Showing 1–50 of 161 results for author: Meng, S

Stacking-sliding and irradiation-direction invariant Floquet altermagnets in A-type antiferromagnetic bilayers

Authors: Zhe Li, Lijuan Li, Mengxue Guan, Sheng Meng

Abstract: Arranging the stacking orders of A-type antiferromagnetic (A-AFM) bilayers offers an accessible pathway to two-dimensional altermagnets, but requires strict symmetry conditions such as layer groups, sliding positions, and twisting angles. Here, we find that circularly polarized light (CPL) irradiation breaks time-reversal symmetry, enabling the development of altermagnets beyond these constraints.… ▽ More Arranging the stacking orders of A-type antiferromagnetic (A-AFM) bilayers offers an accessible pathway to two-dimensional altermagnets, but requires strict symmetry conditions such as layer groups, sliding positions, and twisting angles. Here, we find that circularly polarized light (CPL) irradiation breaks time-reversal symmetry, enabling the development of altermagnets beyond these constraints. Based on symmetrical analysis, our revealments indicate that A-AFM bilayer building-blocks with inversion symmetry exhibit altermagnetism robust to stacking sliding and variations of illumination directions. These bilayers can be constructed from arbitrary ferromagnetic monolayers and guided by the $d$-electron counting rule. Adopting bilayer MnBi$_2$Te$_4$ as a template, out-of-plane illumination with CPL reveals an $f$-wave altermagnetic feature at sliding positions $\left\{E|\left(0,0\right)\right\}$, $\left\{E|\left(\frac{1}{3},\frac{2}{3}\right)\right\}$ and $\left\{E|\left(\frac{2}{3},\frac{1}{3}\right)\right\}$, while a $p$-wave feature is predicted at other sliding positions. Our unveilings popularize the applicability of altermagnets in A-AFM bilayers with inversion symmetry, igniting a new wave of research in this field. △ Less

Submitted 6 December, 2025; originally announced December 2025.

Evidence of a two-dimensional nitrogen crystalline structure on silver surfaces

Authors: Xuegao Hu, Haijun Cao, Zhicheng Gao, Hui Zhou, Daiyu Geng, Dong Li, Jisong Gao, Qiaoxiao Zhao, Zhihao Cai, Peng Cheng, Lan Chen, Sheng Meng, Kehui Wu, Baojie Feng

Abstract: Nitrogen, the most abundant element in Earth's atmosphere, exists as a diatomic gas under standard temperature and pressure. In the two-dimensional (2D) limit, atomically thin nitrogen, termed nitrogene, has been theoretically predicted to form crystalline materials with various polymorphic configurations, exhibiting diverse chemical and physical properties. However, the synthesis of nitrogene has… ▽ More Nitrogen, the most abundant element in Earth's atmosphere, exists as a diatomic gas under standard temperature and pressure. In the two-dimensional (2D) limit, atomically thin nitrogen, termed nitrogene, has been theoretically predicted to form crystalline materials with various polymorphic configurations, exhibiting diverse chemical and physical properties. However, the synthesis of nitrogene has remained elusive due to the strong nitrogen-nitrogen triple bonds. Here, we report experimental evidence of the formation of nitrogen-based crystalline structures compatible with nitrogene on silver surfaces via ion-beam-assisted epitaxy. Through a combination of scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and first-principles calculations, we demonstrate that the nitrogene-like structure adopts a puckered honeycomb lattice. Notably, our calculations predict a nitrogene band gap of up to 7.5 eV, positioning it as a promising candidate for ultraviolet optoelectronic devices and high-k dielectric applications. △ Less

Submitted 3 December, 2025; originally announced December 2025.

Comments: Nature Communications; In Press

Realization of polytype heterostructures via delicate structural transitions from a doped-Mott insulator

Authors: Yanyan Geng, Manyu Wang, Shumin Meng, Shuo Mi, Chang Li, Huiji Hu, Jianfeng Guo, Rui Xu, Fei Pang, Wei Ji, Weichang Zhou, Zhihai Cheng

Abstract: Transition metal dichalcogenides (TMDs) host multiple competing structural and electronic phases, making them an ideal platform for constructing polytype heterostructures with emergent quantum properties. However, controlling phase transitions to form diverse heterostructures inside a single crystal remains challenging. Here, we realize vertical/lateral polytype heterostructures in a hole-doped Mo… ▽ More Transition metal dichalcogenides (TMDs) host multiple competing structural and electronic phases, making them an ideal platform for constructing polytype heterostructures with emergent quantum properties. However, controlling phase transitions to form diverse heterostructures inside a single crystal remains challenging. Here, we realize vertical/lateral polytype heterostructures in a hole-doped Mott insulator via thermal-annealing-induced structural transitions. Raman spectroscopy, atomic force microscopy (AFM) and scanning Kelvin probe force microscopy (SKPM) confirm the coexistence of T-H polytype heterostructures. Atomic-scale scanning tunneling microscopy/spectroscopy (STM/STS) measurements reveal the transparent effect in 1H/1T vertical heterostructures, where the charge density wave (CDW) of the underlying 1T-layer superposes on the top 1H-layer under positive bias. By systematically comparing 1T/1H and 1T/1T interfaces, we demonstrate that the metallic 1H-layer imposes a Coulomb screening effect on the 1T-layer, suppressing the formation of CDW domain walls and forming more ordered electronic states. These results clarify the interfacial coupling between distinct quantum many-body phases and establish a controllable pathway for constructing two-dimensional polytype heterostructures with tunable electronic properties. △ Less

Submitted 2 December, 2025; originally announced December 2025.

