arXiv subjects

Chaoyu He

Publications and source records attributed to Chaoyu He.

At least 19 recordsLinked to original sources

arXiv2026

Phonon-Localization-Driven Decoupling of Dual-Channel Transport for Record-Low Intrinsic Lattice Thermal Conductivity

A fundamental bottleneck in pushing the intrinsic lattice thermal conductivity of inorganic crystalline solids to its lowest limit arises from the inherent competition between the particle-like propagation ($\kappa_{\mathrm{L}}^{\mathrm{P}}$) and wave-like tunneling ($\kappa_{\mathrm{L}}^{\mathrm{C}}$) channels. Herein, we demonstrate that phonon localization provides a robust pathway to decouple the dual-channel transport, achieving record-low $\kappa_{\mathrm{L}}$ in quasi-1D ternary helical crystals. Despite the structural complexity leading to densely populated phonon branches and thus inducing abundant coherent phonons, the weak interchain interactions and heavy elements compress numerous branches into highly localized, nearly dispersionless flat bands. Such strong localization simultaneously suppresses both the diagonal and off-diagonal components of the group velocity, thereby synergistically suppressing $\kappa_{\mathrm{L}}^{\mathrm{P}}$ and $\kappa_{\mathrm{L}}^{\mathrm{C}}$. Taking InSeI as an example, the interchain room-temperature $\kappa_{\mathrm{L}}^{\mathrm{P}}$ and $\kappa_{\mathrm{L}}^{\mathrm{C}}$ are 0.145 and 0.053 W/mK, respectively, yielding an ultralow total $\kappa_{\mathrm{L}}$ of 0.198 W/mK. Weaker interchain interactions further drive the room-temperature $\kappa_{\mathrm{L}}$ of GaSeI and AlSeI to record lows of 0.086 and 0.089 W/mK, respectively; these values even drop to 0.058 and 0.059 W/mK at 900 K. These findings provide useful insights into exploring the thermal conductivity limit in crystals.

cond-mat.mtrl-sci↗

arXiv2026

General Theory for Ferroelectric Control of Spin Splitting in Collinear Antiferromagnets

Electrical control of magnetism is crucial for next-generation spintronics. While recent advances have demonstrated ferroelectric switching in two-dimensional magnets, a general design strategy spanning different dimensionalities remains elusive. Here, we develop a group-theoretical framework for achieving ferroelectric control of spin splitting in collinear antiferromagnets, including altermagnets and compensated ferrimagnets. By systematically classifying switching operators through symmetry analysis, we identify a universal pathway for the simultaneous reversal of electric polarization and nonrelativistic spin splitting.We validate this approach in three representative systems: quasi-one-dimensional $(6,14)$ Zigzag graphene nanoribbons, two-dimensional~\ch{Nb3I8}, and three-dimensional altermagnetic~\ch{MnSe2}. Our work establishes a versatile design paradigm for magnetoelectric devices and expands the functional landscape of low-power spintronic materials beyond the low-dimensional limit.

cond-mat.mtrl-sci↗

arXiv2026

High-Throughput Discovery of Semimetallic Borophenes with Diverse Dirac States Via Transferable Tight-Binding Approach

Borophene has attracted extensive interest due to its structural flexibility and emergent topological electronic states. However, semimetallic borophenes hosting robust Dirac states remain rare among the large number of predicted allotropes. Here, we develop a transferable tight-binding framework for planar borophenes and combine it with a graph- and group-theory-based random generation strategy to perform high-throughput screening of 522 borophene candidates. Eight previously unreported semimetallic borophenes are identified, hosting diverse topological band crossings, including type-I and type-III Dirac cones, Dirac nodal lines, and quadratic nodal points. Notably, quadratic nodal-point semimetals are predicted in borophene for the first time. Symmetry analysis reveals crystalline-symmetry-protected Dirac states, while first-principles calculations confirm their dynamical and thermal stability. These findings establish borophene as a versatile platform for engineering emergent Dirac physics in two dimensions.

