Neural Population Geometry Benefits Computation

Neural population geometry shapes downstream computation. Recent empirical findings in neurobiology suggest that a hyperbolic structure underlies population activity in the hippocampus. Here we provide a theoretical framework for this phenomenon. First, we propose a plausible construction of hippocampal tuning curves that statistically induces hyperbolic geometry. Next, we establish a connection between neural decoding and associative memory by demonstrating that the Modern Hopfield Network update rule computes the minimum mean-squared-error (MMSE) estimator. Finally, we introduce a novel associative memory model defined in hyperbolic space that yields significantly larger capacity than leading models. Our results suggest that animals encode spatial information as a latent hyperbolic cognitive map, improving both memory capacity and decoding accuracy.

q-bio.NC

Quantum Geometry of Moiré Flat Bands Beyond the Valley Paradigm

Flat bands in moiré superlattices provide a fertile ground for correlated and topological phases, governed by their quantum geometric properties. While the valley-based paradigm captures key features in select materials, it breaks down in a growing class of systems lacking valley structure, where exotic phenomena such as twist-angle-tunable numbers of flat bands emerge. In this work, we develop and analyze tight-binding models for twisted heterobilayers of bipartite lattices, with a focus on the role of interlayer hybridization in generating flat-band quantum geometry. We demonstrate that sublattice-selective interlayer tunnelings in twisted dice lattice and graphene heterobilayers induce isolated flat bands at zero energy, whose number is tunable by the twist angle. Most importantly, these flat bands exhibit finite Berry curvature and a quantum metric of the Chern-insulator scale, generated through interlayer hybridization. This establishes a mechanism to induce quantum geometry in moiré flat bands beyond the valley paradigm. Our results chart a route to flat-band quantum geometry engineering in twisted bilayer bipartite lattices, with potential material realizations in oxide heterostructures, molecular lattices, and synthetic quantum matter.

cond-mat.mes-hall

Breakdown of Topological Inheritance and Twist-Induced Quantum Geometry Reconfiguration in Moir\'e Flat Bands

Since the inception of moir\'e quantum matter, a foundational tenet of the field has been that the quantum geometry of emergent flat bands is faithfully inherited from the low-energy valleys of the constituent monolayers. Here, we demonstrate a breakdown of this longstanding tenet in twisted bilayers of loop-current-ordered kagome lattices (tb-LCK). Using microscopic tight-binding modeling, we reveal a twist-induced reconfiguration of quantum geometry where realistic interlayer hybridization quenches topological inheritance from the monolayer. By tuning the loop-current phase, we identify distinct regimes in which the monolayer Berry curvature is either substantially redistributed or entirely suppressed in the moir\'e flat bands. We further show that this quantum geometric collapse is expected to be readily accessible in vanadium-based kagome metals such as AV$_{\text{3}}$Sb$_{\text{5}}$, and that Floquet engineering via waveguide laser illumination offers a practical route to turn topological inheritance on and off. Our findings uncover a universal mechanism for quantum geometric reconstruction, establishing interlayer coupling strength as an independent parameter for tuning band topology beyond the weakly coupled van der Waals heterostructure paradigm.

cond-mat.mes-hall

Extending Topological Bound on Quantum Weight Beyond Symmetry-Protected Topological Phases

The quantum metric encodes the geometric structure of Bloch wave functions and governs a wide range of physical responses. Its Brillouin-zone integral, the quantum weight, appears in the structure factor and provides lower bounds on observables such as the optical gap and dielectric constant. In symmetry-protected topological (SPT) phases, the nontrivial band topology imposes a lower bound on the quantum weight and constraints on the observables. Here, we generalize the topological bound on quantum geometry to encompass systems beyond the SPT phases. We show that topological invariants defined via the projected spectrum lower-bound the quantum weight with a symmetry-breaking correction to the quantum metric. Our proposed bound holds even when the underlying symmetries are broken, and it would be amenable to experimental verification via the optical conductivity sum rule under external fields. We illustrate our theory by adding a nonzero spin-orbit coupling term to a spin Chern insulator model, where we show that our proposed bound applies even though the conventional topological bound does not hold.

