Showing 1–50 of 114 results for author: Gao, M

Topological Phases in Non-Hermitian Nonlinear-Eigenvalue Systems

Authors: Yu-Peng Ma, Ming-Jian Gao, Jun-Hong An

Abstract: The discovery of topological phases has ushered in a new era of condensed matter physics and revealed a variety of natural and artificial materials. They obey the bulk-boundary correspondence (BBC), which guarantees the emergence of boundary states with non-zero topological invariants in the bulk. A wide attention has been paid to extending topological phases to nonlinear and non-Hermitian systems… ▽ More The discovery of topological phases has ushered in a new era of condensed matter physics and revealed a variety of natural and artificial materials. They obey the bulk-boundary correspondence (BBC), which guarantees the emergence of boundary states with non-zero topological invariants in the bulk. A wide attention has been paid to extending topological phases to nonlinear and non-Hermitian systems. However, the BBC and topological invariants of non-Hermitian nonlinear systems remain largely unexplored. Here, we establish a complete BBC and topological characterization of the topological phases in a class of non-Hermitian nonlinear-eigenvalue systems by introducing an auxiliary system. We restore the BBC broken by non-Hermiticity via employing the generalized Brillouin zone on the auxiliary system. Remarkably, we discover that the interplay between non-Hermiticity and nonlinearity creates an exotic complex-band topological phase that coexists with the real-band topological phase. Our results enrich the family of nonlinear topological phases and lay a foundation for exploring novel topological physics in metamaterial systems. △ Less

Submitted 17 November, 2025; originally announced November 2025.

Distortion-Driven Carrier Decoupling in Doped LiMgPO4

Authors: Zhihua Zheng, Xiaolong Yao, Cailian Yu, Menghao Gao, Fangping Ouyang, Shiwu Gao

Abstract: The interplay between lattice distortions and charge carriers governs the properties of many functional oxides. In alkali-doped LiMgPO4, a significant enhancement in dosimetric response is observed, but its microscopic origin is not understood. Using non-adiabatic molecular dynamics, we reveal a fundamental mechanism of carrier decoupling driven by a hierarchy of lattice distortions. We show that… ▽ More The interplay between lattice distortions and charge carriers governs the properties of many functional oxides. In alkali-doped LiMgPO4, a significant enhancement in dosimetric response is observed, but its microscopic origin is not understood. Using non-adiabatic molecular dynamics, we reveal a fundamental mechanism of carrier decoupling driven by a hierarchy of lattice distortions. We show that electrons localize into stable small polarons on an ultrafast timescale, trapped by the strong local potential induced by the dopant, while holes form more delocalized polarons that migrate efficiently through a lattice smoothed by global strain. The stark contrast between the dynamics of trapped electrons and mobile holes explains the suppressed recombination and enhanced energy storage. These results present a clear physical picture of how multiscale lattice distortions can independently control electron and hole transport, offering new insights into the physics of polarons in complex materials. △ Less

Submitted 1 December, 2025; v1 submitted 16 November, 2025; originally announced November 2025.

Electron-phonon coupling of one-dimensional (3,0) carbon nanotube

Authors: Zhenfeng Ouyang, Jing Jiang, Jian-Feng Zhang, Miao Gao, Kai Liu, Zhong-Yi Lu

Abstract: A very recent report claims that ambient-pressure high-temperature ($T_c$) superconductivity was found in boron-doped three-dimensional networks of carbon nanotubes (CNTs). Here, we systematically study the electron-phonon coupling (EPC) of one-dimensional (1D) (3,0) CNT under ambient pressure. Our results show that the EPC constant $λ$ of the undoped 1D (3,0) CNT is 0.70, and reduces to 0.44 afte… ▽ More A very recent report claims that ambient-pressure high-temperature ($T_c$) superconductivity was found in boron-doped three-dimensional networks of carbon nanotubes (CNTs). Here, we systematically study the electron-phonon coupling (EPC) of one-dimensional (1D) (3,0) CNT under ambient pressure. Our results show that the EPC constant $λ$ of the undoped 1D (3,0) CNT is 0.70, and reduces to 0.44 after 1.3 holes/cell doping. Further calculations show that the undoped (3,0) CNT is a two-gap superconductor with a superconducting $T_c$ $\sim$ 33 K under ambient pressure. Additionally, we identify three characteristic phonon modes with strong EPC, establishing that the pristine (3,0) CNT is a high-$T_c$ superconducting unit, and further suggest that searching for those superconducting units with strong EPC phonon mode would be an effective way to discover high-$T_c$ phonon-mediated superconductors. Our study not only provide a crucial and timely theoretical reference for the recent report regarding superconducting CNTs, but also uncover that the pristine (3,0) CNT hosts the highest record of superconducting $T_c$ among the elemental superconductors under ambient pressure. △ Less

Submitted 5 November, 2025; originally announced November 2025.

Comments: 6 pages, 5 figures, 1 table

COFAP: A Universal Framework for COFs Adsorption Prediction through Designed Multi-Modal Extraction and Cross-Modal Synergy

Authors: Zihan Li, Mingyang Wan, Mingyu Gao, Zhongshan Chen, Xiangke Wang, Feifan Zhang

Abstract: Covalent organic frameworks (COFs) are promising adsorbents for gas adsorption and separation, while identifying the optimal structures among their vast design space requires efficient high-throughput screening. Conventional machine-learning predictors rely heavily on specific gas-related features. However, these features are time-consuming and limit scalability, leading to inefficiency and labor-… ▽ More Covalent organic frameworks (COFs) are promising adsorbents for gas adsorption and separation, while identifying the optimal structures among their vast design space requires efficient high-throughput screening. Conventional machine-learning predictors rely heavily on specific gas-related features. However, these features are time-consuming and limit scalability, leading to inefficiency and labor-intensive processes. Herein, a universal COFs adsorption prediction framework (COFAP) is proposed, which can extract multi-modal structural and chemical features through deep learning, and fuse these complementary features via cross-modal attention mechanism. Without Henry coefficients or adsorption heat, COFAP sets a new SOTA by outperforming previous approaches on hypoCOFs dataset. Based on COFAP, we also found that high-performing COFs for separation concentrate within a narrow range of pore size and surface area. A weight-adjustable prioritization scheme is also developed to enable flexible, application-specific ranking of candidate COFs for researchers. Superior efficiency and accuracy render COFAP directly deployable in crystalline porous materials. △ Less

Submitted 3 November, 2025; originally announced November 2025.

Switchable chiral 2x2 pair density wave in pure CsV3Sb5

Authors: Wei Song, Xiao-Yu Yan, Xin Yu, Desheng Wu, Deng Hu, Hailang Qin, Guowei Liu, Hanbin Deng, Chao Yan. Muwei Gao, Zhiwei Wang, Rui Wu, Jia-Xin Yin

Abstract: We investigate electron pairing in a super clean kagome superconductor CsV3Sb5 with a residual resistivity ratio (RRR) of 290. By using the dilution-refrigerator-based scanning tunneling microscopy (STM) at the Synergetic Extreme Condition User Facility (SECUF), we find that the pairing gap exhibits chiral 2x2 modulations, and their chirality can be controlled by magnetic field training. We introd… ▽ More We investigate electron pairing in a super clean kagome superconductor CsV3Sb5 with a residual resistivity ratio (RRR) of 290. By using the dilution-refrigerator-based scanning tunneling microscopy (STM) at the Synergetic Extreme Condition User Facility (SECUF), we find that the pairing gap exhibits chiral 2x2 modulations, and their chirality can be controlled by magnetic field training. We introduce nonmagnetic impurities to observe the complete suppression of 2x2 pairing modulations in presence of persistent 2x2 charge order. This nonmagnetic pair-breaking effect provides phase-sensitive evidence for pair-density-wave (PDW) induced pairing modulations. Our results support switchable chiral 2x2 PDW in this super clean kagome superconductor. △ Less

Submitted 14 October, 2025; originally announced October 2025.

Phase-sensitive evidence for 2x2 pair density wave in a kagome superconductor

Authors: Xiao-Yu Yan, Guowei Liu, Hanbin Deng, Xitong Xu, Haiyang Ma, Hailang Qin, Jun-Yi Zhang, Yuanyuan Zhao, Haitian Zhao, Zhe Qu, Yigui Zhong, Kozo Okazaki, Xiquan Zheng, Yingying Peng, Zurab Guguchia, X. X. Wu, Qianghua Wang, X-H Fan, Wei Song, M-W Gao, Hendrik Hohmann, Matteo Durrnagel, Ronny Thomale, Jia-Xin Yin

Abstract: The pair-density-wave (PDW) exhibits periodic amplitude and sign modulations of the superconducting order parameter. Such a pairing state has been proposed to be sensitive to nonmagnetic scattering. In this work, we observe the nonmagnetic PDW-breaking effect in a kagome superconductor, using scanning tunneling microscopy. We observe 2x2 PDW induced by the coupling between charge order and superco… ▽ More The pair-density-wave (PDW) exhibits periodic amplitude and sign modulations of the superconducting order parameter. Such a pairing state has been proposed to be sensitive to nonmagnetic scattering. In this work, we observe the nonmagnetic PDW-breaking effect in a kagome superconductor, using scanning tunneling microscopy. We observe 2x2 PDW induced by the coupling between charge order and superconductivity. The global PDW is substantially suppressed upon doping the kagome lattice with dilute isovalent nonmagnetic impurities, whereas the charge order and uniform superconductivity remain robust. Spatial correlation analysis further confirms that PDW is distinctly suppressed near dopants. We attribute the PDW suppression to a nonmagnetic PDW breaking effect, arising from phase sign modulation of PDW in the kagome d-orbital hosting Bogoliubov Fermi states. △ Less

Submitted 12 October, 2025; originally announced October 2025.

Learning from the electronic structure of molecules across the periodic table

Authors: Manasa Kaniselvan, Benjamin Kurt Miller, Meng Gao, Juno Nam, Daniel S. Levine

Abstract: Machine-Learned Interatomic Potentials (MLIPs) require vast amounts of atomic structure data to learn forces and energies, and their performance continues to improve with training set size. Meanwhile, the even greater quantities of accompanying data in the Hamiltonian matrix H behind these datasets has so far gone unused for this purpose. Here, we provide a recipe for integrating the orbital inter… ▽ More Machine-Learned Interatomic Potentials (MLIPs) require vast amounts of atomic structure data to learn forces and energies, and their performance continues to improve with training set size. Meanwhile, the even greater quantities of accompanying data in the Hamiltonian matrix H behind these datasets has so far gone unused for this purpose. Here, we provide a recipe for integrating the orbital interaction data within H towards training pipelines for atomic-level properties. We first introduce HELM ("Hamiltonian-trained Electronic-structure Learning for Molecules"), a state-of-the-art Hamiltonian prediction model which bridges the gap between Hamiltonian prediction and universal MLIPs by scaling to H of structures with 100+ atoms, high elemental diversity, and large basis sets including diffuse functions. To accompany HELM, we release a curated Hamiltonian matrix dataset, 'OMol_CSH_58k', with unprecedented elemental diversity (58 elements), molecular size (up to 150 atoms), and basis set (def2-TZVPD). Finally, we introduce 'Hamiltonian pretraining' as a method to extract meaningful descriptors of atomic environments even from a limited number atomic structures, and repurpose this shared embedding space to improve performance on energy-prediction in low-data regimes. Our results highlight the use of electronic interactions as a rich and transferable data source for representing chemical space. △ Less

Submitted 7 December, 2025; v1 submitted 30 September, 2025; originally announced October 2025.