Quasi-steady electron-excitonic complexes coupling in a two-dimensional semiconductor

Authors: Shangkun Mo, Hao Zhong, Keming Zhao, Yunfei Bai, Dingkun Qin, Chunlong Wu, Qiang Wan, Renzhe Li, Cao Peng, Xingzhe Wang, Enting Li, Sheng Meng, Nan Xu

Abstract: Excitons and their complexes govern optical-related behaviors in semiconductors. Here, using angle-resolved photoemission spectroscopy (ARPES), we have elucidated the light-matter interaction mediated by quasi-steady excitonic complexes within a monolayer of the prototypical two-dimensional (2D) semiconductor WSe2. Under continuous incident light, we have observed the generation of quasi-steady ex… ▽ More Excitons and their complexes govern optical-related behaviors in semiconductors. Here, using angle-resolved photoemission spectroscopy (ARPES), we have elucidated the light-matter interaction mediated by quasi-steady excitonic complexes within a monolayer of the prototypical two-dimensional (2D) semiconductor WSe2. Under continuous incident light, we have observed the generation of quasi-steady excitons and their complexes, encompassing ground and excited state excitons, trions, as well as their intricate interplay. We further show spectral evidence of electronic excitation states within the background of quasi-steady excitonic complexes, characterized by valence band (VB) effective mass renormalization, the enhanced spin-orbit coupling (SOC), the formation of an excitonic gap near the Fermi level (EF ) of the conduction band (CB), and intervalley excitonic band folding. Our findings not only unveil a quasi-steady excitonic complex background for the creation of diverse electronic excitations in 2D semiconductors but also offer new insights into the role of excitons in the charge density wave (CDW) formation mechanism and facilitate the advancement of correlated electronic state engineering based on the coupling between electrons and excitonic complexes in a quasi-equilibrium state. △ Less

Submitted 2 December, 2025; originally announced December 2025.

Comments: 6 pages, 4 figures

Light-engineered Multichannel Quantum Anomalous Hall Effect in High-order Topological Plumbene

Authors: Zhe Li, Fangyang Zhan, Haijun Cao, Jingjing Cao, Huisheng Zhang, Sheng Meng

Abstract: Floquet engineering severs as a forceful technique for uncovering high Chern numbers of quantum anomalous Hall (QAH) states with feasible tunability in high-order topologically insulating plumbene, which is readily accessible for experimental investigations. Under the irradiation of righthanded circularly polarized light, we predict a three-stage topological phase transition in plumbene, whether i… ▽ More Floquet engineering severs as a forceful technique for uncovering high Chern numbers of quantum anomalous Hall (QAH) states with feasible tunability in high-order topologically insulating plumbene, which is readily accessible for experimental investigations. Under the irradiation of righthanded circularly polarized light, we predict a three-stage topological phase transition in plumbene, whether it is in a free-standing form or grown on h-BN. Initially, a metallic state evolves into a K(K')-valley-based QAH state with a Chern number of -8, which then decreases to -6 after the valley gap closes. Finally, a band inversion occurs at the $Γ$ point, resulting in a multichannel QAH state with C = -3. The trigonal warping model accounts for both K(K')-valley-based and $Γ$-pointbased QAH states. Additionally, growing plumbene on a non-van-der-Waals substrate eliminates the K(K')-valley-based topology, leaving only the $Γ$-point-based QAH state with C = +3. Our findings propose the tunability of various high Chern numbers derived from high-order topological insulators, aiming to advance the next-generation dissipationless electronic devices. △ Less

Submitted 23 November, 2025; originally announced November 2025.

Floquet-engineered Valley Topology with Anisotropic Response in 1T'-WSe$_2$ and Janus WSeTe monolayers

Authors: Zhe Li, Haijun Cao, Lijuan Li, Huixia Fu, Mengxue Guan, Sheng Meng

Abstract: Valley topology has emerged as a key concept for realizing new classes of quantum states. Here, we investigate Floquet-engineered topological phase transitions in anisotropic 1T'-WSe$_2$ and its Janus derivative WSeTe monolayers, which exhibit valley-degenerate and valley-polarized characteristics, respectively. In 1T'-WSe$_2$, a single topological-phase-transition (TPT) occurs from the quantum-sp… ▽ More Valley topology has emerged as a key concept for realizing new classes of quantum states. Here, we investigate Floquet-engineered topological phase transitions in anisotropic 1T'-WSe$_2$ and its Janus derivative WSeTe monolayers, which exhibit valley-degenerate and valley-polarized characteristics, respectively. In 1T'-WSe$_2$, a single topological-phase-transition (TPT) occurs from the quantum-spin-Hall state (QSH) to the quantum anomalous Hall (QAH) state, involving one spin channel at both valleys simultaneously. In contrast, Janus WSeTe undergoes a two-stage Floquet-driven TPT that occurs within a single valley and sequentially involves two spin components. The intermediate phase manifests as a valley-polarized QAH (vp-QAH) state with a finite valley Chern number, while the final phase evolves into a high-Chern-number QAH state with distinct valley gaps. Furthermore, an in-plane anisotropic response of the TPTs is predicted under oblique light incidence, reflecting the intrinsic low-symmetry nature of the lattice. These findings provide a comprehensive understanding of Floquet-engineered valley-based topological properties and offer guidance for designing light-controllable valleytronic and topological devices. △ Less

Submitted 25 November, 2025; v1 submitted 20 November, 2025; originally announced November 2025.