cond-mat.mes-hall

Nested Feature Spectrum Topology: Tripartite Topological Equivalence of Feature, Entanglement, and Wilson Loop Spectrum

Topological phases of matter are traditionally characterized through symmetry-based classifications. In cases of symmetry breaking, the projected spectrum - obtained by projecting the ground state onto the eigenstates of a pertinent quantum observable, such as spin or orbital angular momentum - provides a clear method for classifying topological phases. This approach underpins well-known frameworks such as spin-resolved topology and feature spectrum topology. Here we introduce nested feature spectrum topology, in which projection operators are applied recursively to subsectors of the feature spectrum, generating a hierarchy of feature spectra. We uncover a fundamental tripartite equivalence among the topology of feature, the entanglement, and the Wilson loop spectra in non-interacting fermionic systems. This equivalence reveals that the feature spectrum encodes the entanglement between sectors of the quantum observable, such as the spin-up and spin-down states in spin-resolved topology. We further prove that spectral flow in the entanglement spectrum and the Wilson loop winding in the feature spectrum are equivalent manifestations of the feature-energy complementarity: the appearance of gapless spectral flow in either energy or projected spectra on the boundary. This complementarity refines the conventional bulk-boundary correspondence by demonstrating that topological boundary modes may persist in the feature spectrum even when energy spectra are gapped. Our results provide a deeper understanding and solid foundation for the origin of band topology in the feature spectrum.

cond-mat.mes-hall

Homeostatic Adaptation of Optimal Population Codes under Metabolic Stress

Information processing in neural populations is inherently constrained by metabolic resource limits and noise properties, with dynamics that are not accurately described by existing mathematical models. Recent data, for example, shows that neurons in mouse visual cortex go into a "low power mode" in which they maintain firing rate homeostasis while expending less energy. This adaptation leads to increased neuronal noise and tuning curve flattening in response to metabolic stress. We have developed a theoretical population coding framework that captures this behavior using two novel, surprisingly simple constraints: an approximation of firing rate homeostasis and an energy limit tied to noise levels via biophysical simulation. A key feature of our contribution is an energy budget model directly connecting adenosine triphosphate (ATP) use in cells to a fully explainable mathematical framework that generalizes existing optimal population codes. Specifically, our simulation provides an energy-dependent dispersed Poisson noise model, based on the assumption that the cell will follow an optimal decay path to produce the least-noisy spike rate that is possible at a given cellular energy budget. Each state along this optimal path is associated with properties (resting potential and leak conductance) which can be measured in electrophysiology experiments and have been shown to change under prolonged caloric deprivation. We analytically derive the optimal coding strategy for neurons under varying energy budgets and coding goals, and show how our method uniquely captures how populations of tuning curves adapt while maintaining homeostasis, as has been observed empirically.

cs.NE

Spectrum from Defocus: Fast Spectral Imaging with Chromatic Focal Stack

Hyperspectral cameras face harsh trade-offs between spatial, spectral, and temporal resolution in inherently low-photon conditions. Computational imaging systems break through these trade-offs with compressive sensing, but have required complex optics and/or extensive compute. We present Spectrum from Defocus (SfD), a chromatic focal sweep method that achieves state-of-the-art hyperspectral imaging with only two off-the-shelf lenses, a grayscale sensor, and less than one second of reconstruction time. By capturing a chromatically-aberrated focal stack that preserves nearly all incident light, and reconstructing it with a fast physics-based iterative algorithm, SfD delivers sharp, accurate hyperspectral images. The combination of photon efficiency, optical simplicity, and physical interpretability makes SfD a promising solution for fast, compact, interpretable hyperspectral imaging.