Breakdown of symmetry constraint in Floquet topological superconductor

Authors: Ming-Jian Gao, Jun-Hong An

Abstract: Topological superconductor is regarded as an ideal candidate for topological quantum computing due to its ability to simulate the enigmatic Majorana fermions that satisfy non-Abelian statistics. Previous studies revealed that symmetry exerts an unbreakable constraint on the existence, classes, and orders of Majorana modes. It severely limits the controllability and application of Majorana modes. H… ▽ More Topological superconductor is regarded as an ideal candidate for topological quantum computing due to its ability to simulate the enigmatic Majorana fermions that satisfy non-Abelian statistics. Previous studies revealed that symmetry exerts an unbreakable constraint on the existence, classes, and orders of Majorana modes. It severely limits the controllability and application of Majorana modes. Here, we propose a Floquet-engineering method to break this symmetry-imposed constraint on topological phases. By applying periodic driving on a system belonging to a symmetry class that prohibits the existence of first-order topological phases, we find that rich first-order Majorana modes are created. Interestingly, exotic hybrid-order topological superconductors with coexisting first-order Majorana boundary modes and second-order Majorana corner modes not only in two different quasienergy gaps but also in one single gap are generated at ease by the periodic driving. Refreshing the prevailing understanding of symmetry constraint on topological phases, our result opens an avenue for the creation of exotic topological superconductors without altering symmetries. It greatly expands the scope of the fabricated materials that host topological superconductor. △ Less

Submitted 24 September, 2025; originally announced September 2025.

Quantum anomalous Hall effect with high Chern number in two dimensional ferromagnets Ti2TeSO

Authors: Panjun Feng, Miao Gao, Xun-Wang Yan, Fengjie Ma

Abstract: Two-dimensional Chern insulators have emerged as crucial platforms for the realization of the quantum anomalous Hall effect, and as such have attracted significant interest in spintronics and topological quantum physics due to their unique coexistence of spontaneous magnetization and nontrivial topological characteristics. Nonetheless, substantial challenges persist in such systems, encompassing s… ▽ More Two-dimensional Chern insulators have emerged as crucial platforms for the realization of the quantum anomalous Hall effect, and as such have attracted significant interest in spintronics and topological quantum physics due to their unique coexistence of spontaneous magnetization and nontrivial topological characteristics. Nonetheless, substantial challenges persist in such systems, encompassing spin entanglement and the possession of only one edge state (Chern number C=1), which significantly hinder their practical applications. Herein, we propose a novel two-dimensional ferromagnetic half-semi-Weyl-metal, monolayer Ti2TeSO, that exhibits exceptional electronic properties. Its majority spin channel possesses only a pair of symmetry-protected Weyl points at the Fermi level, while the states of minority one locate far away from the Fermi level. When spin-orbit coupling is included, a substantial band gap of ~ 92.8 meV is induced at the Weyl points. Remarkably, the emergence of dual dissipationless chiral edge channels and a quantized Hall conductivity plateau at 2e2/h collectively establish monolayer Ti2TeSO as a high-Chern-number insulator with C=2. Furthermore, it is demonstrated that valley polarization can be achieved and controlled through the application of strain and the manipulation of the direction of magnetization. The first-principles calculations, in conjunction with Monte Carlo simulations, yield a Curie temperature of 161 K for monolayer Ti2TeSO, thereby indicating the plausibility of coexistence of valley polarization and topological states at elevated temperatures. These findings could provide a foundation for the development of multi-channels dissipationless transport devices and nonvolatile multistate memory architectures. △ Less

Submitted 2 September, 2025; originally announced September 2025.

Violation of Luttinger's theorem in one-dimensional interacting fermions

Authors: Meng Gao, Yin Zhong

Abstract: Using the density matrix renormalization group method, we systematically investigate the evolution of the Luttinger integral in the one-dimensional generalized $t$-$V$ model as a function of filling and interaction strength, and identify three representative phases. In the weak-coupling regime, the zero-frequency Green's function exhibits a branch-cut structure at the Fermi momentum, and the Lutti… ▽ More Using the density matrix renormalization group method, we systematically investigate the evolution of the Luttinger integral in the one-dimensional generalized $t$-$V$ model as a function of filling and interaction strength, and identify three representative phases. In the weak-coupling regime, the zero-frequency Green's function exhibits a branch-cut structure at the Fermi momentum, and the Luttinger integral accurately reflects the particle density, indicating that the Luttinger theorem holds. As the interaction increases, the spectral weight near the Fermi momentum is gradually suppressed. Interestingly, in the strong coupling regime near half-filling, this singularity is progressively destroyed, accompanied by the emergence of momentum-space zeros in the real part of the Green's function, leading to a novel non-Fermi liquid metallic phase beyond the classic Luttinger liquid paradigm, where the Luttinger surface is no longer defined by a single singularity. While finite spectral weight remains at the original Fermi momentum, the singularity gradually diminishes. Meanwhile, zeros with negligible spectral weight appear away from this momentum, significantly affecting the integral. At exact half-filling, a single-particle gap opens, and the Green's function becomes nearly vanishing across the entire momentum space, indicating the complete suppression of low-energy electronic states consistent with the nature of an insulating charge-density-wave phase. These results suggest that the breakdown of the Luttinger theorem is not triggered by a single mechanism, but rather results from the interplay between interaction-driven evolution of excitation modes and the breaking of particle-hole symmetry, ultimately leading to a continuous reconstruction of the generalized Fermi surface from topologically protected to correlation-driven. △ Less

Submitted 12 September, 2025; v1 submitted 4 June, 2025; originally announced June 2025.

Comments: 11 pages, 8 figures; Accepted for publication in Phys. Rev. B (2025)

Report number: BF15117

Converting $PT$-Symmetric Topological Classes by Floquet Engineering

Authors: Ming-Jian Gao, Jun-Hong An

Abstract: Going beyond the conventional classification rule of Altland-Zirnbauer symmetry classes, $PT$ symmetric topological phases are classified by $(PT)^2=1$ or $-1$. The interconversion between the two $PT$-symmetric topological classes is generally difficult due to the constraint of $(PT)^2$. Here, we propose a scheme to control and interconvert the $PT$-symmetric topological classes by Floquet engine… ▽ More Going beyond the conventional classification rule of Altland-Zirnbauer symmetry classes, $PT$ symmetric topological phases are classified by $(PT)^2=1$ or $-1$. The interconversion between the two $PT$-symmetric topological classes is generally difficult due to the constraint of $(PT)^2$. Here, we propose a scheme to control and interconvert the $PT$-symmetric topological classes by Floquet engineering. We find that it is the breakdown of the $\mathbb{Z}_2$ gauge, induced by the $π$ phase difference between different hopping rates, by the periodic driving that leads to such an interconversion. Relaxing the system from the constraint of $(PT)^2$, rich exotic topological phases, e.g., the coexisting $PT$-symmetric first-order real Chern insulator and second-order topological insulators not only in different quasienergy gaps, but also in one single gap, are generated. In contrast to conventional Floquet topological phases, our result provides a way to realize exotic topological phases without changing symmetries. It enriches the family of topological phases and gives an insightful guidance for the development of multifunctional quantum devices. △ Less

Submitted 20 April, 2025; originally announced April 2025.

Comments: 7 pages and 4 figures

Multiple topological corner states in the continuum of extended kagome lattice

Authors: Shun-Peng Zhang, Ming-Jian Gao, Wei Jia, Jun-Hong An

Abstract: The kagome lattice is renowned for its exotic electronic properties, such as flat bands, Dirac points, and Van Hove singularities. These features have provided a fertile ground for exploring exotic quantum phenomena. Here, we discover that a breathing kagome lattice with long-range hoppings can host multiple zero-energy corner states, which emerge as topologically protected bound states in the con… ▽ More The kagome lattice is renowned for its exotic electronic properties, such as flat bands, Dirac points, and Van Hove singularities. These features have provided a fertile ground for exploring exotic quantum phenomena. Here, we discover that a breathing kagome lattice with long-range hoppings can host multiple zero-energy corner states, which emerge as topologically protected bound states in the continuum (BICs). This result demonstrates that additional hopping control can induce further non-trivial physics of the kagome lattice. Since the zero-energy corner states in the continuum are intertwined with a substantial number of zero-energy bulk states, we also develop a momentum-space topological characterization theory to precisely quantify the number of corner states, revealing a general bulk-corner correspondence. Furthermore, we uncover three distinct types of topological phase transitions (TPTs) for the BICs driven by shifts in the spatial localization of zero-energy bulk and/or edge states. These TPTs are exactly captured by our characterization theory. This work provides deep insights into the topological physics of the kagome lattice and broadens the understanding of its electronic properties △ Less

Submitted 18 September, 2025; v1 submitted 1 April, 2025; originally announced April 2025.

Comments: 8+6 pages,4+1 figures

Journal ref: Phys. Rev. B 112, L121106 (2025)

Type-II quantum spin Hall insulator

Authors: Panjun Feng, Chao-Yang Tan, Miao Gao, Xun-Wang Yan, Zheng-Xin Liu, Peng-Jie Guo, Fengjie Ma, Zhong-Yi Lu

Abstract: Quantum spin Hall effect is usually realized in two-dimensional materials with time-reversal symmetry, but whether it can be realized without symmetry protection remains unexplored. Here, we propose type-II quantum spin Hall insulator with quantized spin Hall conductivity, whose edge states with opposite chirality and polarization, distributed in different Brillouin zone regions, connect the condu… ▽ More Quantum spin Hall effect is usually realized in two-dimensional materials with time-reversal symmetry, but whether it can be realized without symmetry protection remains unexplored. Here, we propose type-II quantum spin Hall insulator with quantized spin Hall conductivity, whose edge states with opposite chirality and polarization, distributed in different Brillouin zone regions, connect the conduction and valence bands at the boundary. Thus, the type-II quantum spin Hall insulator does not require any symmetry protection other than translational symmetry. Then, based on symmetry analysis and the first-principles electronic structure calculations, we demonstrate that type-II quantum spin Hall insulator can be realized in both altermagnetic materials and Luttinger compensated magnetic materials. Furthermore, based on lattice model, we find that as long as $U(1)$ symmetry exists, type-II quantum spin Hall insulator phase can always exist stably. However, if $U(1)$ symmetry is broken, type-II quantum spin Hall insulator phase transforms into an obstructed atomic insulator phase as spin-orbit coupling effect is enhanced. Therefore, our work not only proposes a new mechanism for realizing the quantum spin Hall effect, but also enriches the types of unconventional magnetic topological phases. △ Less

Submitted 31 March, 2025; v1 submitted 17 March, 2025; originally announced March 2025.

