arXiv subjects

Huiyuan Zheng

Publications and source records attributed to Huiyuan Zheng.

15 recordsLinked to original sources

arXiv2026

Topological Charge-Transfer Excitons

Excitons possess internal structure absent from single-particle Bloch particles, allowing their band topology to emerge from the bound-state structure rather than being inherited from their constituents. This raises the question of how the internal structure of a bound state can provide a microscopic origin of exciton topology. Here we show that the real-space embedding of charge-transfer excitons can generate an intrinsic manifold of symmetry-related off-site composite orbitals whose coupling supports topological exciton bands. Lateral electron-hole separation embeds the localized exciton on the bond connecting its constituent sites rather than on either site. We demonstrate this mechanism in a honeycomb lattice, where three bond-centered charge-transfer exciton orbitals form a Kagome lattice. By solving the Bethe-Salpeter equation, we show that this emergent multi-orbital manifold supports a topological exciton flat band upon time-reversal symmetry breaking, even when the electron and hole bands are topologically trivial. The resulting band exhibits nearly uniformly distributed quantum geometry, favorable for interaction-driven bosonic states. Our results establish a general route toward topological bands of localized composite bound states and unconventional strongly correlated bosonic phases.

cond-mat.mes-hall↗

arXiv2026

Electrically Programmable Correlated Topology and Magnetism in a Moiré Trilayer

Strong electron-electron interactions underlie a wide range of quantum many-body phenomena, including magnetism, superconductivity, and charge fractionalization. A central goal is to achieve in situ control over lattice geometry, bandwidth, and band topology within a single platform. Here we realize such an electrically programmable quantum many-body system in an alternating twisted trilayer MoTe$_2$, where an out-of-plane displacement field continuously modifies the layer polarization, effective lattice, and topology of the moiré bands. At zero displacement field, the system realizes a triangular lattice hosting a correlated insulator at one hole per moiré unit cell ($ν= -1$). Doping this state produces strongly asymmetric magnetic responses: double-exchange-like ferromagnetism for $|ν| > 1$, and signatures of spin polarons and antiferromagnetism for $|ν| < 1$. At large displacement field, interlayer hybridization reconstructs the electronic structure into a honeycomb lattice with a flat Chern band, supporting integer and fractional Chern insulators. Magneto-optical measurements further reveal the signatures of gap closure and Landau-level formation from a spin-polarized Fermi surface near the crossover between the two regimes. These results establish a unified, electrically tunable platform in which correlated magnetism and topological states emerge from a single controllable band structure.

cond-mat.mes-hall↗

arXiv2026

Evaluating Stochastic Collapse and Implicit Bias in Multimodal Large Language Models

Current evaluations for Multimodal Large Language Models (MLLMs) overwhelmingly focus on utility-driven objectives, leaving model behavior under logic-neutral scenarios largely underexplored. Stochasticity is essential in scenarios where multiple actions are equally valid, such as recommending travel itineraries or daily schedules where multiple options have similar utility. In such settings, deterministic policies may lead to repetitive behaviors and reduced coverage of valid alternatives. To bridge this gap, we propose RandomBench, a benchmark designed to evaluate whether MLLMs can maintain distributionally neutral behavior when selecting among equivalent options. We further introduce three metrics, including RI, BCI, BII, to quantify entropy and distributional bias. Experiments reveal a pervasive phenomenon termed Stochastic Collapse, where MLLMs fail to maintain uniform randomness under explicit random instructions, with top-1 probabilities reaching 97% from the ideal one quarter baseline and RI dropping to 0.068 in Claude Sonnet 4.6. Extensive ablation studies further demonstrate that these deviations persist across languages and representation formats, highlighting the robustness of distributional collapse in logic-neutral decision settings.