Ultrafast μeV-Precision Bandgap Engineering in Low-Dimensional Topological Insulators

Authors: Peng Tan, Yuantao Chen, Yuqi Zhang, Hanyan Cheng, Guoyu Xian, Ming Cheng, Minghong Sun, Jiaxin Yin, Feifan Wang, Yaxian Wang, Yanjun Liu, Mingyuan Huang, Zhiwei Wang, Yugui Yao, Sheng Meng, Li Huang, Yanan Dai

Abstract: Precise and ultrafast control of electronic band structures is a central challenge for advancing quantum functional materials and devices. Conventional approaches--such as chemical doping, lattice strain, or external gating--offer robust stability but remain confined to the quasi-static regime, far from the intrinsic femto- to picosecond dynamics that govern many-body interactions. Here, using cry… ▽ More Precise and ultrafast control of electronic band structures is a central challenge for advancing quantum functional materials and devices. Conventional approaches--such as chemical doping, lattice strain, or external gating--offer robust stability but remain confined to the quasi-static regime, far from the intrinsic femto- to picosecond dynamics that govern many-body interactions. Here, using cryogenic transient reflectance spectroscopy, we realize dynamic bandgap engineering in the anisotropic topological insulator $α$-Bi$_4$Br$_4$ with unprecedented micro-electron-volt ($μ$eV) precision. The exceptional sensitivity arises from the cooperative action of long-lived topological carriers, stabilized by restricted bulk-to-edge scattering phase space, together with symmetry-resolved coherent phonons that modulate inter-chain hopping. These channels jointly modify Coulomb screening and interband transitions, enabling both gradual and oscillatory control of the electronic structure. Supported by first-principles and tight-binding theory, we further demonstrate a dual-pump coherent control strategy for continuous, mode-selective tuning of electronic energies with $μ$eV accuracy. This framework paves the way for ultrafast on-demand band-structure engineering, pointing toward new frontiers in quantum optoelectronics, precision measurement in molecular and biological systems, and attosecond control of matter. △ Less

Submitted 20 November, 2025; originally announced November 2025.

Deep Learning Assisted Prediction of Electrochemical Lithiation State in Spinel Lithium Titanium Oxide Thin Films

Authors: Devin Chugh, Bhagath Sreenarayanan, Steven Suwito, Ganesh Raghavendran, Bing Joe Hwang, Ying Shirley Meng, Weinien Su

Abstract: Machine Learning (ML) and Deep Learning (DL) based framework have evolved rapidly and generated considerable interests for predicting the properties of materials. In this work, we utilize ML-DL framework to predict the electrochemical lithiation state and associated electrical conductivity of spinel Li4Ti5O12 (LTO) thin films using Raman spectroscopy data. Raman spectroscopy, with its rapid, non-d… ▽ More Machine Learning (ML) and Deep Learning (DL) based framework have evolved rapidly and generated considerable interests for predicting the properties of materials. In this work, we utilize ML-DL framework to predict the electrochemical lithiation state and associated electrical conductivity of spinel Li4Ti5O12 (LTO) thin films using Raman spectroscopy data. Raman spectroscopy, with its rapid, non-destructive, and high-resolution capabilities, is leveraged to monitor dynamic electrochemical changes in LTO films. A comprehensive dataset of 3,272 Raman spectra, representing lithiation states from 0% to 100%, was collected and preprocessed using advanced techniques including cosmic ray removal, smoothing, baseline correction, normalization, and data augmentation. Classical machine learning models such as Support Vector Machine (SVM), Linear Discriminant Analysis (LDA), and Random Forest (RF) were evaluated alongside a Convolutional Neural Network (CNN). While traditional models achieved moderate to high accuracy, they struggled with generalization and noise sensitivity. In contrast, the CNN demonstrated superior performance, achieving over 99.5% accuracy and robust predictions on unseen samples. The CNN model effectively captured non-linear spectral features and showed resilience to experimental variability. This pipeline not only enables accurate lithiation state classification but also facilitates conductivity estimation, offering a scalable approach for real-time battery material characterization and potential extension to other spectroscopic datasets. △ Less

Submitted 18 November, 2025; originally announced November 2025.

Unveiling the delicate "hidden" interface conditions in WS2 flakes by advanced atomic force microscopy

Authors: Yanyan Geng, Chang Li, Shuo Mi, Manyu Wang, Xinen Han, Huiji Hu, Yunzhen Wang, Haojie You, Shumin Meng, Hanxiang Wu, Jianfeng Guo, Shiyu Zhu, Yanjun Li, Yasuhiro Sugawara, Sabir Hussain, Fei Pang, Rui Xu, Zhihai Cheng

Abstract: The delicate interfacial conditions and behaviors play critical roles in determining the valuable physical properties of two-dimensional materials and their heterostructures on substrates. However, directly probing these complex interface conditions remains challenging. Here, we reveal the coupled in-plane strain and out-of-plane bonding conditions in strain-engineered WS2 flakes by combining dual… ▽ More The delicate interfacial conditions and behaviors play critical roles in determining the valuable physical properties of two-dimensional materials and their heterostructures on substrates. However, directly probing these complex interface conditions remains challenging. Here, we reveal the coupled in-plane strain and out-of-plane bonding conditions in strain-engineered WS2 flakes by combining dual-harmonic electrostatic force microscopy (DH-EFM) and scanning microwave impedance microscopy (sMIM). A striking contradiction is observed between the compressive-strain-induced larger bandgap (lower electrical conductivity) detected by DH-EFM, and the enhanced conductivity probed by sMIM. Comparative measurements under different sMIM modes demonstrate that this contradiction originates from a tip-loading-force-induced dynamic puckering effect, which is governed by the interfacial bonding strength. Furthermore, the progressive accumulation and subsequent release of conductivity during forward/backward sMIM-contact scans further confirms this dynamic puckering behavior, revealing pronounced differences in interface conditions between the open- and closed-ring regions of WS2. This work resolves the correlation between electrical properties and interface conditions, and provides fundamental insights for interface-engineered devices. △ Less

Submitted 6 November, 2025; v1 submitted 27 October, 2025; originally announced October 2025.