Comments: 6pages, 4 figures

Prediction of two-dimensional ferromagnetic VO$_2$ layers in the hexagonal and tetragonal phases

Authors: Lihui Han, Lujia Tian, Bing-Xin Liu, Zong-liang Li, Miao Gao, Fengjie Ma, Xun-Wang Yan

Abstract: Ferromagnetism in the two-dimensional materials is of great significance and has become an emerging topic. The ferromagnetic VS$_2$ and VSe$_2$ monolayers have been experimentally synthesized, and O element belongs to the same group as S and Se elements. Thus, whether there exists the ferromagnetic VO$_2$ monolayer is a necessary and urgent question. Using first-principles methods within the frame… ▽ More Ferromagnetism in the two-dimensional materials is of great significance and has become an emerging topic. The ferromagnetic VS$_2$ and VSe$_2$ monolayers have been experimentally synthesized, and O element belongs to the same group as S and Se elements. Thus, whether there exists the ferromagnetic VO$_2$ monolayer is a necessary and urgent question. Using first-principles methods within the framework of density functional theory, we predict two kinds of VO$_2$ monolayers with the hexagonal and tetragonal phases and investigate their structural stability, electronic and magnetic properties, and ferromagnetic phase transition. The computational results demonstrate that the two two-dimensional structural phases are stable and possess the ferromagnetic ground states, and they are half-metal with large energy gap. In addition, by solving the Heisenberg model with the Monte Carlo simulation methods, the ferromagnetic phase transition at 270 K in the hexagonal phase is determined. These findings not only predict a new type of intrinsic half-metallic ferromagnet with a high Curie temperature but also fill in an important gap that are lacking in the series of studies from VO$_2$, VS$_2$, VSe$_2$, to VTe$_2$. △ Less

Submitted 13 March, 2025; originally announced March 2025.

Huge Stress-induced Adiabatic Temperature Change in a High-Toughness All-d-metal Heusler Alloy

Authors: Rui Cai, Zhiyang Wei, Hongjie Ren, Hanyang Qian, Xinyu Zhang, Yao Liu, Xiang Lu, Wen Sun, Meng Gao, Enke Liu, Jian Liu, Guowei Li

Abstract: The elastocaloric effect (eCE), referring to the thermal effect triggered by a uniaxial stress, provides a promising and versatile routine for green and high efficient thermal management. However, current eCE materials generally suffer from relatively low eCE and poor mechanical properties, hindering their practical applications. Here, we report a exceptionally huge eCE with a directly measured ad… ▽ More The elastocaloric effect (eCE), referring to the thermal effect triggered by a uniaxial stress, provides a promising and versatile routine for green and high efficient thermal management. However, current eCE materials generally suffer from relatively low eCE and poor mechanical properties, hindering their practical applications. Here, we report a exceptionally huge eCE with a directly measured adiabatic temperature change of up to 57.2 K in a dual-phase all-d-metal Heusler Mn50Ni37.5Ti12.5 polycrystalline alloy, revealing an extra contribution to the latent heat during the stress-induced martensitic transformation from B2 to L10, and breaking the record of adiabatic temperature change for elastocaloric alloys. Moreover, thanks to the combined strengthening effect of d-d hybridization and well-dispersed secondary cubic γ phase, the alloy can endure a uniaxial stress up to 1760 MPa. Such an abnormal huge eCE is attributed to the combination of the enhanced entropy change associated with a stress-induced B2 to L10 martensitic transformation under higher stress, in contrast with the thermally induced B2 to 5-layer modulated structure one, and the high transformation fraction due to the multi-point nucleation facilitated by the γ phase dispersed in the main phase. This work provides insight into making full use of the transformation heat to enhance the caloric effect for high-efficient thermal management systems. △ Less

Submitted 3 March, 2025; originally announced March 2025.

Bright hybrid excitons in molecularly tunable bilayer crystals

Authors: Tomojit Chowdhury, Aurélie Champagne, Patrick Knüppel, Zehra Naqvi, Ariana Ray, Mengyu Gao, David A. Muller, Nathan Guisinger, Kin Fai Mak, Jeffrey B. Neaton, Jiwoong Park

Abstract: Bilayer crystals, built by stacking crystalline monolayers, generate interlayer potentials that govern excitonic phenomena but are constrained by fixed covalent lattices and orientations. Replacing one layer with an atomically thin molecular crystal overcomes this limitation, as diverse functional groups enable tunable molecular lattices and interlayer potentials, tailoring a wide range of exciton… ▽ More Bilayer crystals, built by stacking crystalline monolayers, generate interlayer potentials that govern excitonic phenomena but are constrained by fixed covalent lattices and orientations. Replacing one layer with an atomically thin molecular crystal overcomes this limitation, as diverse functional groups enable tunable molecular lattices and interlayer potentials, tailoring a wide range of excitonic properties. Here, we report hybrid excitons in four-atom-thick hybrid bilayer crystals (HBCs), directly synthesized with single-crystalline perylene diimide (PDI) molecular crystal atop WS2 monolayers. These excitons arise from a hybridized bilayer band structure, revealed by lattice-scale first-principles calculations, inheriting properties from both monolayers. They exhibit bright photoluminescence with near-unity polarization above and below the WS2 bandgap, along with spectral signatures of exciton delocalization, supported by theory, while their energies and intensities are tuned by modifying the HBC composition by synthesis. Our work introduces a molecule-based 2D quantum materials platform for bottom-up design and control of optoelectronic properties. △ Less

Submitted 19 February, 2025; originally announced February 2025.

Comments: arXiv admin note: text overlap with arXiv:2412.12027

Liquid crystalline structures formed by sphere-rod amphiphilic molecules in solvents

Authors: Nilanthi P. Haputhanthrige, Yifan Zhou, Jingfan Wei, Min Gao, Tianbo Liu, Oleg D. Lavrentovich

Abstract: Self-assembly of amphiphilic molecules is an important phenomenon attracting a broad range of research. In this work, we study the self-assembly of KTOF4 sphere-rod amphiphilic molecules in mixed water-dioxane solvents. The molecules are of a T-shaped geometry, comprised of a hydrophilic spherical Keggin-type cluster attached by a flexible bridge to the center of a hydrophobic rod-like oligodialky… ▽ More Self-assembly of amphiphilic molecules is an important phenomenon attracting a broad range of research. In this work, we study the self-assembly of KTOF4 sphere-rod amphiphilic molecules in mixed water-dioxane solvents. The molecules are of a T-shaped geometry, comprised of a hydrophilic spherical Keggin-type cluster attached by a flexible bridge to the center of a hydrophobic rod-like oligodialkylfluorene (OF), which consists of four OF units. Transmission electron microscopy (TEM) uncovers self-assembled spherical structures of KTOF4 in dilute solutions. These spheres are filled with smectic-like layers of KTOF4 separated by layers of the solution. There are two types of layer packings: (i) concentric spheres and (ii) flat layers. The concentric spheres form when the dioxane volume fraction in the solution is 35-50 vol%. The flat layers are formed when the dioxane volume fraction is either below (20 and 30 vol%.) or above (55 and 60 vol%.) the indicated range. The layered structures show no in-plane orientational order and thus resemble thermotropic smectic A liquid crystals and their lyotropic analogs. The layered packings reveal edge and screw dislocations. Evaporation of the solvent produces a bulk birefringent liquid crystal phase with textures resembling the ones of uniaxial nematic liquid crystals. These findings demonstrate that sphere-rod molecules produce a variety of self-assembled structures that are controlled by the solvent properties. △ Less

Submitted 10 February, 2025; originally announced February 2025.

Comments: This article has 21 pages, 7 figures and one Table

MSC Class: 00 ACM Class: E.1

Topological semimetal with coexisting nodal points and nodal lines

Authors: Bing-Bing Luo, Ming-Jian Gao, Jun-Hong An

Abstract: Featuring exotic quantum transport and surface states, topological semimetals can be classified into nodal-point, nodal-line, and nodal-surface semimetals according to the degeneracy and dimensionality of their nodes. However, a topological semimetal that possesses both nodal points and nodal lines is rarely reported. Here, we propose a scheme to construct this type of topological semimetal, which… ▽ More Featuring exotic quantum transport and surface states, topological semimetals can be classified into nodal-point, nodal-line, and nodal-surface semimetals according to the degeneracy and dimensionality of their nodes. However, a topological semimetal that possesses both nodal points and nodal lines is rarely reported. Here, we propose a scheme to construct this type of topological semimetal, which simultaneously exhibits hinge Fermi arcs and drumhead surface states. Then, by applying periodic driving on the system, we find a hybrid-order topological semimetal with nodal points and rich nodal-line structures and its conversion into a first-order topological semimetal, which are absent in a static system. Our results enrich the family of topological semimetals, and establish a foundation for further exploration of their potential applications. △ Less

Submitted 16 July, 2025; v1 submitted 23 January, 2025; originally announced January 2025.

Journal ref: Phys. Rev. B 112, 045126 (2025)

The critical role of entropy in glass transition kinetics

Authors: Lijian Song, Meng Gao, Juntao Huo, Li-Min Wang, Yuanzheng Yue, Jun-Qiang Wang

Abstract: Glass transition is a reversible transition that occurs in most amorphous materials. However, the nature of glass transition remains far from being clarified. A key to understand the glass transition is to clarify what determines the glass transition temperature (Tg) and liquid fragility (m). Here the glass transition thermodynamics for 150 different glass-forming systems are studied statistically… ▽ More Glass transition is a reversible transition that occurs in most amorphous materials. However, the nature of glass transition remains far from being clarified. A key to understand the glass transition is to clarify what determines the glass transition temperature (Tg) and liquid fragility (m). Here the glass transition thermodynamics for 150 different glass-forming systems are studied statistically. It is found that the activation characters in the energy landscape are crucial to precisely portray the glass transition and, in particular, both the activation free energy (G*) and the activation entropy (S*) play critical roles. G* determines Tg, Tg=G*/290+25.5, while S* determines m, m=S*/Rln10+15 with R is gas constant. Based on the Boltzmann definition of entropy, the fragility is an indication of the number of the degeneracy of the evolution paths. This explains why the nano-confined, low-dimension or high-pressured glasses exhibit stronger characteristics, which has been a puzzling phenomenon for a long time. △ Less

Submitted 20 January, 2025; originally announced January 2025.