cs.CV

Wavefunction-Free Approach for Predicting Nonlinear Responses in Weyl Semimetals

By sidestepping the intractable calculations of many-body wavefunctions, density functional theory (DFT) has revolutionized the prediction of ground states of materials. However, predicting nonlinear responses--critical for next-generation quantum devices--still relies heavily on explicit wavefunctions, limiting computational efficiency. In this letter, using the circular photogalvanic effect (CPGE) in Weyl semimetals as a representative example, we realize a 1000-fold computational speedup by eliminating the explicit dependence on wavefunctions. Our approach leverages the one-to-one correspondence between free parameters of Weyl fermions and the associated responses to obtain precise wavefunction-free formulations. Applying our methodology, we systematically investigated known Weyl semimetals and revealed that Ta$_3$S$_2$ exhibits photocurrents an order of magnitude greater than those observed in TaAs, with potential for an additional order-of-magnitude enhancement under strain. To further demonstrate the generality of our approach, we obtained a wavefunction-free formula for the Berry-curvature dipole in Weyl semimetals. Our work paves the way for substantially more efficient screening and optimization of nonlinear electromagnetic properties in topological quantum materials.

cond-mat.mtrl-sci

Tunable Competing Electronic Orders in Double Quantum Spin Hall Superlattices

Competing superconducting (SC) and density-wave orders are of key importance in generating unconventional superconductivity and emergent electronic responses. Quasi-one-dimensional models provide insight into these competing orders and suggest higher-dimensional realizations through coupled-wire constructions, but analysis of such systems remains limited. Recent studies suggest that double helical edge states (DHESs) in double quantum spin Hall insulators (DQSHIs) form a two-channel Luttinger liquid that exhibits SC and spin density wave (SDW) phases and their $π$-junction analogs. Here, we analyze weakly coupled DHESs from the surface of a periodically stacked layered structure consisting of DQSHIs and dielectrics, where inter-edge interactions approximately develop a tunable helical sliding Luttinger liquid (HSLL) order. Using a renormalization-group analysis, we construct phase diagrams and identify a regime of HSLL parameters that favor competing two-dimensional $π$-SC and $π$-SDW orders. We identify parameter regimes where the competing orders could be realized experimentally in nanoscale devices. Our study suggests a promising materials platform for exploring tunable $π$-SC and $π$-SDW orders in double quantum spin Hall superlattices.

cond-mat.mes-hall

Geometry-Driven Moiré Engineering in Twisted Bilayers of High-Pseudospin Fermions

Moiré engineering offers new pathways for manipulating emergent states in twisted layered materials and lattice-mismatched heterostructures. With the key role of the geometry of the underlying lattice in mind, here we introduce the watermill lattice, a two-dimensional structure with low-energy states characterized by massless pseudospin-3/2 fermions with high winding numbers. Its twisted bilayer is shown to exhibit magic angles, where four isolated flat bands emerge around the Fermi level, featuring elevated Wilson-loop windings and enhanced quantum geometric effects, such as an increase in the ratio of the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature to the mean-field critical temperature under a weak Bardeen-Cooper-Schrieffer (BCS) pairing. We discuss how the watermill lattice could be realized in the MXene and group-IV materials. Our study highlights the potential of exploiting lattice geometry in moiré engineering to uncover novel quantum phenomena and tailor emergent electronic properties in materials.

cond-mat.mtrl-sci

Engineering Majorana Kramers Pairs In Synthetic High Spin Chern Insulators

High spin-Chern-number topological phases provide a promising low-dimensional platform for realizing double-helical edge states. In this letter, we show how these edge states can host a variety of phases driven by electron interaction effects, including multi-channel helical Luttinger liquid, spin density wave, superconducting phases, and a new type of $\pi$-junction analog of the latter two, where the transitions between the phases can be controlled. The superconducting phase in the interacting system is shown to be adiabatically connected to a time-reversal-symmetric topological superconductor in the non-interacting DIII class. This connection stabilizes Majorana Kramers pairs as domain wall states at the interface between the superconducting and $\pi$-spin-density wave phases, with the latter exhibiting a time-reversal-symmetric spin-density wave phase. We discuss the possibility of realizing our proposed scheme for generating Majorana Kramers pairs in a cold-atom based platform with existing techniques, and how it could offer potential advantages over other approaches.