cond-mat.mtrl-sci↗

arXiv2026

Domain-Direct Band Gaps: Classification and Material Realization

The conventional classification of direct band-gap semiconductors relies on point-like extrema in momentum space. Here, we introduce the concept of domain-direct band gaps, where the conduction-band minimum (CBM) and valence-band maximum (VBM) form extended manifolds in the Brillouin zone. We demonstrate this concept through the material realization of an extreme two-dimensional-two-dimensional (2D-2D) domain-direct band gap in twisted diamond. First-principles calculations show that both the CBM and VBM exhibit nearly flat 2D manifolds in the kx-ky plane with minimal energy variation (a few meV), yielding a direct band gap of 3.264 eV. In contrast, strong dispersion along the out-of-plane kz direction induces anisotropic carrier dynamics, with strongly suppressed in-plane Fermi velocities (down to about 10$^1$-10$^3$ m/s in certain directions) and much larger out-of-plane velocities (about 10$^6$ m/s). The nearly flat CBM and VBM manifolds enhance the joint density of states, leading to a pronounced optical absorption peak at the band gap onset. This new type of domain-direct gap, coupled with strong directional anisotropy, opens up opportunities for anisotropic optoelectronic applications. Our results establish domain-direct band gaps as a new class of semiconductors, demonstrating their feasibility in real materials.

cond-mat.mtrl-sci↗

arXiv2025

Unique Hierarchical Rotational Dynamics Induces Ultralow Lattice Thermal Conductivity in Cyanide-bridged Framework Materials

The pursuit of materials combining light constituent elements with ultralow lattice thermal conductivity ($\kappa_{\mathrm{L}}$) is crucial to advancing technologies like thermoelectrics and thermal barrier coatings, yet it remains a formidable challenge to date. Herein, we achieve ultralow $\kappa_{\mathrm{L}}$ in lightweight cyanide-bridged framework materials (CFMs) through the rational integration of properties such as the hierarchical vibrations exhibited in superatomic structures and rotational dynamics exhibited in perovskites. Unique hierarchical rotation behavior leads to multiple negative peaks in Gr\"uneisen parameters across a wide frequency range, thereby inducing pronounced negative thermal expansion and strong cubic anharmonicity in CFMs. Meanwhile, the synergistic effect between large four-phonon scattering phase space (induced by phonon quasi-flat bands and wide bandgaps) and strong quartic anharmonicity (associated with rotation modes) leads to giant quartic anharmonic scattering rates in these materials. Consequently, the $\kappa_{\mathrm{L}}$ of these CFMs decreases by one to two orders of magnitude compared to the known perovskites or perovskite-like materials with equivalent average atomic masses. For instance, the Cd(CN)$_{2}$, NaB(CN)$_{4}$, LiIn(CN)$_{4}$, and AgX(CN)$_{4}$ (X = B, Al, Ga, In) exhibit ultralow room-temperature $\kappa_{\mathrm{L}}$ values ranging from 0.35 to 0.81 W/mK. This work not only establishes CFMs as a novel and rich platform for studying extreme phonon anharmonicity, but also provides a new paradigm for achieving ultralow thermal conductivity in lightweight materials via the conscious integration of hierarchical and rotational dynamics.

cond-mat.mtrl-sci↗

arXiv2025

Two-dimensional flat-bands in Moire-diamonds

The discovery of flat-bands in magic-angle twisted bilayer graphene has underscored the potential of moire engineering for correlated states, but such phases are notoriously difficult to realize and highly fragile against perturbations. Here, we propose an alternative route to flat-bands by introducing sp3 hybridization in twisted graphite. Instead of relying on fine-tuned magic angles, our approach identifies flat-band states at relatively large twist angles with short moire periods. In this regime, sp3-induced reconstructions generate electronic states that, once formed, are locked by substantial energy barriers, rendering them robust against external perturbations. Using twisted graphite as a prototype, we uncover a series moire-diamond that host two-dimensional flat conduction of valence bands, where carriers are localized within specific momentum planes but remain dispersive along orthogonal directions. The emergence of dimensional flat-bands opens a new platform for flat-band-driven correlated physics and suggests opportunities for designing quantum materials with highly directional electronic functionalities.

cond-mat.mes-hall↗

arXiv2025

First-principles predictions of the diversity in atomic structures and electronic properties of the reconstructed Si(111)-7x7 surface