Machine Learning Relationships between Nanoporous Structures and Electrochemical Performance in MOF Supercapacitors

Authors: Zhenxiang Wang, Taizheng Wu, Liang Zeng, Jiaxing Peng, Ding Yu, Ming Gao, Guang Feng

Abstract: The development of supercapacitors is impeded by the unclear relationships between nanoporous electrode structures and electrochemical performance, primarily due to challenges in decoupling the complex interdependencies of various structural descriptors. While machine learning (ML) techniques offer a promising solution, their application is hindered by the lack of large, unified databases. Herein,… ▽ More The development of supercapacitors is impeded by the unclear relationships between nanoporous electrode structures and electrochemical performance, primarily due to challenges in decoupling the complex interdependencies of various structural descriptors. While machine learning (ML) techniques offer a promising solution, their application is hindered by the lack of large, unified databases. Herein, we use constant-potential molecular simulation to construct a unified supercapacitor database with hundreds of metal-organic framework (MOF) electrodes. Leveraging this database, well-trained decision-tree-based ML models achieve fast, accurate, and interpretable predictions of capacitance and charging rate, experimentally validated by a representative case. SHAP analyses reveal that specific surface area (SSA) governs gravimetric capacitance while pore size effects are minimal, attributed to the strong dependence of electrode-ion coordination on SSA rather than pore size. SSA and porosity, respectively, dominate volumetric capacitance in 1D-pore and 3D-pore MOFs, pinnacling the indispensable effects of pore dimensionality. Meanwhile, porosity is found to be the most decisive factor in the charging rate for both 1D-pore and 3D-pore MOFs. Especially for 3D-pore MOFs, an exponential increase with porosity is observed in both ionic conductance and in-pore ion diffusion coefficient, ascribed to loosened ion packing. These findings provide profound insights for the design of high-performance supercapacitor electrodes. △ Less

Submitted 15 January, 2025; originally announced January 2025.

Molecular tuning of excitons in four-atom-thick hybrid bilayer crystals

Authors: Tomojit Chowdhury, Aurélie Champagne, Patrick Knüppel, Zehra Naqvi, Mengyu Gao, Nathan Guisinger, Kin Fai Mak, Jeffrey B. Neaton, Jiwoong Park

Abstract: Bilayer crystals, formed by stacking monolayers of two-dimensional (2D) crystals, create interlayer potentials that govern excitonic phenomena but are constrained by their fixed covalent lattices. Replacing one layer with an atomically thin molecular crystal overcomes this limitation, as precise control of functional groups enables tunable 2D molecular lattices and, consequently, electronic struct… ▽ More Bilayer crystals, formed by stacking monolayers of two-dimensional (2D) crystals, create interlayer potentials that govern excitonic phenomena but are constrained by their fixed covalent lattices. Replacing one layer with an atomically thin molecular crystal overcomes this limitation, as precise control of functional groups enables tunable 2D molecular lattices and, consequently, electronic structures. Here, we report molecular tuning of lattices and excitons in four-atom-thick hybrid bilayer crystals (HBCs), synthesized as monolayers of perylene-based molecular and transition metal dichalcogenide (TMD) single crystals. In HBCs, we observe an anisotropic photoluminescence signal exhibiting characteristics of both molecular and TMD excitons, directly tuned by molecular geometry and HBC composition. Ab initio calculations reveal that this anisotropic emission arises from hybrid excitons, which inherit properties from both layers through a hybridized bilayer band structure. Our work establishes a synthetically derived, molecule-based 2D quantum materials platform with the potential for engineering interlayer potentials. △ Less

Submitted 8 February, 2025; v1 submitted 16 December, 2024; originally announced December 2024.

Comments: This revised version of the paper has undergone substantial modifications, and a portion of these changes has been submitted as a separate work under the ID arXiv:2502.01629

Discovery of an Antiferromagnetic Topological Nodal-line Kondo Semimetal

Authors: D. F. Liu, Y. F. Xu, H. Y. Hu, J. Y. Liu, T. P. Ying, Y. Y. Lv, Y. Jiang, C. Chen, Y. H. Yang, D. Pei, D. Prabhakaran, M. H. Gao, J. J. Wang, Q. H. Zhang, F. Q. Meng, B. Thiagarajan, C. Polley, M. Hashimoto, D. H. Lu, N. B. M. Schröter, V. N. Strocov, A. Louat, C. Cacho, D. Biswas, T. -L. Lee , et al. (12 additional authors not shown)

Abstract: The symbiosis of strong interactions, flat bands, topology and symmetry has led to the discovery of exotic phases of matter, including fractional Chern insulators, correlated moiré topological superconductors, and Dirac and Weyl semimetals. Correlated metals, such as those present in Kondo lattices, rely on the screening of local moments by a sea of non-magnetic conduction electrons. Here, we repo… ▽ More The symbiosis of strong interactions, flat bands, topology and symmetry has led to the discovery of exotic phases of matter, including fractional Chern insulators, correlated moiré topological superconductors, and Dirac and Weyl semimetals. Correlated metals, such as those present in Kondo lattices, rely on the screening of local moments by a sea of non-magnetic conduction electrons. Here, we report on a unique topological Kondo lattice compound, CeCo2P2, where the Kondo effect - whose existence under the magnetic Co phase is protected by PT symmetry - coexists with antiferromagnetic order emerging from the flat bands associated with the Co atoms. Remarkably, this is the only known Kondo lattice compound where magnetic order occurs in non-heavy electrons, and puzzlingly, at a temperature significantly higher than that of the Kondo effect. Furthermore, at low temperatures, the emergence of the Kondo effect, in conjunction with a glide-mirror-z symmetry, results in a nodal line protected by bulk topology near the Fermi energy. These unusual properties, arising from the interplay between itinerant and correlated electrons from different constituent elements, lead to novel quantum phases beyond the celebrated topological Kondo insulators and Weyl Kondo semimetals. CeCo2P2 thus provides an ideal platform for investigating narrow bands, topology, magnetism, and the Kondo effect in strongly correlated electron systems. △ Less

Submitted 21 November, 2024; originally announced November 2024.

Comments: 17pages,4 figures

Open Materials 2024 (OMat24) Inorganic Materials Dataset and Models

Authors: Luis Barroso-Luque, Muhammed Shuaibi, Xiang Fu, Brandon M. Wood, Misko Dzamba, Meng Gao, Ammar Rizvi, C. Lawrence Zitnick, Zachary W. Ulissi

Abstract: The ability to discover new materials with desirable properties is critical for numerous applications from helping mitigate climate change to advances in next generation computing hardware. AI has the potential to accelerate materials discovery and design by more effectively exploring the chemical space compared to other computational methods or by trial-and-error. While substantial progress has b… ▽ More The ability to discover new materials with desirable properties is critical for numerous applications from helping mitigate climate change to advances in next generation computing hardware. AI has the potential to accelerate materials discovery and design by more effectively exploring the chemical space compared to other computational methods or by trial-and-error. While substantial progress has been made on AI for materials data, benchmarks, and models, a barrier that has emerged is the lack of publicly available training data and open pre-trained models. To address this, we present a Meta FAIR release of the Open Materials 2024 (OMat24) large-scale open dataset and an accompanying set of pre-trained models. OMat24 contains over 110 million density functional theory (DFT) calculations focused on structural and compositional diversity. Our EquiformerV2 models achieve state-of-the-art performance on the Matbench Discovery leaderboard and are capable of predicting ground-state stability and formation energies to an F1 score above 0.9 and an accuracy of 20 meV/atom, respectively. We explore the impact of model size, auxiliary denoising objectives, and fine-tuning on performance across a range of datasets including OMat24, MPtraj, and Alexandria. The open release of the OMat24 dataset and models enables the research community to build upon our efforts and drive further advancements in AI-assisted materials science. △ Less

Submitted 16 October, 2024; originally announced October 2024.

Comments: 19 pages

A quasi-ohmic back contact achieved by inserting single-crystal graphene in flexible Kesterite solar cells

Authors: Yixiong Ji, Wentong Yang, Di Yan, Wei Luo, Jialu Li, Shi Tang, Jintao Fu, James Bullock, Mei Gao, Xin Li, Zhancheng Li, Jun Yang, Xingzhan Wei, Haofei Shi, Fangyang Liu, Paul Mulvaney

Abstract: Flexible photovoltaics with a lightweight and adaptable nature that allows for deployment on curved surfaces and in building facades have always been a goal vigorously pursued by researchers in thin-film solar cell technology. The recent strides made in improving the sunlight-to-electricity conversion efficiency of kesterite Cu$_{2}$ZnSn(S, Se)$_{4}$ (CZTSSe) suggest it to be a perfect candidate.… ▽ More Flexible photovoltaics with a lightweight and adaptable nature that allows for deployment on curved surfaces and in building facades have always been a goal vigorously pursued by researchers in thin-film solar cell technology. The recent strides made in improving the sunlight-to-electricity conversion efficiency of kesterite Cu$_{2}$ZnSn(S, Se)$_{4}$ (CZTSSe) suggest it to be a perfect candidate. However, making use of rare Mo foil in CZTSSe solar cells causes severe problems in thermal expansion matching, uneven grain growth, and severe problems at the back contact of the devices. Herein, a strategy utilizing single-crystal graphene to modify the back interface of flexible CZTSSe solar cells is proposed. It will be shown that the insertion of graphene at the Mo foil/CZTSSe interface provides strong physical support for the subsequent deposition of the CZTSSe absorber layer, improving the adhesion between the absorber layer and the Mo foil substrate. Additionally, the graphene passivates the rough sites on the surface of the Mo foil, enhancing the chemical homogeneity of the substrate, and resulting in a more crystalline and homogeneous CZTSSe absorber layer on the Mo foil substrate. The detrimental reaction between Mo and CZTSSe has also been eliminated. Through an analysis of the electrical properties, it is found that the introduction of graphene at the back interface promotes the formation of a quasi-ohmic contact at the back contact, decreasing the back contact barrier of the solar cell, and leading to efficient collection of charges at the back interface. This investigation demonstrates that solution-based CZTSSe photovoltaic devices could form the basis of cheap and flexible solar cells. △ Less

Submitted 28 August, 2024; originally announced August 2024.