cs.CL↗

arXiv2026

Giant and Helical Exciton Dipole from Berry Curvature in Flat Chern Bands

We show that excitons forming between moiré flat Chern bands possess a substantial electric dipole moment comparable to the moiré lattice parameter times the elementary charge ($\sim10^2$ Debye). At a hole filling factor of one in twisted MoTe$_2$, the dipole moment of the lowest-energy exciton branch develops in-plane helical texture in momentum space from the intrinsic Berry curvature of electron and hole. By solving the Bethe-Salpeter equations, we demonstrate that an out-of-plane displacement field induces a Frenkel-to-Wannier exciton transition, accompanied by a reversal of the dipole texture helicity. The resulting attractive exciton dipole-dipole interactions lead to quadrupolar biexcitons that can be probed via two-photon spectroscopy. Our findings establish band topology as a tunable knob to engineer exciton dipole moments and pave the way to manipulate many-body interactions in the terahertz regime.

cond-mat.mes-hall↗

arXiv2026

Valleytronics in 2D Materials Roadmap

Valleytronics exploits non-equivalent energy extrema in the electronic band structure of crystalline solids -- the valley degree of freedom -- to encode, manipulate, and read out information. The advent of 2D materials, first graphene and then transition-metal dichalcogenides, made valley control practical through optical, electrical, and magnetic routes. This foundation has enabled remarkable progress in recent years spanning established frontiers, such as valley exciton physics and valley Hall effects, as well as emerging directions including lightwave valleytronics, nanophotonic integration, flat-band valleytronics, and spin-valley qubits. In parallel, there are sustained efforts to scale up valleytronic materials and to predict new valleytronic platforms. This Roadmap brings together perspectives from leading experts to chart the key opportunities and challenges at the forefront of 2D material valleytronics. Each section captures a snapshot of progress in a key research area, identifies critical open challenges, and outlines pathways toward future valleytronics breakthroughs.

cond-mat.mes-hall↗

arXiv2025

What's Wrong with Your Code Generated by Large Language Models? An Extensive Study

The increasing development of LLMs in code generation has drawn significant attention among researchers. To enhance LLM-based code generation ability, current efforts are predominantly directed towards collecting high-quality datasets and leveraging diverse training technologies. However, there is a notable lack of comprehensive studies examining the limitations and boundaries of existing methods. To bridge this gap, we conducted an extensive empirical study evaluating the performance of three leading closed-source LLMs and six popular open-source LLMs on three commonly used benchmarks. Our investigation, which evaluated the length, cyclomatic complexity and API number of the generated code, revealed that these LLMs face challenges in generating successful code for more complex problems, and tend to produce code that is shorter yet more complicated as compared to canonical solutions. Additionally, we developed a taxonomy of bugs for incorrect codes that includes three categories and ten sub-categories, and analyzed the root cause for common bug types. To better understand the performance of LLMs in real-world projects, we also manually created a real-world benchmark RWPB. We analyzed bugs on RWPB to highlight distinct differences in bug distributions between actual scenarios and existing benchmarks. Finally, we propose a novel training-free iterative method that introduces self-critique, enabling LLMs to critique and correct their generated code based on bug types and compiler feedback. Our comprehensive and extensive study provides insights into the current limitations of LLM-based code generation and opportunities for enhancing the accuracy and quality of the generated code.

cs.SE↗

arXiv2025

Layer Coherence Origin of Planar Hall Effect: from Charge to Multipole and Valley

We uncover a new origin of the planar Hall effect - as an intrinsic property of layer coherent electrons - that exists even in bilayer and trilayer atomically thin limit. It reforms the existing theories requiring three-dimensional orbital motion, or strong spin-orbit coupling of certain forms, which are absent in van der Waals thin films. We exemplify that the effect can be triggered by strain and interlayer sliding in twisted structures with rich tunability and strong magnitudes. Furthermore, this layer coherence mechanism broadens the conceptual framework to include planar multipole Hall effect, and valley Hall effect induced by in-plane pseudo-magnetic field, outreaching the existing mechanisms. The layer mechanism also provides a new route towards quantized Hall response upon a topological phase transition induced by in-plane magnetic field. These results unveil the unexplored potential of quantum layertronics and moiré flat band for planar transport in 2D materials.