The 7x7 reconstruction of Si(111) surface is widely understood by the dimer-adatom-stacking-fault model (DAS), but the predicted metallicity of DAS contradicts experimental signs of insulation. It is still challenge to predict DAS-like reconstructions by traditional method to solve such a puzzle. Here, we show that low-energy reconstructions of Si(111)-7x7 surface with (DAS-d8-T12, DAS-d8-T9H3-A, DAS-d8-T9H3-B and DAS-d8-T6H6) and without (AB-d10-T12, AB-d10-T9H3, AA-d10-T12 and AA-d10-T9H3) stacking-fault can be quickly discovered by graph theory as implemented in RG2 code for crystal structure prediction. They exhibit comparable stability to the DAS (DAS-d8-T12) model and similar STM patterns, offering a plausible explanation for the observed Si(111)-7x7 reconstruction. All these reconstructions exhibit metallic behavior in the nonmagnetic (NM) state with isolated narrow bands crossing the Fermi level in varying occupancy. And they are further confirmed as ferromagnetic (FM) metals (DAS-d8-T9H3-B), half-metals (DAS-d8-T12, AB-d10-T9H3, AA-d10-T12 and AA-d10-T9H3), half-semimetals (DAS-d8-T9H3-A and DAS-d8-T6H6) and even insulators (AB-d10-T12), depending their occupancies of the NM band structures. These findings not only demonstrate the rich electromagnetic phases of reconstructed Si(111) surfaces and their potential for spintronic applications, but also provide a plausible physical explanation for the metal-insulator transition observed on the Si(111) surface.

cond-mat.mes-hall↗

arXiv2025

Natural and Intrinsic Vacancies in two-dimensional g-C$_3$N$_4$ for Trapping Isolated B and C Atoms as Color Centers

Color centers are vital for quantum information processing, but traditional ones often suffer from instability, difficulty in realization, and precise control of locations. In contrast, natural intrinsic vacancy-based color centers in two-dimensional systems offer enhanced stability and tunability. In this work, we demonstrate that g-C$_3$N$_4$ with natural intrinsic vacancies is highly suitable for trapping B/C atoms to form stable color centers as qubits. With easily identifiable vacancies, B/C atoms are expectable to be placed at the vacancy sites in g-C$_3$N$_4$ through STM manipulation. The vacancy sites are confirmed as the most stable adsorption positions, and once atoms are adsorbed, they are protected by diffusion barriers from thermal diffusions. The most stable charge states are C$_V^{+2}$/B$_V^{+2}$, C$_V^{+1}$/B$_V^{+1}$, and C$_V^0$/B$_V^0$ in turn, with charge transition levels of 0.39 eV and 2.49 eV, respectively. Specifically, the defect levels and net spin of C$_V$/B$_V$ can be adjusted by charge states. C$_V$, C$_V^{+1}$, C$_V^{+2}$, B$_V^{+1}$, and B$_V^{+2}$ exhibit optically allowable defect transition levels. The zero-phonon lines suggest fluorescence wavelengths fall within the mid-infrared band, ideal for qubit operations of stable initialization and readout. Furthermore, the Zero-field splitting (ZFS) parameter and the characteristic hyperfine tensor are provided as potential fingerprints for electron paramagnetic resonance (EPR) experiments.

cond-mat.mes-hall↗

arXiv2024

Isolated zero-energy flat-bands and intrinsic magnetism in carbon monolayers

Flat-band in twisted graphene bilayer has garnered widespread attention, and whether flat-bands can be realized in carbon monolayer is an interesting topic worth exploring in condensed matter physics. In this work, we demonstrate that, based on the theory of compact localized states, a series of two-dimensional carbon allotropes with flat-bands can be achieved. Two of them named as 191-8-66-C-r567x-1 and 191-10-90-C-r567x-1 are confirmed to be dynamically stable carbon phases with isolated or weakly overlapped flat-bands at the Fermi-level. The maximum Fermi velocities of the flat-band electrons are evaluated to be 1x10^4 m/s and 0.786x10^4 m/s, both of which are lower than the Fermi velocity of the flat-band electrons in magic-angle graphene (4x10^4 m/s). Furthermore, 191-8-66-C-r567x-1 has been confirmed to be a flat-band related magnetic half-metal with a magnetic moment of 1.854 miuB per cell, while 191-10-90-C-r567x-1 is a flat-band related magnetic normal metal with a magnetic moment of 1.663 miuB per cell. These results not only show that flat-bands can be constructed in carbon monolayer, but also indicate the potential for achieving metal-free magnetic materials with light elements based on flat-band theory.