Screening of half-Heuslers with temperature-induced band convergence and enhanced thermoelectric properties

Authors: Jinyang Xi, Zirui Dong, Menghan Gao, Jun Luo, Jiong Yang

Abstract: Enhancing band convergence is an effective way to optimize the thermoelectric (TE) properties of materials. However, the temperature-induced band renormalization is commonly ignored. By employing the recently-developed electron-phonon renormalization (EPR) method, the nature of band renormalization in half-Heusler (HH) compounds TiCoSb and NbFeSb is revealed, and the key factors for temperature-in… ▽ More Enhancing band convergence is an effective way to optimize the thermoelectric (TE) properties of materials. However, the temperature-induced band renormalization is commonly ignored. By employing the recently-developed electron-phonon renormalization (EPR) method, the nature of band renormalization in half-Heusler (HH) compounds TiCoSb and NbFeSb is revealed, and the key factors for temperature-induced conduction band convergence in HH are found out. Using these as the screening criteria, 3 out of 274 HHs (TiRhBi, TiPtSn, NbPtTl) are then stood out from our MatHub-3d database. Taking TiPtSn as the example, it shows the conduction band convergence at mid-high temperature, and further resulting in enhanced Seebeck coefficient S: e.g., at 600 K with electron concentration 10^20 cm^-3, the predicted S with and without renormalized band is 352.83 uV/K and 289.52 uV/K, respectively. Herein, the former is closer to our measurement value of 338.79 uV/K. Besides, the effective masses obtained from calculation and experiment are both enlarged with temperature, indicating the existence of band convergence. Our work demonstrates for the first time the significance of adding the temperature effect on electronic structure in the design of potential high-performance TE materials. △ Less

Submitted 29 June, 2024; originally announced July 2024.

Revisiting first-principles thermodynamics by quasiharmonic approach: Application to study thermal expansion of additively-manufactured Inconel 625

Authors: Shun-Li Shang, Rushi Gong, Michael C. Gao, Darren C. Pagan, Zi-Kui Liu

Abstract: An innovative method is developed for accurate determination of thermodynamic properties as a function of temperature by revisiting the density functional theory (DFT) based quasiharmonic approach (QHA). The present methodology individually evaluates the contributions from static total energy, phonon, and thermal electron to free energy for increased efficiency and accuracy. The Akaike information… ▽ More An innovative method is developed for accurate determination of thermodynamic properties as a function of temperature by revisiting the density functional theory (DFT) based quasiharmonic approach (QHA). The present methodology individually evaluates the contributions from static total energy, phonon, and thermal electron to free energy for increased efficiency and accuracy. The Akaike information criterion with a correction (AICc) is used to select models and model parameters for fitting each contribution as a function of volume. Using the additively manufactured Inconel alloy 625 (IN625) as an example, predicted temperature-dependent linear coefficient of thermal expansion (CTE) agrees well with dilatometer measurements and values in the literature. Sensitivity and uncertainty are also analyzed for the predicted IN625 CTE due to different structural configurations used by DFT, and hence different equilibrium properties determined. △ Less

Submitted 15 May, 2024; originally announced May 2024.

Comments: This manuscript includes both the main text and the supplementary material, but without the supplementary Excel file

Unveiling Higher-Order Topology via Polarized Topological Charges

Authors: Wei Jia, Bao-Zong Wang, Ming-Jian Gao, Jun-Hong An

Abstract: Higher-order topological phases (HOTPs) host exotic topological states that go beyond the traditional bulk-boundary correspondence. Up to now, there is still a lack of experimentally measurable momentum-space topological characterization for the HOTPs, which is not conducive to revealing the essential properties of these topological states and also restricts their detection in quantum simulation s… ▽ More Higher-order topological phases (HOTPs) host exotic topological states that go beyond the traditional bulk-boundary correspondence. Up to now, there is still a lack of experimentally measurable momentum-space topological characterization for the HOTPs, which is not conducive to revealing the essential properties of these topological states and also restricts their detection in quantum simulation systems. Here, we propose a concept of polarized topological charges to characterize chiral-symmetric HOTPs in momentum space, which further facilitates a feasible experimental scheme to detect the HOTPs in $^{87}$Rb cold atomic system. Remarkably, our characterization theory not only shows that the second-order (third-order) topological phases are determined by a quarter (negative eighth) of the total polarized topological charges, but also reveals that the higher-order topological phase transitions are identified by the creation or annihilation of polarized topological charges. Particularly, these polarized topological charges can be measured by pseudospin structures of the systems. Due to theoretical simplicity and observational intuitiveness, this work shall advance the broad studies of the HOTPs in both theory and experiment. △ Less

Submitted 3 December, 2024; v1 submitted 8 May, 2024; originally announced May 2024.

Comments: 8+9 pages, 4+3 figures. References are updated.Typos are corrected

Journal ref: Phys. Rev. B 110, L201117 (2024)

A method for quantifying the generalization capabilities of generative models for solving Ising models

Authors: Qunlong Ma, Zhi Ma, Ming Gao

Abstract: For Ising models with complex energy landscapes, whether the ground state can be found by neural networks depends heavily on the Hamming distance between the training datasets and the ground state. Despite the fact that various recently proposed generative models have shown good performance in solving Ising models, there is no adequate discussion on how to quantify their generalization capabilitie… ▽ More For Ising models with complex energy landscapes, whether the ground state can be found by neural networks depends heavily on the Hamming distance between the training datasets and the ground state. Despite the fact that various recently proposed generative models have shown good performance in solving Ising models, there is no adequate discussion on how to quantify their generalization capabilities. Here we design a Hamming distance regularizer in the framework of a class of generative models, variational autoregressive networks (VAN), to quantify the generalization capabilities of various network architectures combined with VAN. The regularizer can control the size of the overlaps between the ground state and the training datasets generated by networks, which, together with the success rates of finding the ground state, form a quantitative metric to quantify their generalization capabilities. We conduct numerical experiments on several prototypical network architectures combined with VAN, including feed-forward neural networks, recurrent neural networks, and graph neural networks, to quantify their generalization capabilities when solving Ising models. Moreover, considering the fact that the quantification of the generalization capabilities of networks on small-scale problems can be used to predict their relative performance on large-scale problems, our method is of great significance for assisting in the Neural Architecture Search field of searching for the optimal network architectures when solving large-scale Ising models. △ Less

Submitted 6 May, 2024; originally announced May 2024.

Comments: 10 pages, 7 figures

Journal ref: Mach. Learn.: Sci. Technol. 5 (2024) 025011

Message Passing Variational Autoregressive Network for Solving Intractable Ising Models

Authors: Qunlong Ma, Zhi Ma, Jinlong Xu, Hairui Zhang, Ming Gao

Abstract: Many deep neural networks have been used to solve Ising models, including autoregressive neural networks, convolutional neural networks, recurrent neural networks, and graph neural networks. Learning a probability distribution of energy configuration or finding the ground states of a disordered, fully connected Ising model is essential for statistical mechanics and NP-hard problems. Despite tremen… ▽ More Many deep neural networks have been used to solve Ising models, including autoregressive neural networks, convolutional neural networks, recurrent neural networks, and graph neural networks. Learning a probability distribution of energy configuration or finding the ground states of a disordered, fully connected Ising model is essential for statistical mechanics and NP-hard problems. Despite tremendous efforts, a neural network architecture with the ability to high-accurately solve these fully connected and extremely intractable problems on larger systems is still lacking. Here we propose a variational autoregressive architecture with a message passing mechanism, which can effectively utilize the interactions between spin variables. The new network trained under an annealing framework outperforms existing methods in solving several prototypical Ising spin Hamiltonians, especially for larger spin systems at low temperatures. The advantages also come from the great mitigation of mode collapse during the training process of deep neural networks. Considering these extremely difficult problems to be solved, our method extends the current computational limits of unsupervised neural networks to solve combinatorial optimization problems. △ Less

Submitted 9 April, 2024; originally announced April 2024.

Comments: 18 pages, 14 figures

Journal ref: Communications Physics 7, 236 (2024)

Absence of phonon-mediated superconductivity in La$_3$Ni$_2$O$_7$ under pressure

Authors: Zhenfeng Ouyang, Miao Gao, Zhong-Yi Lu

Abstract: A recent experimental study announced the emergence of superconductivity in La$_3$Ni$_2$O$_7$ under pressure, with the highest observed superconducting transition temperature ($T_c$) reaching approximately 80 K beyond 14 GPa. While extensive studies have been devoted to the electronic correlations and potential superconducting pairing mechanisms, there lack investigations into the phonon propertie… ▽ More A recent experimental study announced the emergence of superconductivity in La$_3$Ni$_2$O$_7$ under pressure, with the highest observed superconducting transition temperature ($T_c$) reaching approximately 80 K beyond 14 GPa. While extensive studies have been devoted to the electronic correlations and potential superconducting pairing mechanisms, there lack investigations into the phonon properties and electron phonon coupling. Using density functional theory in conjunction with Wannier interpolation techniques, we study the phonon properties and electron phonon interactions in La$_3$Ni$_2$O$_7$ under 29.5 GPa. Our findings reveal that the electron phonon coupling is insufficient to solely explain the observed high superconducting $T_c$ $\sim$ 80 K in La$_3$Ni$_2$O$_7$. And the calculated strong Fermi surface nesting may explain the experimental observed charge density wave transition in La$_3$Ni$_2$O$_7$. Our calculations substantiate La$_3$Ni$_2$O$_7$ is an unconventional superconductor. △ Less

Submitted 21 March, 2024; originally announced March 2024.

Comments: 5 pages, 4 figures

Journal ref: npj Quantum Mater. 9, 80 (2024)

Two-dimensional Kagome-in-Honeycomb materials (MN$_4$)$_3$C$_{32}$ (M=Pt or Mn)

Authors: Jingping Dong, Miao Gao, Xun-Wang Yan, Fengjie Ma, Zhong-Yi Lu

Abstract: We propose two novel two-dimensional (2D) topological materials, (PtN$_4$)$_3$C$_{32}$ and (MnN$_4$)$_3$C$_{32}$, with a special geometry that we named as kagome-in-honeycomb (KIH) lattice structure, to illustrate the coexistence of the paradigmatic states of kagome physics, Dirac fermions and flat bands, that are difficult to be simultaneously observed in three-dimensional realistic systems. In s… ▽ More We propose two novel two-dimensional (2D) topological materials, (PtN$_4$)$_3$C$_{32}$ and (MnN$_4$)$_3$C$_{32}$, with a special geometry that we named as kagome-in-honeycomb (KIH) lattice structure, to illustrate the coexistence of the paradigmatic states of kagome physics, Dirac fermions and flat bands, that are difficult to be simultaneously observed in three-dimensional realistic systems. In such system, MN$_4$(M=Pt or Mn) moieties are embedded in honeycomb graphene sheet according to kagome lattice structure, thereby resulting in a KIH lattice. Using the first-principles calculations, we have systemically studied the structural, electronic, and topological properties of these two materials. In the absence of spin-orbit coupling (SOC), they both exhibit the coexistence of Dirac/quadratic-crossing cone and flat band near the Fermi level. When SOC is included, a sizable topological gap is opened at the Dirac/quadratic-crossing nodal point. For nonmagnetic (PtN$_4$)$_3$C$_{32}$, the system is converted into a $\mathbb{Z}_2$ topological quantum spin Hall insulator defined on a curved Fermi level, while for ferromagnetic (MnN$_4$)$_3$C$_{32}$, the material is changed from a half-semi-metal to a quantum anomalous Hall insulator with nonzero Chern number and nontrivial chiral edge states. Our findings not only predict a new family of 2D quantum materials, but also provide an experimentally feasible platform to explore the emergent kagome physics, topological quantum Hall physics, strongly correlated phenomena, and theirs fascinating applications. △ Less

Submitted 5 March, 2024; originally announced March 2024.