cond-mat.str-el

Topological Orbital Hall Effect

The orbital Hall effect (OHE) is attracting recent interest due to its fundamental science implications and potential applications in orbitronics and spintronics. Unlike the spin Hall effect, the connection between the OHE and band topology is not well understood. Here we present a novel approach for understanding the OHE based on analyzing the projected orbital angular momentum (POAM) spectrum. By considering monolayers of group IV elements, we demonstrate that the Wannier charge centers of the POAM spectrum display topologically nontrivial windings. The orbital Hall conductivity is found to form a plateau within the band gap as a direct consequence of the Chern number carried by the POAM spectrum. The topological orbital Hall phase is shown to yield a new form of bulk-boundary correspondence, which features gapless states in the POAM spectrum and induces nonzero orbital textures at the boundaries that should be amenable to experimental verification through ARPES measurements. Our study presents a systematic method for investigating the topological OHE and provides a pathway for its broader exploration in two-dimensional materials.

cond-mat.mes-hall

Feature-energy duality of topological boundary states in multilayer quantum spin Hall insulator

Gapless topological boundary states characterize nontrivial topological phases arising from the bulk-boundary correspondence in symmetry-protected topological materials, such as the emergence of helical edge states in a two-dimensional $\mathbb{Z}_2$ topological insulator. However, the incorporation of symmetry-breaking perturbation terms in the Hamiltonian leads to the gapping of these edge bands, resulting in missing these crucial topological boundary states. In this work, we systematically investigate the robustness of bulk-boundary correspondence in the quantum spin Hall insulator via recently introduced feature spectrum topology. Our findings present a comprehensive understanding of feature-energy duality, illustrating that the aggregate number of gapless edge states in the energy-momentum ($\it{E-k}$) map and the non-trivial edge states in the $\hat{S}_z$ feature spectrum equals the spin Chern number of multilayer quantum spin Hall insulator. We identify a van der Waals material bismuth bromide $\rm(Bi_4Br_4)$ as a promising candidate through first-principles calculations. Our work not only unravels the intricacies of bulk-boundary correspondence but also charts a course for exploring quantum spin Hall insulators with high spin-Chern number.

cond-mat.mtrl-sci

Time-Reversal Soliton Pairs In Even Spin-Chern-Number Higher-Order Topological Insulators

Solitons formed through the one-dimensional mass-kink mechanism on the edges of two-dimensional systems with non-trivial topology play an important role in the emergence of higher-order (HO) topological phases. In this connection, the existing work in time-reversal symmetric systems has focused on gapping the edge Dirac cones in the presence of particle-hole symmetry, which is not suited to the common spin-Chern insulators. Here, we address the emergence of edge solitons in spin-Chern number of $2$ insulators, in which the edge Dirac cones are gapped by perturbations preserving time-reversal symmetry but breaking spin-$U(1)$ symmetry. Through the mass-kink mechanism, we thus explain the appearance of pairwise corner modes and predict the emergence of extra charges around the corners. By tracing the evolution of the mass term along the edge, we demonstrate that the in-gap corner modes and the associated extra charges can be generated through the $S_z$-mixing spin-orbit coupling via the mass-kink mechanism. We thus provide strong evidence that an even spin-Chern-number insulator is an HO topological insulator with protected corner charges.

cond-mat.mes-hall

Feature Spectrum Topology

Topology is a fundamental aspect of quantum physics, and it has led to key breakthroughs and results in various fields of quantum materials. In condensed matters, this has culminated in the recent discovery of symmetry-protected topological phases. However, symmetry-based topological characterizations rely heavily on symmetry analysis and are incapable of detecting the topological phases in systems where the symmetry is broken, thus missing a large portion of interesting topological physics. Here, we propose a new approach to understanding the topological nature of quantum materials, which we call feature spectrum topology. In this framework, the ground-state is separated into different partitions by the eigenspectrum of a feature, a particular chosen internal quantum degree of freedom, such as spin or pseudo-spin, and the topological properties are determined by analysis of these ground-state partitions. We show that bulk-boundary correspondence guarantees gapless spectral flows in either one of the energy or feature spectrum. Most importantly, such 'feature-energy duality' of gapless spectral flows serves as a fundamental manifestation of a topological phase, thereby paving a new way towards topological characterizations beyond symmetry considerations. Our development reveals the topological nature of a quantum ground state hidden outside symmetry-based characterizations, hence, providing a platform for a more refined search of unconventional topological materials.

cond-mat.mtrl-sci