cond-mat.mes-hall↗

arXiv2024

Switchable Ferroelectricity in Subnano Silicon Thin Films

Recent advancements underscore the critical need to develop ferroelectric materials compatible with silicon. We systematically explore possible ferroelectric silicon quantum films and discover a low-energy variant (hex-OR-2*2-P) with energy just 1 meV/atom above the ground state (hex-OR-2*2). Both hex-OR-2*2 and hex-OR-2*2-P are confirmed to be dynamically and mechanically stable semiconductors with indirect gaps of 1.323 eV and 1.311 eV, respectively. The ferroelectric hex-OR-2*2-P exhibits remarkable in-plane spontaneous polarization up to 120 Pc/m and is protected by a potential barrier (13.33 meV/atom) from spontaneously transitioning to hex-OR-22. To simulate the switching ferroelectricity in electric fields of the single-element silicon bilayer, we develop a method that simultaneously learns interatomic potentials and Born effective charges (BEC) in a single equivariant model with a physically informed loss. Our method demonstrates good performance on several ferroelectrics. Simulations of hex-OR-2*2-P silicon suggest a depolarization temperature of approximately 300 K and a coercive field of about 0.05 V/Å. These results indicate that silicon-based ferroelectric devices are feasible, and the ground state phase of the silicon bilayer (hex-OR-2*2) is an ideal system. Our findings highlight the promise of pure silicon ferroelectric materials for future experimental synthesis and applications in memory devices, sensors, and energy converters.

cond-mat.mtrl-sci↗

arXiv2023

Vertical Ferroelectricity in Van der Waals Materials: Models and Devices

Ferroelectricity has a wide range of applications in functional electronics and is extremely important for the development of next-generation information storage technology, but it is difficult to achieve due to its special symmetry requirements. In this letter, based on van derWaals stacking, a generic model is proposed for realizing ferroelectric devices, where a freely movable center layer is packaged in two fixed and symmetrically stacked layers. In this model, the ferroelectric phase transition can be realized between the two equivalent and eccentric ground stacking-states with opposite polarizations. By means of first-principles calculations, taking the h-BN/h-BN/h-BN and h-BN/Graphene/h-BN as feasible models, we carefully evaluate the magnitude of ferroelectricity. The corresponding polarizations are estimated as 1.83 and 1.35 pC/m, respectively, which are comparable to the sliding ferroelectricity. Such a new tri-layer model of vertical ferroelectricity can be constructed by arbitrary van derWaals semiconducting materials, and usually holds low switching barrier. Optimized material combinations with remarkable polarization are highly expectable to be discovered from the huge candidate set for future information storage.

cond-mat.mtrl-sci↗

arXiv2022

Flat-band based ferromagnetic semiconducting state in the graphitic C$_4$N$_3$ monolayer

A new set of lattice-models based on the hexagonal $\sqrt{N}\times\sqrt{N}$ super-cells of the well-known honeycomb lattice with single-hole defect (HL-D-1/2N) are proposed to realize the nontrivial isolated flat-bands. Through performing both tight-binding and density functional theory calculations, we demonstrate that the experimentally realized graphitic carbon nitride (Adv. Mater., 22, 1004, 2010; Nat. Commun., 9, 3366, 2018), the HL-D-1/8 based C$_4$N$_3$, is a perfect system to host such flat bands. For the flat high-energy P-6m2 C$_4$N$_3$ structure, it displays the ferromagnetic half-metallicity which is not related to the isolated flat bands. However, the P-6m2 C$_4$N$_3$ structure is dynamically unstable. Using a structure searching method based on group and graph theory, we find that a new corrugated Pca21 C4N3 structure has the lowest energy among all known C$_4$N$_3$ structures. This Pca21 C$_4$N$_3$ structure is an intrinsic ferromagnetic half-semiconductor (Tc$\approx$241 K) with one semiconducting spin-channel (1.75 eV) and one insulating spin-channel (3.64 eV), which is quite rare in the two-dimensional (2D) systems. Its ferromagnetic semiconducting property originates from the isolated p$_z$-state flat-band as the corrugation shift the flat band upward to the Fermi level. Interestingly, this Pca21 C$_4$N$_3$ structure is found to be piezoelectric and ferroelectric, which makes C$_4$N$_3$ an unusual transition-metal-free 2D multiferroic.