cond-mat.mes-hall↗

arXiv2025

Förster valley-orbit coupling and topological lattice of hybrid moiré excitons

Hybrid exciton in moiré superlattices of two-dimensional (2D) semiconductors inherits the electric dipole, strong moiré trapping, and stacking optical selection rules from its interlayer part, whereas the intralayer part is intended for enhancing optical coupling strength. Here, we show that electron-hole Coulomb exchange, or Förster coupling, within the intralayer component qualitatively alters the properties of moiré excitons, enabling their coherent hopping between moiré traps laterally separated over 10 nm and/or across layers, where their kinetic propagation is completely suppressed. Valley-flip hopping channels are found as significant as the valley-conserving ones, leading to rich possibilities to tailor valley-orbit-couplings and introduce non-trivial topology to the moiré exciton superlattice. In twisted MoTe$_2$ where hybrid moiré excitons feature a symmetry protection from radiative recombination, we show that Förster valley-orbit-coupling can give rise to a rich topological phase diagram.

cond-mat.mes-hall↗

arXiv2025

Beyond Boundaries: Learning a Universal Entity Taxonomy across Datasets and Languages for Open Named Entity Recognition

Open Named Entity Recognition (NER), which involves identifying arbitrary types of entities from arbitrary domains, remains challenging for Large Language Models (LLMs). Recent studies suggest that fine-tuning LLMs on extensive NER data can boost their performance. However, training directly on existing datasets neglects their inconsistent entity definitions and redundant data, limiting LLMs to dataset-specific learning and hindering out-of-domain adaptation. To address this, we present B2NERD, a compact dataset designed to guide LLMs' generalization in Open NER under a universal entity taxonomy. B2NERD is refined from 54 existing English and Chinese datasets using a two-step process. First, we detect inconsistent entity definitions across datasets and clarify them by distinguishable label names to construct a universal taxonomy of 400+ entity types. Second, we address redundancy using a data pruning strategy that selects fewer samples with greater category and semantic diversity. Comprehensive evaluation shows that B2NERD significantly enhances LLMs' Open NER capabilities. Our B2NER models, trained on B2NERD, outperform GPT-4 by 6.8-12.0 F1 points and surpass previous methods in 3 out-of-domain benchmarks across 15 datasets and 6 languages. The data, models, and code are publicly available at https://github.com/UmeanNever/B2NER.

cs.CL↗

arXiv2024

TransferTOD: A Generalizable Chinese Multi-Domain Task-Oriented Dialogue System with Transfer Capabilities

Task-oriented dialogue (TOD) systems aim to efficiently handle task-oriented conversations, including information collection. How to utilize TOD accurately, efficiently and effectively for information collection has always been a critical and challenging task. Recent studies have demonstrated that Large Language Models (LLMs) excel in dialogue, instruction generation, and reasoning, and can significantly enhance the performance of TOD through fine-tuning. However, current datasets primarily cater to user-led systems and are limited to predefined specific scenarios and slots, thereby necessitating improvements in the proactiveness, diversity, and capabilities of TOD. In this study, we present a detailed multi-domain task-oriented data construction process for conversations, and a Chinese dialogue dataset generated based on this process, TransferTOD, which authentically simulates human-computer dialogues in 30 popular life service scenarios. Leveraging this dataset, we trained a model called TransferTOD-7B using full-parameter fine-tuning, showcasing notable abilities in slot filling and questioning. Our work has demonstrated its strong generalization capabilities in various downstream scenarios, significantly enhancing both data utilization efficiency and system performance. The data is released in https://github.com/KongLongGeFDU/TransferTOD.

cs.AI↗

arXiv2024

Interlayer electric multipoles induced by in-plane field from quantum geometric origins

We show that interlayer charge transfer in 2D materials can be driven by an in-plane electric field, giving rise to electrical multipole generation in linear and second order of in-plane field. The linear and nonlinear effects have quantum geometric origins in the Berry curvature and quantum metric respectively, defined in extended parameter spaces characteristic of layered materials. We elucidate their symmetry characters, and demonstrate sizable dipole and quadrupole polarizations respectively in twisted bilayers and trilayers of transition metal dichalcogenides. Furthermore, we show that the effect is strongly enhanced during the topological phase transition tuned by interlayer translation. The effects point to a new electric control on layer quantum degree of freedom.

cond-mat.mes-hall↗

arXiv2023

Extraordinary Thermoelectric Properties of Topological Surface States in Quantum-Confined Cd3As2 Thin Films