Comments: 21 pages, 5 figures

Deciphering the dynamics of the light-induced phase transition in VO$_2$

Authors: S. Mandal, P. Kumar, H. Y. Kim, Z. Pi, J. Xu, D. Chen, D. Kazenwadel, P. Baum, S. Meng, E. Goulielmakis

Abstract: Vanadium dioxide (VO$_2$) is central in the study of ultrafast photoinduced insulator-to-metal phase transitions in strongly correlated materials, and a primary candidate for next-generation light-driven devices. However, the physical mechanism underlying its phase transition remains unresolved. Here, we use single-cycle light transients to perform phase-resolved ultrafast spectroscopy on VO$_2$ c… ▽ More Vanadium dioxide (VO$_2$) is central in the study of ultrafast photoinduced insulator-to-metal phase transitions in strongly correlated materials, and a primary candidate for next-generation light-driven devices. However, the physical mechanism underlying its phase transition remains unresolved. Here, we use single-cycle light transients to perform phase-resolved ultrafast spectroscopy on VO$_2$ crystals. Our experiments reveal two processes: a structural transformation from the insulating monoclinic M1-VO$_2$ phase to the excited metallic rutile R*-VO$_2$ phase, followed by electron thermalization and relaxation dynamics intrinsic to the newly formed excited metallic phase. △ Less

Submitted 22 September, 2025; originally announced September 2025.

Laser-engineered $Γ$-point Topology in Trigonal Bismuthene

Authors: Zhe Li, Haijun Cao, Sheng Meng

Abstract: The $Γ$-point topology represents a significant segment in the family of topological insulators. Here we provide a comprehensive prediction of light-induced $Γ$-point-based topological manipulation in trigonal bismuthene and its derivatives. Our findings unveil a two-stage process of topological phase transitions (TPT) as the laser intensity increases. Initially, a quantum-spin-Hall or metallic st… ▽ More The $Γ$-point topology represents a significant segment in the family of topological insulators. Here we provide a comprehensive prediction of light-induced $Γ$-point-based topological manipulation in trigonal bismuthene and its derivatives. Our findings unveil a two-stage process of topological phase transitions (TPT) as the laser intensity increases. Initially, a quantum-spin-Hall or metallic state transitions to a quantum-anomalous-Hall (QAH) state ($C$ = $\pm$3), followed by another TPT that yields a compensated Chern-insulating state ($C$ = 0). The trigonal warping model accounts for these states, describing the $C_{3z}$-rotational band-inversion process, which is determined by $\pm$1 orders of replica bands. Notably, this high Chern-number QAH state persists over a broad range of laser parameters, maintaining functionality beyond room temperature as evidenced by the large global gaps ($\geq$ 60 meV). Our work provides a comprehensive roadmap towards the designer $Γ$-point topology under laser excitation, facilitating applications of artificial topological materials. △ Less

Submitted 11 September, 2025; v1 submitted 9 September, 2025; originally announced September 2025.

FastTrack: a fast method to evaluate mass transport in solid leveraging universal machine learning interatomic potential

Authors: Hanwen Kang, Tenglong Lu, Zhanbin Qi, Jiandong Guo, Sheng Meng, Miao Liu

Abstract: We introduce a rapid, accurate framework for computing atomic migration barriers in crystals by combining universal machine learning force fields (MLFFs) with 3D potential energy surface sampling and interpolation. Our method suppresses periodic self interactions via supercell expansion, builds a continuous PES from MLFF energies on a spatial grid, and extracts minimum energy pathways without pred… ▽ More We introduce a rapid, accurate framework for computing atomic migration barriers in crystals by combining universal machine learning force fields (MLFFs) with 3D potential energy surface sampling and interpolation. Our method suppresses periodic self interactions via supercell expansion, builds a continuous PES from MLFF energies on a spatial grid, and extracts minimum energy pathways without predefined NEB images. Across twelve benchmark electrode and electrolyte materials including LiCoO2, LiFePO4, and LGPS our MLFF-derived barriers lie within tens of meV of DFT and experiment, while achieving ~10^2 x speedups over DFT-NEB. We benchmark GPTFF, CHGNet, and MACE, show that fine-tuning on PBE/PBE+U data further enhances accuracy, and provide an open-source package for high-throughput materials screening and interactive PES visualization. △ Less

Submitted 14 August, 2025; originally announced August 2025.

Journal ref: AI Sci. 1 015004 (2025)

Observation of quasi-steady dark excitons and gap phase in a doped semiconductor

Authors: Shangkun Mo, Yunfei Bai, Chunlong Wu, Xingxia Cui, Guangqiang Mei, Qiang Wan, Renzhe Li, Cao Peng, Keming Zhao, Dingkun Qin, Shuming Yu, Hao Zhong, Xingzhe Wang, Enting Li, Yiwei Li, Limin Cao, Min Feng, Sheng Meng, Nan Xu

Abstract: Exciton plays an important role in optics and optics-related behaviors and leads to novel correlated phases like charge order, exciton insulator, and exciton-polariton condensation. Dark exciton shows distinct properties from bright one. However, it cannot be directly detected by conventional optic measurements. The electronic modulation effect of dark excitons in quasi-equilibrium distribution, c… ▽ More Exciton plays an important role in optics and optics-related behaviors and leads to novel correlated phases like charge order, exciton insulator, and exciton-polariton condensation. Dark exciton shows distinct properties from bright one. However, it cannot be directly detected by conventional optic measurements. The electronic modulation effect of dark excitons in quasi-equilibrium distribution, critical for electronic devices in working status, is still elusive. Here, using angle-resolved photoemission spectroscopy, we report creating, detecting, and controlling dark excitons in the quasi-equilibrium distribution in a doped semiconductor SnSe2. Surprisingly, we observe an excitonic gap phase, with a conduction band opening an anisotropic gap. Our results broaden the scope of dark excitons, extending their studies from the picosecond timescale in the ultrafast photoemission process to conditions occurring under quasi-equilibrium. We reveal the light-matter interaction in the engineering of electronic structures and provide a new way to realize the excitonic gap phase in semiconductors with large band gaps. △ Less

Submitted 11 July, 2025; originally announced July 2025.