cond-mat.mtrl-sci↗

arXiv2022

I4/mcm-Si$_{48}$: An Ideal Topological Nodal-Line Semimetal

Topological semimetals (TSMs) have attracted numerous attention due to their exotic physical properties and great application potentials. Silicon-based TMSs are of particularly importance because of their high abundance, nontoxicity and natural compatibility with current semiconductor industry. In this work, an ideal low-energy topological nodal-line semimetal (TNLSM) silicon (I4/mcm-Si$_{48}$) with a clean band crossing at Fermi level is screened from thousands of silicon allotropes by the transferable tight-binding and DFT-HSE calculations. The results of formation energy, phonon dispersion, ab initio molecular dynamics and elastic constants show that I4/mcm-Si48 possesses good stability and is more stable than several synthetized silicon structures. By analyzing the symmetry, it reveals that the topological nodal-line of I4/mcm-Si48 is protected by mirror symmetry and inversion, time-reversal and SU(2) spin-rotation symmetries, and the nearly flat drumhead-like surface spectrum is observed. Furthermore, I4/mcm-Si48 exhibits exotic photoelectric properties and the Dirac fermions with high Fermi velocity (3.4$\sim$4.36$\times$10$^5$ m/s) can be excited by low energy photons. Our study provides a promising topological nodal-line semimetal for fundamental research and potential practical applications in semiconductor-compatible high-speed photoelectric devices.

cond-mat.mtrl-sci↗

arXiv2021

Ground state configuration of hydrogenated Biphenylene sheet: structure, stabilities, electronic and mechanical properties from first-principles calculations

Based on first-principles calculations, the ground state configuration (Cmma-CH) of hydrogenated Biphenylene sheet (Science, 372, 852, 2021) is carefully identified from hundreds of possible candidates generated by RG2 code (Phys. Rev. B., 97, 014104, 2018). Cmma-CH contains four benzene molecules in its crystalline cell and all of them are inequivalent due to its Cmma symmetry. The hydrogen atoms in Cmma-CH bond to carbon atoms in each benzene with a boat-like (boat-1:DDUDDU) up/down sequence and reversed boat-1 (UUDUUD) sequence in adjacent benzene rings. It is energetically less stable than the previously proposed allotropes (chair, tricycle, stirrup, boat-1, boat-2 and twist-boat) of hydrogenated graphene, but its formation energy from hydrogenating Biphenylene sheet is remarkably lower than those for hydrogenating graphene to graphane. Our results confirm that Cmma-CH is mechanically and dynamically stable 2D hydrocarbon phase which is expectable to be experimentally realized by hydrogenating the synthesized Biphenylene sheet. The HSE06 based band structures show that Cmma-CH is an indirect band gap insulators with a gap of 4.645 eV.

cond-mat.mtrl-sci↗

arXiv2020

New structure canditates for the experimentally synthesized heptazine-based and triazine-based two dimensional graphitic carbon nitride

The widely used crystal structures for both heptazine-based and triazine-based two-dimensional (2D) graphitic carbon nitride (g-C$_3$N$_4$) are the flat P-6m2 configurations. However, the experimentally synthesized 2D g-C$_3$N$_4$ possess thickness ranging in 0.2-0.5 nm, indicating that the theoretically used flat P-6m2 configurations are not the correct ground states. In this work, we propose three new corrugated structures P321, P3m1 and Pca21 with energies of 66 (86), 77 (87) and 78 (89) meV/atom lower than that of the corresponding heptazine-based (triazine-based) g-C$_3$N$_4$ in flat P-6m2 configuration, respectively. These corrugated structures have very similar periodic patterns to the flat P-6m2 ones and they are difficult to be distinguished from each other according to their top-views. The optimized thicknesses of the three corrugated structures ranging in 1.347-3.142 Å are in good agreement with the experimental results. The first-principles results show that these corrugated structural candidates are also semiconductors with band gaps slightly larger than those of the correspondingly flat P-6m2 ones. Furthermore, they possess also suitable band edge positions for sun-light-driven water-splitting at both $pH=0$ and $pH=7$ environments. Our results show that these three new structures are more promising candidates for the experimentally synthesized g-C$_3$N$_4$.