Comments: 6 figures. arXiv admin note: text overlap with arXiv:2207.03703

Majorana modes in trapped-ion system and their Floquet engineering

Authors: Ming-Jian Gao, Yu-Peng Ma, Jun-Hong An

Abstract: Obeying non-Abelian statistics, Majorana fermions holds a promise to implement fault-tolerant quantum computing. It was found that Majorana fermions can be simulated by the zero-energy excitation in a nanowire with strong spin-orbit coupling interacting with an $s$-wave superconductor under a magnetic field. However, the signal of Majorana fermion in that system is obscured by the disorder in the… ▽ More Obeying non-Abelian statistics, Majorana fermions holds a promise to implement fault-tolerant quantum computing. It was found that Majorana fermions can be simulated by the zero-energy excitation in a nanowire with strong spin-orbit coupling interacting with an $s$-wave superconductor under a magnetic field. However, the signal of Majorana fermion in that system is obscured by the disorder in the nanowire and the confinement potential at the wire end. Thus, more controllable platforms are desired to simulate Majorana fermions. We here propose an alternative scheme to simulate the Majorana fermions in a trapped-ion system. Our dimerized-ion configuration permits us to generate the Majorana modes not only at zero energy but also at the nonzero ones, which enlarge the family of Majorana modes and supply another qubit carrier for quantum computing. We also investigate the controllability of the Majorana modes by Floquet engineering. It is found that a widely tunable number of Majorana modes are created on demand by applying a periodic driving on the trapped-ion system. Enriching the platforms for simulating Majorana fermions, our result would open another avenue for realizing fault-tolerant quantum computing. △ Less

Submitted 22 May, 2024; v1 submitted 21 January, 2024; originally announced January 2024.

Journal ref: Phys. Rev. B 109, 184518 (2024)

Twisted van der Waals Quantum Materials: Fundamentals, Tunability and Applications

Authors: Xueqian Sun, Manuka Suriyage, Ahmed Khan, Mingyuan Gao, Jie Zhao, Boqing Liu, Mehedi Hasan, Sharidya Rahman, Ruosi Chen, Ping Koy Lam, Yuerui Lu

Abstract: Twisted vdW quantum materials have emerged as a rapidly developing field of 2D semiconductors. These materials establish a new central research area and provide a promising platform for studying quantum phenomena and investigating the engineering of novel optoelectronic properties such as single-photon emission, non-linear optical response, magnon physics, and topological superconductivity. These… ▽ More Twisted vdW quantum materials have emerged as a rapidly developing field of 2D semiconductors. These materials establish a new central research area and provide a promising platform for studying quantum phenomena and investigating the engineering of novel optoelectronic properties such as single-photon emission, non-linear optical response, magnon physics, and topological superconductivity. These captivating electronic and optical properties result from, and can be tailored by, the interlayer coupling using moiré patterns formed by vertically stacking atomic layers with controlled angle misorientation or lattice mismatch. Their outstanding properties and the high degree of tunability position them as compelling building blocks for both compact quantum-enabled devices and classical optoelectronics. This article offers a comprehensive review of recent advancements in the understanding and manipulation of twisted van der Waals structures and presents a survey of the state-of-the-art research on moiré superlattices, encompassing interdisciplinary interests. It delves into fundamental theories, synthesis and fabrication, and visualization techniques, and the wide range of novel physical phenomena exhibited by these structures, with a focus on their potential for practical device integration in applications ranging from quantum information to biosensors, and including classical optoelectronics such as modulators, light emitting diodes (LEDs), lasers, and photodetectors. It highlights the unique ability of moiré superlattices to connect multiple disciplines, covering chemistry, electronics, optics, photonics, magnetism, topological and quantum physics. This comprehensive review provides a valuable resource for researchers interested in moiré superlattices, shedding light on their fundamental characteristics and their potential for transformative applications in various fields. △ Less

Submitted 18 December, 2023; originally announced December 2023.

Comments: 179 pages, 42 figures, Chemical Reviews

MSC Class: 85-06 ACM Class: J.2

Ab-initio tensile tests applied to BCC refractory alloys

Authors: Vishnu Raghuraman, Saro San, Michael C. Gao, Michael Widom

Abstract: Refractory metals exhibit high strength at high temperature, but often lack ductility. Multiprinciple element alloys such as high entropy alloys offer the potential to improve ductility while maintaining strength, but we don't know $a-priori$ what compositions will be suitable. A number of measures have been proposed to predict the ductility of metals, notably the Pugh ratio, the Rice-Thomson D-pa… ▽ More Refractory metals exhibit high strength at high temperature, but often lack ductility. Multiprinciple element alloys such as high entropy alloys offer the potential to improve ductility while maintaining strength, but we don't know $a-priori$ what compositions will be suitable. A number of measures have been proposed to predict the ductility of metals, notably the Pugh ratio, the Rice-Thomson D-parameter, among others. Here we examine direct $ab-initio$ simulation of deformation under tensile strain, and we apply this to a variety of Nb- and Mo-based binary alloys and to several quaternary alloy systems. Our results exhibit peak stresses for elastic deformation, beyond which defects such as lattice slip, stacking faults, transformation, and twinning, relieve the stress. The peak stress grows strongly with increasing valence electron count. Correlations are examined among several physical properties, including the above-mentioned ductility parameters. △ Less

Submitted 5 December, 2023; v1 submitted 29 November, 2023; originally announced November 2023.

Comments: 22 pages, 13 figures, accepted for publication in Physical Review Materials

Engineering rich two-dimensional higher-order topological phases by flux and periodic driving

Authors: Ming-Jian Gao, Jun-Hong An

Abstract: Nodal-line semimetals are commonly believed to exist in $\mathcal{PT}$ symmetric or mirror-rotation symmetric systems. Here, we find a flux-induced parameter-dimensional second-order nodal-line semimetal (SONLS) in a two-dimensional system without $\mathcal{PT}$ and mirror-rotation symmetries. It has coexisting hinge Fermi arcs and drumhead surface states. Meanwhile, we discover a flux-induced sec… ▽ More Nodal-line semimetals are commonly believed to exist in $\mathcal{PT}$ symmetric or mirror-rotation symmetric systems. Here, we find a flux-induced parameter-dimensional second-order nodal-line semimetal (SONLS) in a two-dimensional system without $\mathcal{PT}$ and mirror-rotation symmetries. It has coexisting hinge Fermi arcs and drumhead surface states. Meanwhile, we discover a flux-induced second-order topological insulator (SOTI). We then propose a Floquet engineering scheme to create exotic parameter-dimensional hybrid-order nodal-line semimetals with abundant nodal-line structures and widely tunable numbers of corner states in a SONLS and SOTI, respectively. Our results break the perception of SONLSs and supply a convenient way to artificially synthesize exotic topological phases by periodic driving. △ Less

Submitted 8 December, 2023; v1 submitted 4 September, 2023; originally announced September 2023.

Journal ref: Phys. Rev. B 108, L241402 (2023)

Predictions and correlation analyses of Ellingham diagrams in binary oxides

Authors: Shun-Li Shang, Shuang Lin, Michael C. Gao, Darrell G. Schlom, Zi-Kui Liu

Abstract: Knowing oxide-forming ability is vital to gain desired or avoid deleterious oxides formation through tuning oxidizing environment and materials chemistry. Here, we have conducted a comprehensive thermodynamic analysis of 137 binary oxides using the presently predicted Ellingham diagrams. It is found that the active elements to form oxides easily are the f-block elements (lanthanides and actinides)… ▽ More Knowing oxide-forming ability is vital to gain desired or avoid deleterious oxides formation through tuning oxidizing environment and materials chemistry. Here, we have conducted a comprehensive thermodynamic analysis of 137 binary oxides using the presently predicted Ellingham diagrams. It is found that the active elements to form oxides easily are the f-block elements (lanthanides and actinides), elements in the groups II, III, and IV (alkaline earth, Sc, Y, Ti, Zr, and Hf), and Al and Li; while the noble elements with their oxides nonstable and easily reduced are coinage metals (Cu, Ag, and especially Au), Pt-group elements, and Hg and Se. Machine learning based sequential feature selection indicates that oxide-forming ability can be represented by electronic structures of pure elements, for example, their d- and s-valence electrons, Mendeleev numbers, and the groups, making the periodic table a useful tool to tailor oxide-forming ability. The other key elemental features to correlate oxide-forming ability are thermochemical properties such as melting points and standard entropy at 298 K of pure elements. It further shows that the present Ellingham diagrams enable qualitatively understanding and even predicting oxides formed in multicomponent materials, such as the Fe-20Cr-20Ni alloy (in wt.%) and the equimolar high entropy alloy of AlCoCrFeNi, which are in accordance with thermodynamic calculations using the CALPHAD approach and experimental observations in the literature. △ Less

Submitted 10 August, 2023; originally announced August 2023.

Structural, electronic, magnetic properties of Cu-doped lead-apatite Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$O

Authors: Jianfeng Zhang, Huazhen Li, Ming Yan, Miao Gao, Fengjie Ma, Xun-Wang Yan, Z. Y. Xie

Abstract: The recent report of superconductivity in the Cu-doped PbPO compound stimulates the extensive researches on its physical properties. Herein, the detailed atomic and electronic structures of this compound are investigated, which are the necessary information to explain the physical properties, including possible superconductivity. By the first-principles electronic structure calculations, we find t… ▽ More The recent report of superconductivity in the Cu-doped PbPO compound stimulates the extensive researches on its physical properties. Herein, the detailed atomic and electronic structures of this compound are investigated, which are the necessary information to explain the physical properties, including possible superconductivity. By the first-principles electronic structure calculations, we find that the partial replacement of Pb at $4f$ site by Cu atom, instead of Pb at $6h$ site, plays a crucial role in dominating the electronic state at Fermi energy. The $3d$ electronic orbitals of Cu atom emerge near the Fermi energy and exhibit strong spin-polarization, resulting in the local moment around the doped Cu atom. Particularly, the ground state of Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$O (x = 1) is determined to be a semiconducting phase, in good agreement with the experimental measurements. △ Less

Submitted 8 August, 2023; originally announced August 2023.