Comments: 16 pages, 5 figures

Atomic to mesoscale hierarchical structures and magnetic states in an anisotropic layered ferromagnet FePd2Te2

Authors: Shuo Mi, Manyu Wang, Bingxian Shi, Songyang Li, Xiaoxiao Pei, Yanyan Geng, Shumin Meng, Rui Xu, Li Huang, Wei Ji, Fei Pang, Peng Cheng, Jianfeng Guo, Zhihai Cheng

Abstract: Two-dimensional (2D) magnetic materials have predominantly exhibited easy-axis or easy-plane anisotropy and display a high sensitivity to the underlying crystal structure and lattice symmetry. Recently, an in-plane anisotropic 2D ferromagnet of FePd2Te2 has been discovered with intriguing structure and quasi-one-dimensional spin system. Here, we report a real-space investigation of its twinning st… ▽ More Two-dimensional (2D) magnetic materials have predominantly exhibited easy-axis or easy-plane anisotropy and display a high sensitivity to the underlying crystal structure and lattice symmetry. Recently, an in-plane anisotropic 2D ferromagnet of FePd2Te2 has been discovered with intriguing structure and quasi-one-dimensional spin system. Here, we report a real-space investigation of its twinning structure and magnetic states using atomic/magnetic force microscopy (AFM/MFM) combined with scanning tunneling microscopy (STM). The atomic to mesoscale hierarchical structures with the orthogonal and corrugated compressive /tensile(C/T) regions are directly observed due to the intrinsic twinning-domain characteristic. The structure-related intact ferromagnetic (FM), field-induced polarized-FM states and their transitions are comparatively discussed at the mesoscale with the corresponding macroscopic magnetic measurements. Temperature- and field-dependent evolution of magnetic phase are further investigated at the FM and PM states, and summarized to obtain a unique H-T phase diagram of FePd2Te2. Our work provides key results for understanding the complicated magnetic properties of FePd2Te2, and suggests new directions for manipulating magnetic states through the atomic and mesoscale structure engineering. △ Less

Submitted 10 June, 2025; originally announced June 2025.

Photostriction-tunable Polarization and Structural Dynamics in Interlayer Sliding Ferroelectrics

Authors: Kun Yang, Jianxin Yu, Jia Zhang, Sheng Meng, Jin Zhang

Abstract: Two-dimensional ferroelectrics with robust polarization offer promising opportunities for non-volatile memory, field-effect transistors, and optoelectronic devices. However, the impact of lattice deformation on polarization and photoinduced structural response remains poorly understood. Here, we employ first-principles calculations to demonstrate photodoping-induced lattice expansion in rhombohedr… ▽ More Two-dimensional ferroelectrics with robust polarization offer promising opportunities for non-volatile memory, field-effect transistors, and optoelectronic devices. However, the impact of lattice deformation on polarization and photoinduced structural response remains poorly understood. Here, we employ first-principles calculations to demonstrate photodoping-induced lattice expansion in rhombohedrally stacked bilayer MoS2, revealing a strong coupling between photodoping carrier and lattice structure. We identify a pronounced photostrictive response in sliding ferroelectrics, wherein electron-hole excitation leads to substantial in-plane expansion, increased interlayer spacing, and enhanced ferroelectric polarization. This strain-induced modulation drives significant bandgap renormalization. The photostriction-tunable polarization and structural dynamics arise from the strong electromechanical coupling inherent to the non-centrosymmetric rhombohedral stacking. The findings provide critical insights into the nonthermal lattice expansion governing sliding ferroelectrics at atomic-scale timescales, while simultaneously laying the groundwork for next-generation electronic and memory technologies by leveraging lattice-tunable polarization switching. △ Less

Submitted 29 May, 2025; originally announced May 2025.

Comments: 4 figures

Tailoring composite skyrmionic spin textures in an above-room-temperature ferromagnet Fe3-xGaTe2

Authors: Songyang Li, Jianfeng Guo, Zizhao Gong, Guojing Hu, Shuo Mi, Chang Li, Yanyan Geng, Manyu Wang, Shumin Meng, Shiyu Zhu, Fei Pang, Wei Ji, Rui Xu, Haitao Yang, Zhihai Cheng

Abstract: Realizing room-temperature tunable skyrmionic objects in van der Waals ferromagnet offers unparalleled prospects for future spintronics. Here, we report an experimental investigation on the emergence and evolution of skyrmionic spin textures in the non-stoichiometric Fe3-xGaTe2 using magnetic force microscopy. The iron-deficiency-specific magnetic states of stripe, striped skyrmionium and striped… ▽ More Realizing room-temperature tunable skyrmionic objects in van der Waals ferromagnet offers unparalleled prospects for future spintronics. Here, we report an experimental investigation on the emergence and evolution of skyrmionic spin textures in the non-stoichiometric Fe3-xGaTe2 using magnetic force microscopy. The iron-deficiency-specific magnetic states of stripe, striped skyrmionium and striped skyrmion sack are observed. Through zero-field-cooling and field-cooling measurements, we observed distinct topological transitions and trivial transitions (distinguished by changes in topological charge) emerging during the stepwise evolution of topological spin textures, which enabled us to develop an evolution pathway model. Leveraging this model, the room-temperature stable composite topological spin textures of skyrmionium, skyrmion bag and sack states are further controllably realized via the exclusive topological-transition path (regulated by magnetic field and DMI intensity). Our work provides valuable insights into the room-temperature realization of topological spin textures in Fe3-xGaTe2, and inspires further exploration of their potential applications in heterostructure spintronics. △ Less

Submitted 8 May, 2025; originally announced May 2025.