Topological insulators and semimetals have been shown to possess intriguing thermoelectric properties promising for energy harvesting and cooling applications. However, thermoelectric transport associated with the Fermi arc topological surface states on topological Dirac semimetals remains less explored. In this work, we systematically examine thermoelectric transport in a series of topological Dirac semimetal Cd3As2 thin films grown by molecular beam epitaxy. Surprisingly, we find significantly enhanced Seebeck effect and anomalous Nernst effect at cryogenic temperatures when the Cd3As2 layer is thin. Combining angle-dependent quantum oscillation analysis, magnetothermoelectric measurement, transport modelling and first-principles simulation, we isolate the contributions from bulk and surface conducting channels and attribute the unusual thermoeletric properties to the topological surface states. Our analysis showcases the rich thermoelectric transport physics in quantum-confined topological Dirac semimetal thin films and suggests new routes to achieving high thermoelectric performance at cryogenic temperatures.

cond-mat.mtrl-sci↗

arXiv2023

Dynamically tunable moiré Rydberg excitons in a monolayer semiconductor on twisted bilayer graphene

Moiré excitons are emergent optical excitations in 2D semiconductors with deep moiré superlattice potentials. While these excitations have been realized in several platforms, a system with dynamically tunable moiré potential to tailor the moiré exciton properties is yet to be realized. Here, we present a continuously tunable moiré potential in a monolayer WSe2 that is enabled by its proximity to twisted bilayer graphene (TBG) near the magic-angle. Due to its flat electronic bands, charge distribution is highly localized and forms a triangular lattice in TBG. Tuning the local charge density via electrostatic gating, TBG thus provides a spatially varying and dynamically tunable dielectric superlattice for modulating monolayer exciton wavefunctions. By performing optical reflection spectroscopy, we observe emergent moiré exciton Rydberg branches in monolayer WSe2 with increased energy splitting upon doping TBG. The twist-angle dependence reveals that the observation is due to a hybridization between bright and dark Rydberg states enabled by the moiré potential. Further, at the magic-angle near 1.1°, the moiré Rydberg excitons form a sawtooth pattern with doping owing to the formation of strongly correlated states in the TBG. Our study provides a new platform for engineering moiré excitons as well as optical accessibility to the electronic states with small correlation gaps in TBG.

cond-mat.mes-hall↗

arXiv2022

Anomalous magneto-optical response and chiral interface of dipolar excitons at twisted valleys

An anomalous magneto-optical spectrum is discovered for dipolar valley excitons in twisted double layer transition metal dichalcogenides (TMD), where in-plane magnetic field induces a sizable multiplet splitting of exciton states inside the light cone. Chiral dispersions of the split branches make possible efficient optical injection of unidirectional exciton current. We also find an analog effect with a modest heterostrain replacing the magnetic field for introducing large splitting and chiral dispersions in the light cone. Angular orientation of photo-injected exciton flow can be controlled by strain, with left-right unidirectionality selected by circular polarisation.

cond-mat.mes-hall↗

arXiv2021

Twist versus heterostrain control of optical properties of moiré exciton minibands

We investigate the optical properties of interlayer excitons in heterobilayer transition metal dichalcogenides where moiré pattern is introduced by heterostrain, in comparison with that introduced by twisting (and/or lattice mismatch). Besides being a cause of the moiré texture, strain also effectively introduces a constant gauge potential on either electron or hole, which shifts the dispersion of kinetic energy with respect to the excitonic crystal momenta in the moiré superlattices.This leads to distinct exciton mini-band dispersions and light coupling properties from the twisting induced moiré, even if the excitonic moiré superlattice potentials have the similar real-space profile for the two cases. For strain that breaks the three-fold rotational symmetry at the atomic scale,the exciton wave packets trapped at the superlattice potential minima have elliptically polarized valley optical selection rules, in contrast to the circularly polarized ones in the twisting moiré. We investigate the evolution of the excitonic mini-bands and the optical dipoles of the bright states inside the light cones with the decrease of the moiré periodicity, upon which the excitonic wavefunctions evolve from localized wave packets to the extended Bloch states. Furthermore, moiré exciton properties under the interplay of twisting and heterostrain are also discussed.

cond-mat.mes-hall↗