Superconductivity at ambient pressure in hole-doped LuH$_3$

Authors: Zhenfeng Ouyang, Miao Gao, Zhong-Yi Lu

Abstract: Very recently, a report on possible room-temperature superconductivity in N-doped lutetium hydrides near 1 GPa pressure has drawn lots of attentions. To date, the superconductivity is not confirmed under relatively low pressure in subsequent studies. Based on the density functional theory first-principles calculations, we extensively investigate the influence of charge doping on the stability and… ▽ More Very recently, a report on possible room-temperature superconductivity in N-doped lutetium hydrides near 1 GPa pressure has drawn lots of attentions. To date, the superconductivity is not confirmed under relatively low pressure in subsequent studies. Based on the density functional theory first-principles calculations, we extensively investigate the influence of charge doping on the stability and superconductivity of LuH$_3$ at ambient pressure. Although electron doping can not stabilize LuH$_3$, we find that the room-pressure stability of LuH$_3$ can be achieved by doping holes with concentrations in between 0.15-0.30 holes/cell. Moreover, our calculations reveal a positive dependence of superconducting transition temperature on the number of doped holes, with the highest value close to 54 K. These findings suggest that realizing superconductivity in hole-doped LuH$_3$ at ambient pressure is not impossible, although the transition temperature is still far away from the room temperature. △ Less

Submitted 24 June, 2023; originally announced June 2023.

Comments: 6 pages, 6 figures

Microscopic resolution of superconducting electrons in ultrahigh-pressed hydrogen sulfide

Authors: Jian-Feng Zhang, Bo Zhan, Miao Gao, Kai Liu, Xinguo Ren, Zhong-Yi Lu, Tao Xiang

Abstract: We investigate the electronic and phonon properties of hydrogen sulfide (SH$_3$) under ultrahigh pressure to elucidate the origin of its high-T$_c$ superconductivity. Contrary to the prevailing belief that the metalized S-H $σ$ bond is responsible, our analysis, based on the anisotropic Migdal-Eliashberg equation and the crystal orbital Hamilton population (COHP) calculation, reveals that the H-H… ▽ More We investigate the electronic and phonon properties of hydrogen sulfide (SH$_3$) under ultrahigh pressure to elucidate the origin of its high-T$_c$ superconductivity. Contrary to the prevailing belief that the metalized S-H $σ$ bond is responsible, our analysis, based on the anisotropic Migdal-Eliashberg equation and the crystal orbital Hamilton population (COHP) calculation, reveals that the H-H $σ$-antibonding states play a dominant role in the large electron-phonon coupling that leads to the superconducting pairing in SH$_3$. Furthermore, by partially restricting the vibration of S atoms, we demonstrate that the S-H bonds provide subsidiary contributions to the pairing interaction. These findings shed light on the importance of the previously overlooked H-H $σ^*$ bonds in driving high-T$_c$ superconductivity in SH$_3$ and offer insights into the relationship between metallic H-H covalent antibonding and high-T$_c$ superconductivity in other hydrogen-rich materials under high pressure. △ Less

Submitted 6 June, 2023; originally announced June 2023.

Comments: 11 pages, 9 figures, 59 references

Single-shot spatial instability and electric control of polariton condensates at room temperature

Authors: Ying Gao, Xuekai Ma, Xiaokun Zhai, Chunzi Xing, Meini Gao, Haitao Dai, Hao Wu, Tong Liu, Yuan Ren, Xiao Wang, Anlian Pan, Wei Hu, Stefan Schumacher, Tingge Gao

Abstract: In planar microcavities, the transverse-electric and transverse-magnetic (TE-TM) mode splitting of cavity photons arises due to their different penetration into the Bragg mirrors and can result in optical spin-orbit coupling (SOC). In this work, we find that in a liquid crystal (LC) microcavity filled with perovskite microplates, the pronounced TE-TM splitting gives rise to a strong SOC that leads… ▽ More In planar microcavities, the transverse-electric and transverse-magnetic (TE-TM) mode splitting of cavity photons arises due to their different penetration into the Bragg mirrors and can result in optical spin-orbit coupling (SOC). In this work, we find that in a liquid crystal (LC) microcavity filled with perovskite microplates, the pronounced TE-TM splitting gives rise to a strong SOC that leads to the spatial instability of microcavity polariton condensates under single-shot excitation. Spatially varying hole burning and mode competition occurs between polarization components leading to different condensate profiles from shot to shot. The single-shot polariton condensates become stable when the SOC vanishes as the TE and TM modes are spectrally well separated from each other, which can be achieved by application of an electric field to our LC microcavity with electrically tunable anisotropy. Our findings are well reproduced and traced back to their physical origin by our detailed numerical simulations. With the electrical manipulation our work reveals how the shot-to-shot spatial instability of spatial polariton profiles can be engineered in anisotropic microcavities at room temperature, which will benefit the development of stable polariton-based optoeletronic and light-emitting devices. △ Less

Submitted 2 May, 2023; originally announced May 2023.

Cubic C$_{20}$: An intrinsic superconducting carbon allotrope

Authors: Ying Yu, Xun-Wang Yan, Fengjie Ma, Miao Gao, Zhong-Yi Lu

Abstract: Finding intrinsic carbon superconductor is an interesting topic. Based on density functional first-principles calculations, we first study the phonon-mediated superconductivity in a cubic metallic carbon allotrope, namely sc-C$_{20}$, which has been synthesized in experiment. The electron-phonon coupling is accurately computed with Wannier interpolation method. By solving the Eliashberg equations,… ▽ More Finding intrinsic carbon superconductor is an interesting topic. Based on density functional first-principles calculations, we first study the phonon-mediated superconductivity in a cubic metallic carbon allotrope, namely sc-C$_{20}$, which has been synthesized in experiment. The electron-phonon coupling is accurately computed with Wannier interpolation method. By solving the Eliashberg equations, we predict that sc-C$_{20}$ is an intrinsic carbon superconductor, without introducing any guest atoms or doping, whose transition temperature is determined to be about 24 K. Our findings enrich the family of carbon-based superconductors. △ Less

Submitted 24 June, 2023; v1 submitted 28 January, 2023; originally announced January 2023.

Comments: 4 pages, 5 figures

Journal ref: Appl. Phys. Express 16, 063003 (2023)

Stabilizing a hydrogen-rich superconductor at 1 GPa by the charge-transfer modulated virtual high-pressure effect

Authors: Miao Gao, Peng-Jie Guo, Huan-Cheng Yang, Xun-Wang Yan, Fengjie Ma, Zhong-Yi Lu, Tao Xiang, Hai-Qing Lin

Abstract: Applying pressure around megabar is indispensable in the synthesis of high-temperature superconducting hydrides, such as SH$_3$ and LaH$_{10}$. Stabilizing the high-pressure phase of hydride around ambient condition is a severe challenge. Based on the density-functional theory calculations, we give the first example that the structure of hydride CaBH$_5$ predicted above 280 GPa, can maintain its d… ▽ More Applying pressure around megabar is indispensable in the synthesis of high-temperature superconducting hydrides, such as SH$_3$ and LaH$_{10}$. Stabilizing the high-pressure phase of hydride around ambient condition is a severe challenge. Based on the density-functional theory calculations, we give the first example that the structure of hydride CaBH$_5$ predicted above 280 GPa, can maintain its dynamical stability with pressure down to 1 GPa, by modulating the charge transfer from metal atoms to hydrogen atoms via the replacement of Ca with alkali metal atoms e.g. Cs, in which the [BH$_5$]$^{2-}$ anion shrinks along $c$ axis and expands in the $ab$ plane, experiencing an anisotropic virtual high pressure. This mechanism, namely charge transfer modulated virtual high pressure effect, plays a vital role in enhancing the structural stability and leading to the reemergence of ambient-pressure-forbidden [BH$_5$]$^{2-}$ anion around 1 GPa in CsBH$_5$. Moreover, we find that CsBH$_5$ is a strongly coupled superconductor, with transition temperature as high as 98 K, well above the liquid-nitrogen temperature. Our findings provide a novel mechanism to reduce the critical pressure required by hydrogen-rich compound without changing its crystal structure, and also shed light on searching ambient-pressure high-temperature superconductivity in metal borohydrides. △ Less

Submitted 5 May, 2023; v1 submitted 28 January, 2023; originally announced January 2023.

Comments: accepted for publication as a Letter in Phys. Rev. B

Journal ref: Phys. Rev. B 107, L180501 (2023)

High-temperature ferromagnetism and strong $π$-conjugation feature in two-dimensional manganese tetranitride

Authors: Ming Yan, Z. Y. Xie, Miao Gao

Abstract: Two-dimensional (2D) magnetic materials have attracted tremendous research interest because of the promising application in the next-generation microelectronic devices. Here, by the first-principles calculations, we propose a two-dimensional ferromagnetic material with high Curie temperature, manganese tetranitride MnN$_4$ monolayer, which is a square-planar lattice made up of only one layer of at… ▽ More Two-dimensional (2D) magnetic materials have attracted tremendous research interest because of the promising application in the next-generation microelectronic devices. Here, by the first-principles calculations, we propose a two-dimensional ferromagnetic material with high Curie temperature, manganese tetranitride MnN$_4$ monolayer, which is a square-planar lattice made up of only one layer of atoms. The structure is demonstrated to be stable by the phonon spectra and the molecular dynamic simulations, and the stability is ascribed to the $π$-d conjugation between $π$ orbital of N=N bond and Mn $d$ orbital. More interestingly, the MnN$_4$ monolayer displays robust 2D ferromagnetism, which originates from the strong exchange couplings between Mn atoms due to the $π$-d conjugation. The high critical temperature of 247 K is determined by solving the Heisenberg model with the Monte Carlo method. △ Less

Submitted 27 November, 2022; originally announced November 2022.

Journal ref: Chin. Phys. B 32, 037104 (2023)

Two-dimensional anisotropic Dirac materials PtN4C2 and Pt2N8C6 with quantum spin and valley Hall effects

Authors: Jingping Dong, Chuhan Wang, Xinlei Zhao, Miao Gao, Xun-Wang Yan, Fengjie Ma, Zhong-Yi Lu

Abstract: We propose two novel two-dimensional topological Dirac materials, planar PtN4C2 and Pt2N8C6, which exhibit graphene-like electronic structures with linearly dispersive Dirac-cone states exactly at the Fermi level. Moreover, the Dirac cone is anisotropic, resulting in anisotropic Fermi velocities and making it possible to realize orientation-dependent quantum devices. Using the first-principles ele… ▽ More We propose two novel two-dimensional topological Dirac materials, planar PtN4C2 and Pt2N8C6, which exhibit graphene-like electronic structures with linearly dispersive Dirac-cone states exactly at the Fermi level. Moreover, the Dirac cone is anisotropic, resulting in anisotropic Fermi velocities and making it possible to realize orientation-dependent quantum devices. Using the first-principles electronic structure calculations, we have systemically studied the structural, electronic, and topological properties. We find that spin-orbit coupling opens a sizable topological band gap so that the materials can be classified as quantum spin Hall insulators as well as quantum valley Hall insulators. Helical edge states that reside in the insulating band gap connecting the bulk conduction and valence bands are observed. Our work not only expands the Dirac cone material family, but also provides a new avenue to searching for more two-dimensional topological quantum spin and valley Hall insulators. △ Less

Submitted 8 July, 2022; originally announced July 2022.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Materials 6, 074202 (2022)

Engineering second-order nodal-line semimetals by breaking $\mathcal{PT}$ symmetry and periodic driving

Authors: Ming-Jian Gao, Hong Wu, Jun-Hong An

Abstract: Hosting unique drumhead surface states enclosed by nodal lines, topological nodal-line semimetals exhibit novel transport phenomena. Thus, the exploration of topological semimetals with different nodal-line structures has attracted much attention. In this paper, we first find a second-order nodal line semimetal (SONLS), which has coexisting hinge Fermi arcs and drumhead surface states, in a… ▽ More Hosting unique drumhead surface states enclosed by nodal lines, topological nodal-line semimetals exhibit novel transport phenomena. Thus, the exploration of topological semimetals with different nodal-line structures has attracted much attention. In this paper, we first find a second-order nodal line semimetal (SONLS), which has coexisting hinge Fermi arcs and drumhead surface states, in a $\mathcal{PT}$-symmetry broken system. Then, without changing the intrinsic parameters, we artificially create exotic hybrid-order nodal-line semimetals hosted by different quasienergy gaps and rich nodal-line structures including nodal chains, crossing ring nodal nets, crossing line nodes, and nodal nets by applying a periodic driving on our SONLS. Enriching the classification of topological semimetals, such Floquet engineered high tunability of the orders and nodal-line structures of the SONLS sets up a foundation for exploring its further applications. △ Less

Submitted 18 January, 2023; v1 submitted 3 July, 2022; originally announced July 2022.