Comments: 26 pages, 5 figures

Light-driven lattice metastability for enhanced superconductivity in FeSe/SrTiO3

Authors: Qiang Zou, Zhan Su, Andres Tellez Mora, Na Wu, Joseph Benigno, Christopher L. Jacobs, Aldo H. Romero, Subhasish Mandal, Yaxian Wang, Sheng Meng, Michael Weinert, Hua Zhou, Lian Li, Cheng Cen

Abstract: Driven quantum materials with on demand properties controlled by external stimuli are critical for emergent quantum technology. In optically tunable superconducting heterostructures, the lattice responses at the buried interface may hold the key to the light susceptibility but is very challenging to detect. In this work, a nondestructive synchrotron-based X-ray scattering phase-retrieval technique… ▽ More Driven quantum materials with on demand properties controlled by external stimuli are critical for emergent quantum technology. In optically tunable superconducting heterostructures, the lattice responses at the buried interface may hold the key to the light susceptibility but is very challenging to detect. In this work, a nondestructive synchrotron-based X-ray scattering phase-retrieval technique is implemented in monolayer-FeSe/SrTiO3 heterostructures to capture the three-dimensional interfacial atomic displacements in-situ as the interface superconductivity is actively manipulated by light. It is found that the interlayer sliding between FeSe and SrTiO3 can drastically alter how the lattice responds to the light. In domains with selected stacking configurations, the interface transforms the very weak photoexcitation in SrTiO3 into significant Fe-atom displacements in FeSe and generate metastable interfacial structures that can lead to a persistent superconductivity enhancement. These findings demonstrate an effective strategy for achieving greatly amplified light-lattice coupling for efficient quantum phase manipulations at designed interfaces. △ Less

Submitted 24 April, 2025; originally announced April 2025.

Photostriction Facilitates Relaxation of Lattice Distortion in Two-Dimensional Perovskites

Authors: Jin Zhang, Kun Yang, Jianxin Yu, Jia Zhang, Sheng Meng, Xinghua Shi, Wei-Hai Fang

Abstract: The photostriction effect, a light-induced mechanical deformation in materials, originates from the intricate interplay between lattice structure and electronic excitation. In photovoltaic semiconductors, this effect plays a crucial role in shaping non-equilibrium structural responses, yet its fundamental mechanism remains elusive. Here, we uncover lattice expansion and structural reconfiguration… ▽ More The photostriction effect, a light-induced mechanical deformation in materials, originates from the intricate interplay between lattice structure and electronic excitation. In photovoltaic semiconductors, this effect plays a crucial role in shaping non-equilibrium structural responses, yet its fundamental mechanism remains elusive. Here, we uncover lattice expansion and structural reconfiguration in two-dimensional (2D) perovskites driven by photoinduced excitation using first-principles calculations. Our findings reveal that the photoinduced carriers lead to a substantial lattice expansion by about 2%. The expanded lattice facilitates strain relaxation with the amplitude of 20% by increasing interatomic distances and reducing internal stresses, thereby enhancing structural stability. The lattice dynamics can be systematically engineered through photodoping density, unveiling a new pathway to modulate light-matter interactions in 2D perovskites. These insights not only advance the understanding of optically driven structural dynamics but also offer a guiding principle for optimizing next-generation high-efficiency photovoltaic devices and optoelectronics. △ Less

Submitted 15 March, 2025; originally announced March 2025.

Comments: 17 pages, 4 figures

Intrinsic exciton transport and recombination in single-crystal lead bromide perovskite

Authors: Zhixuan Bi, Yunfei Bai, Ying Shi, Tao Sun, Heng Wu, Haochen Zhang, Yuhang Cui, Danlei Zhu, Yubin Wang, Miao-Ling Lin, Yaxian Wang, Dongxin Ma, Ping-Heng Tan, Sheng Meng, Qihua Xiong, Luyi Yang

Abstract: Photogenerated carrier transport and recombination in metal halide perovskites are critical to device performance. Despite considerable efforts, sample quality issues and measurement techniques have limited the access to their intrinsic physics. Here, by utilizing high-purity CsPbBr3 single crystals and contact-free transient grating spectroscopy, we directly monitor exciton diffusive transport fr… ▽ More Photogenerated carrier transport and recombination in metal halide perovskites are critical to device performance. Despite considerable efforts, sample quality issues and measurement techniques have limited the access to their intrinsic physics. Here, by utilizing high-purity CsPbBr3 single crystals and contact-free transient grating spectroscopy, we directly monitor exciton diffusive transport from 26 to 300 K. As the temperature (T) increases, the carrier mobility (μ) decreases rapidly below 100 K wtih a μ~T^{-3.0} scaling, and then follows a more gradual μ~T^{-1.7} trend at higher temperatures. First-principles calculations perfectly reproduce this experimental trend and reveal that optical phonon scattering governs carrier mobility shifts over the entire temperature range, with a single longitudinal optical mode dominating room-temperature transport. Time-resolved photoluminescence further identifies a substantial increase in exciton radiative lifetime with temperature, attributed to increased exciton population in momentum-dark states caused by phonon scattering. Our findings unambiguously resolve previous theory-experiment discrepancies, providing benchmarks for future optoelectronic design. △ Less

Submitted 10 May, 2025; v1 submitted 3 March, 2025; originally announced March 2025.