cond-mat.mtrl-sci↗

arXiv2020

Theoretical prediction of a low-energy Stone-Wales graphene with intrinsic type-III Dirac-cone

Based on first-principles method we predict a new low-energy Stone-Wales graphene SW40, which has an orthorhombic lattice with Pbam symmetry and 40 carbon atoms in its crystalline cell forming well-arranged Stone-Wales patterns. The calculated total energy of SW40 is just about 133 meV higher than that of graphene, indicating its excellent stability exceeds all the previously proposed graphene allotropes. We find that SW40 processes intrinsic Type-III Dirac-cone (Phys. Rev. Lett., 120, 237403, 2018) formed by band-crossing of a local linear-band and a local flat-band, which can result in highly anisotropic Fermions in the system. Interestingly, such intrinsic type-III Dirac-cone can be effectively tuned by inner-layer strains and it will be transferred into Type-II and Type-I Dirac-cones under tensile and compressed strains, respectively. Finally, a general tight-binding model was constructed to understand the electronic properties nearby the Fermi-level in SW40. The results show that type-III Dirac-cone feature can be well understood by the $π$-electron interactions between adjacent Stone-Wales defects.

physics.comp-ph↗

arXiv2019

Stone-Wales graphene: A Two Dimensional Carbon Semi-Metal with Magic Stability

A two-dimensional carbon allotrope, Stone-Wales graphene, is identified in stochastic group and graph constrained searches and systematically investigated by first-principles calculations. Stone-Wales graphene consists of well-arranged Stone-Wales defects, and it can be constructed through a 90$^\circ$ bond-rotation in a $\sqrt{8}$$\times$$\sqrt{8}$ super-cell of graphene. Its calculated energy relative to graphene, +149 meV/atom, makes it more stable than the most competitive previously suggested graphene allotropes. We find that Stone-Wales graphene based on a $\sqrt{8}$ super-cell is more stable than those based on $\sqrt{9} \times \sqrt{9}$, $\sqrt{12} \times \sqrt{12}$ and $\sqrt{13} \times \sqrt{13}$ super-cells, and is a "magic size" that can be further understood through a simple "energy splitting and inversion" model. The calculated vibrational properties and molecular dynamics of SW-graphene confirm that it is dynamically stable. The electronic structure shows SW-graphene is a semimetal with distorted, strongly anisotropic Dirac cones.

cond-mat.mtrl-sci↗

arXiv2018

Complex low energy tetrahedral polymorphs of group IV elements from first-principles

The energy landscape of carbon is exceedingly complex, hosting diverse and important metastable phases, including diamond, fullerenes, nanotubes and graphene. Searching for structures, especially those with large unit cells, in this landscape is challenging. Here we use a combined stochastic search strategy employing two algorithms (AIRSS and RG2) to apply connectivity constraints to unit cells containing up to 100 carbon atoms. We uncover three low energy carbon polymorphs (Pbam-32, P6/mmm and I-43d) with new topologies, containing 32, 36 and 94 atoms in their primitive cells, respectively. Their energies relative to diamond are 96 meV/atom, 131 meV/atom and 112 meV/atom, respectively, which suggests potential metastability. These three carbon allotropes are mechanically and dynamically stable, insulating carbon crystals with superhard mechanical properties. The I43d structure possesses a direct band gap of 7.25 eV, which is the widest gap in the carbon allotrope family. Silicon, germanium and tin versions of Pbam-32, P6/mmm and I-43d also show energetic, dynamical and mechanical stability. The computed electronic properties show that they are potential materials for semiconductor and photovoltaic applications.

cond-mat.mtrl-sci↗