Comments: 7 pages and 4 figures

Journal ref: Phys. Rev. B 107, 035128 (2023)

Two-dimensional binary transition metal nitride $M$N$_4$ ($M$ = V, Cr, Mn, Fe, Co) with a graphene-like structure and strong magnetic properties

Authors: Shuo Zhang, Panjun Feng, Dapeng Liu, Hongfen Wu, Miao Gao, Tongshuai Xu, Xun-Wang Yan, Z. Y. Xie

Abstract: Binary transition metal nitride with a graphene-like structure and strong magnetic properties is rare. Based on the first-principles calculations, we design two kinds of $M$N$_4$ ($M$ =transition metal) monolayers, which are transition metal nitrides with a planar structure, made up of $M$N$_4$ units aligned in the rhombic and square patterns. The two structural lattices have robust stability and… ▽ More Binary transition metal nitride with a graphene-like structure and strong magnetic properties is rare. Based on the first-principles calculations, we design two kinds of $M$N$_4$ ($M$ =transition metal) monolayers, which are transition metal nitrides with a planar structure, made up of $M$N$_4$ units aligned in the rhombic and square patterns. The two structural lattices have robust stability and good compatibility with different metal atoms, and the underlying mechanism is the combination of $sp^2$ hybridization, coordinate bond, and $π$ conjugation. With the metal atom changing from V, Cr, Mn, Fe to Co, the total charge of $M$N$_4$ system increases by one electron in turn, which results in continuous adjustability of the electronic and magnetic properties. The planar ligand field is another feature of the two $M$N$_4$ lattices, which brings about the special splitting of five suborbitals of 3$d$ metal atom and gives rise to strong magnetism. Moreover, room-temperature ferromagnetism in square-CoN$_4$ monolayer with the Curie temperatures of 321 K is determined by solving the Heisenberg model combined with Monte Carlo method. △ Less

Submitted 3 October, 2022; v1 submitted 28 April, 2022; originally announced April 2022.

Journal ref: Phys. Rev. B 106, 235402 (2022)

Prediction of single-atom-thick transition metal nitride CrN$_4$ with a square-planar network and high-temperature ferromagnetism

Authors: Dapeng Liu, Panjun Feng, Shuo Zhang, Miao Gao, Fengjie Ma, Xun-Wang Yan, Z. Y. Xie

Abstract: Single-atom-thick two-dimensional materials such as graphene usually have a hexagonal lattice while the square-planar lattice is uncommon in the family of two-dimensional materials. Here, we demonstrate that single-atom-thick transition metal nitride CrN$_4$ monolayer is a stable free-standing layer with a square-planar network. The stability of square-planar geometry is ascribed to the combinat… ▽ More Single-atom-thick two-dimensional materials such as graphene usually have a hexagonal lattice while the square-planar lattice is uncommon in the family of two-dimensional materials. Here, we demonstrate that single-atom-thick transition metal nitride CrN$_4$ monolayer is a stable free-standing layer with a square-planar network. The stability of square-planar geometry is ascribed to the combination of N=N double bond, Cr-N coordination bond, and $π$-d conjugation, in which the double $π$-d conjugation is rarely reported in previous studies. This mechanism is entirely different from that of the reported two-dimensional materials, leading to lower formation energy and more robust stability compared to the synthesized g-C$_3$N$_4$ monolayer. On the other hand, CrN$_4$ layer has a ferromagnetic ground state, in which the ferromagnetic coupling between two Cr atoms is mediated by electrons of the half-filled large $π$ orbitals from $π$-d conjugation. The high-temperature ferromagnetism in CrN$_4$ monolayer is confirmed by solving the Heisenberg model with Monte Carlo method. △ Less

Submitted 22 September, 2022; v1 submitted 10 March, 2022; originally announced March 2022.

Comments: https://link.aps.org/doi/10.1103/PhysRevB.106.125421

Journal ref: Physical Review B 106, 125421 (2022)

Achieving high-temperature ferromagnetism by means of magnetic ions dimerization

Authors: Panjun Feng, Shuo Zhang, Dapeng Liu, Miao Gao, Fengjie Ma, Xun-Wang Yan, Z. Y. Xie

Abstract: Magnetic two-dimensional materials have potential application in next-generation electronic devices and have stimulated extensive interest in condensed matter physics and material fields. However, how to realize high-temperature ferromagnetism in two-dimensional materials remains a great challenge in physics. Herein, we propose an effective approach that the dimerization of magnetic ions in two-… ▽ More Magnetic two-dimensional materials have potential application in next-generation electronic devices and have stimulated extensive interest in condensed matter physics and material fields. However, how to realize high-temperature ferromagnetism in two-dimensional materials remains a great challenge in physics. Herein, we propose an effective approach that the dimerization of magnetic ions in two-dimensional materials can enhance the exchange coupling and stabilize the ferromagnetism. Manganese carbonitride Mn$_2$N$_6$C$_6$ with a planar monolayer structure is taken as an example to clarify the method, in which two Mn atoms are gathered together to form a ferromagnetic dimer of Mn atoms and further these dimers are coupled together to form the overall ferromagnetism of the two-dimensional material. In Mn$_2$N$_6$C$_6$ monolayer, the near-room-temperature ferromagnetism with the Curie temperature of 272.3 K is determined by solving Heisenberg model using Monte Carlo simulations method. △ Less

Submitted 12 June, 2022; v1 submitted 3 March, 2022; originally announced March 2022.

Comments: Journal of Physical Chemistry C Cite This: https://doi.org/10.1021/acs.jpcc.2c02593

Journal ref: J. Phys. Chem. C 126, 10139 (2022)

Ultrathin quantum light source enabled by a nonlinear van der Waals crystal with vanishing interlayer-electronic-coupling

Authors: Qiangbing Guo, Xiao-Zhuo Qi, Meng Gao, Sanlue Hu, Lishu Zhang, Wenju Zhou, Wenjie Zang, Xiaoxu Zhao, Junyong Wang, Bingmin Yan, Mingquan Xu, Yun-Kun Wu, Goki Eda, Zewen Xiao, Huiyang Gou, Yuan Ping Feng, Guang-Can Guo, Wu Zhou, Xi-Feng Ren, Cheng-Wei Qiu, Stephen J. Pennycook, Andrew T. S. Wee

Abstract: Interlayer electronic coupling in two-dimensional (2D) materials enables tunable and emergent properties by stacking engineering. However, it also brings significant evolution of electronic structures and attenuation of excitonic effects in 2D semiconductors as exemplified by quickly degrading excitonic photoluminescence and optical nonlinearities in transition metal dichalcogenides when monolayer… ▽ More Interlayer electronic coupling in two-dimensional (2D) materials enables tunable and emergent properties by stacking engineering. However, it also brings significant evolution of electronic structures and attenuation of excitonic effects in 2D semiconductors as exemplified by quickly degrading excitonic photoluminescence and optical nonlinearities in transition metal dichalcogenides when monolayers are stacked into van der Waals structures. Here we report a novel van der Waals crystal, niobium oxide dichloride, featuring a vanishing interlayer electronic coupling and scalable second harmonic generation intensity of up to three orders higher than that of exciton-resonant monolayer WS2. Importantly, the strong second-order nonlinearity enables correlated parametric photon pair generation, via a spontaneous parametric down-conversion (SPDC) process, in flakes as thin as ~46 nm. To our knowledge, this is the first SPDC source unambiguously demonstrated in 2D layered materials, and the thinnest SPDC source ever reported. Our work opens an avenue towards developing van der Waals material-based ultracompact on-chip SPDC sources, and high-performance photon modulators in both classical and quantum optical technologies. △ Less

Submitted 8 February, 2022; originally announced February 2022.

Journal ref: Nature 613, 53-59 (2023)

Nonlinear deformation and elasticity of BCC refractory metals and alloys

Authors: Vishnu Raghuraman, Michael Widom, Michael C. Gao

Abstract: Application of isotropic pressure or uniaxial strain alters the elastic properties of materials; sufficiently large strains can drive structural transformations. Linear elasticity describes stability against infinitesimal strains, while nonlinear elasticity describes the response to finite deformations. It was previously shown that uniaxial strain along [100] drives refractory metals and alloys to… ▽ More Application of isotropic pressure or uniaxial strain alters the elastic properties of materials; sufficiently large strains can drive structural transformations. Linear elasticity describes stability against infinitesimal strains, while nonlinear elasticity describes the response to finite deformations. It was previously shown that uniaxial strain along [100] drives refractory metals and alloys towards mechanical instabilities. These include an extensional instability, and a symmetry-breaking orthorhombic distortion caused by a Jahn-Teller-Peierls instability that splays the cubic lattice vectors. Here, we analyze these transitions in depth. Eigenvalues and eigenvectors of the Wallace tensor identify and classify linear instabilities in the presence of strain. We show that both instabilities are discontinuous, leading to discrete jumps in the lattice parameters. We provide physical intuition for the instabilities by analyzing the changes in first principles energy, stress, bond lengths and angles upon application of strain. Electronic band structure calculations show differential occupation of bonding and anti-bonding orbitals, driven by the changing bond lengths and leading to the structural transformations. Strain thresholds for these instabilities depend on the valence electron count. △ Less

Submitted 21 April, 2022; v1 submitted 2 February, 2022; originally announced February 2022.

Comments: 31 pages, 17 figures, 1 table, accepted to Physical Review Materials