Journal ref: ACS Nano 19, 19989-20000 (2025)

Flat bands and temperature-driven phase transition in quasi-one-dimensional zigzag chains

Authors: Jisong Gao, Haijun Cao, Xuegao Hu, Hui Zhou, Zhihao Cai, Qiaoxiao Zhao, Dong Li, Zhicheng Gao, Shin-ichiro Ideta, Kenya Shimada, Peng Cheng, Lan Chen, Kehui Wu, Sheng Meng, Baojie Feng

Abstract: Flat-band materials have garnered extensive attention due to their captivating properties associated with strong correlation effects. While flat bands have been discovered in several types of 2D materials, their existence in 1D systems remains elusive. Here, we propose a 1D frustrated lattice, specifically the 1D zigzag lattice, as a platform for hosting flat bands. This lattice can be experimenta… ▽ More Flat-band materials have garnered extensive attention due to their captivating properties associated with strong correlation effects. While flat bands have been discovered in several types of 2D materials, their existence in 1D systems remains elusive. Here, we propose a 1D frustrated lattice, specifically the 1D zigzag lattice, as a platform for hosting flat bands. This lattice can be experimentally realized by growing CuTe chains on Cu(111). The presence of flat bands was confirmed by tight-binding model analysis, first-principles calculations, and angle-resolved photoemission spectroscopy measurements. In addition, we discovered a temperature-driven phase transition at approximately 250 K. Detailed analyses demonstrate that the system has a Tomonaga-Luttinger liquid behavior, accompanied by spin-charge separation effects. Our work unveils new prospects for investigating strongly correlated electron behaviors and topological properties in the 1D limit. △ Less

Submitted 3 March, 2025; originally announced March 2025.

Journal ref: Physical Review Letters 134, 086202 (2025)

Photoexcitation-induced Stacking Transition Assisted by Intralayer Reconstruction in Charge-Density-Wave Materials

Authors: Jin Zhang, Yang Yang, Jia Zhang, Mengxue Guan, Jiyu Xu, Kun Yang, Xinghua Shi, Sheng Meng

Abstract: Laser excitation has emerged as an effective tool for probing microscopic interactions and manipulating phases of matter. Among charge density wave (CDW) materials, 1T-TaS2 has garnered significant attention due to its diverse stacking orders and photoexcited responses. However, the mechanisms driving transitions among different stacking orders and the microscopic out-of-equilibrium dynamics remai… ▽ More Laser excitation has emerged as an effective tool for probing microscopic interactions and manipulating phases of matter. Among charge density wave (CDW) materials, 1T-TaS2 has garnered significant attention due to its diverse stacking orders and photoexcited responses. However, the mechanisms driving transitions among different stacking orders and the microscopic out-of-equilibrium dynamics remain unclear. We elucidate that photoexcitation can introduce interlayer stacking order transitions facilitated by laser-induced intralayer reconstruction in 1T-TaS2. Importantly, our finding reveals a novel pathway to introduce different phases through laser excitations, apparently distinct from thermally-induced phase transitions via interlayer sliding. In particular, photoexcitation is able to considerably change potential energy surfaces and evoke collective lattice dynamics. Consequently, the laser-induced intralayer reconstruction plays a crucial role in interlayer stacking-order transition, offering a new method to create exotic stackings and quantum phases. The exploration opens up great opportunities for manipulating CDW phases and electronic properties on the femtosecond timescale. △ Less

Submitted 25 February, 2025; originally announced February 2025.

Comments: 10 pages, 4 figures

Large moiré superstructure of stacked incommensurate charge density waves

Authors: B. Q. Lv, Yifan Su, Alfred Zong, Qiaomei Liu, Dong Wu, Noah F. Q. Yuan, Zhengwei Nie, Jiarui Li, Suchismita Sarker, Sheng Meng, Jacob P. C. Ruff, N. L. Wang, Nuh Gedik

Abstract: Recent advances in van der Waals heterostructures have opened the new frontier of moiré physics, whereby tuning the interlayer twist angle or adjusting lattice parameter mismatch have led to a plethora of exotic phenomena such as unconventional superconductivity and fractional quantum spin Hall effect. We extend the concept of moiré engineering to materials that host incommensurate orders, where w… ▽ More Recent advances in van der Waals heterostructures have opened the new frontier of moiré physics, whereby tuning the interlayer twist angle or adjusting lattice parameter mismatch have led to a plethora of exotic phenomena such as unconventional superconductivity and fractional quantum spin Hall effect. We extend the concept of moiré engineering to materials that host incommensurate orders, where we discovered a long-period, thermally-hysteretic moiré superlattice in a layered charge density wave (CDW) compound, EuTe$_\text{4}$. Using high-momentum-resolution X-ray diffraction performed on ultrathin flakes, we found two coexisting, incommensurate CDWs with slightly mismatched in-plane wavevectors. The interaction between these two CDWs leads to their joint commensuration with the high-symmetry lattice as well as a large moiré superstructure with an in-plane period of 13.6~nm. Due to different out-of-plane orders of the incommensurate CDWs, the moiré superstructure exhibits a clear thermal hysteresis, accounting for the large hysteresis observed in electrical resistivity and numerous metastable states induced by light or electrical pulses. Our findings pave the way for a new development in moiré engineering based on an incommensurate lattice. They further highlight the important role of interlayer ordering in determining the macroscopic properties of these stacked incommensurate structures. △ Less

Submitted 16 January, 2025; originally announced January 2025.

Comments: 9 pages, 4 figures

Guidelines for Correlative Imaging and Analysis of Reactive Lithium Metal Battery Materials

Authors: Shuang Bai, Zhao Liu, Diyi Cheng, Bingyu Lu, Nestor J. Zaluzec, Ganesh Raghavendran, Shen Wang, Thomas S. Marchese, Brandon van Leer, Letian Li, Lin Jiang, Adam Stokes, Joseph P. Cline, Rachel Osmundsen, Paul Barends, Alexander Bright, Minghao Zhang, Ying Shirley Meng

Abstract: To unlock the full potential of lithium metal batteries, a deep understanding of lithium metal reactivity and its solid electrolyte interphase is essential. Correlative imaging, combining focused ion beam and electron microscopy offers a powerful approach for multi-scale characterization. However, the extreme reactivity of lithium metal and its SEI presents challenges in investigating deposition a… ▽ More To unlock the full potential of lithium metal batteries, a deep understanding of lithium metal reactivity and its solid electrolyte interphase is essential. Correlative imaging, combining focused ion beam and electron microscopy offers a powerful approach for multi-scale characterization. However, the extreme reactivity of lithium metal and its SEI presents challenges in investigating deposition and stripping mechanisms. In this work, we systematically evaluated the storage stability of lithium metal in glovebox before and after electrochemical deposition. We then assessed different FIB ion sources for their impact on lithium metal lamella preparation for transmission electron microscopy. Furthermore, we examined cryogenic-TEM transfer methods, optimizing for minimal contamination during sample handling. Contrary to prior assumptions, we demonstrate that high resolution imaging of pure lithium metal at room temperature is achievable using inert gas transfer with an electron dose rate exceeding 1000 e/A2/s, without significant detectable damage. In contrast, SEI components, such as Li2CO3 and LiF display much greater sensitivity to electron beams, requiring cryogenic conditions and precise dose control for nano/atomic scale imaging. We quantified electron dose limits for these SEI components to track their structural evolution under irradiation. Based on these findings, we propose a robust protocol for lithium metal sample handling - from storage to atomic-level characterization - minimizing damage and contamination. This work paves the way for more accurate and reproducible studies, accelerating the development of next-generation lithium metal batteries by ensuing the preservation of native material properties during analysis. △ Less

Submitted 26 December, 2024; originally announced December 2024.

Comments: 31 pages, 6 figures

Ultrafast Polarization Switching via Laser-activated Ionic Migration in Ferroelectric CuInP$_2$S$_6$

Authors: Jin Zhang, Kun Yang, Jianxin Yu, Huixia Fu, Zijing Ding, Xinghua Shi, Sheng Meng

Abstract: As a layered ferroelectric material, CuInP2S6 has garnered significant attention for its robust ferroelectric state and potential applications in memory devices. In this work, we demonstrate that with short laser pulses ultrafast reversible polarization switching within hundreds of femtoseconds can be achieved in ferroelectric CuInP$_2$S$_6$. Specifically, photoexcitation triggers collective ionic… ▽ More As a layered ferroelectric material, CuInP2S6 has garnered significant attention for its robust ferroelectric state and potential applications in memory devices. In this work, we demonstrate that with short laser pulses ultrafast reversible polarization switching within hundreds of femtoseconds can be achieved in ferroelectric CuInP$_2$S$_6$. Specifically, photoexcitation triggers collective ionic migration and ferroelectricity reversal in CuInP$_2$S$_6$, revealing a novel pathway to access different ferroelectric phases through optical excitation. Our findings indicate that laser pulses substantially alter the transition barriers, promoting ionic transport facilitated by the photodoping effect. This laser-induced ionic migration proves critical for enabling polarization transitions, offering a novel pathway to explore and control exotic quantum phases. These insights open exciting possibilities for manipulating ferroelectric states and electronic properties on an unprecedented ultrafast timescale. △ Less

Submitted 13 December, 2024; originally announced December 2024.

Grain Selection Growth of Soft Metal in Electrochemical Processes

Authors: Minghao Zhang, Karnpiwat Tantratian, So-Yeon Ham, Zhuo Wang, Mehdi Chouchane, Ryosuke Shimizu, Shuang Bai, Hedi Yang, Zhao Liu, Letian Li, Amir Avishai, Lei Chen, Ying Shirley Meng

Abstract: Soft metals like lithium and sodium play a critical role in battery technology owing to their high energy density. Texture formation by grain selection growth of soft metals during electrochemical processes is a crucial factor affecting power and safety. Developing a framework to understand and control grain growth is a multifaceted challenge. Here, a general thermodynamic theory and phase-field m… ▽ More Soft metals like lithium and sodium play a critical role in battery technology owing to their high energy density. Texture formation by grain selection growth of soft metals during electrochemical processes is a crucial factor affecting power and safety. Developing a framework to understand and control grain growth is a multifaceted challenge. Here, a general thermodynamic theory and phase-field model are formulated to study grain selection growth of soft metals. Our study focuses on the interplay between surface energy and atomic mobility-related intrinsic strain energy in grain selection growth. Differences in grain selection growth arise from the anisotropy in surface energy and diffusion barrier of soft metal atoms. Our findings highlight the kinetic limitations of solid-state Li metal batteries, which originate from load stress-induced surface energy anisotropy. These insights lead to the development of an amorphous LixSi1-x (0.50<x<0.79) seed layer, improving the critical current density at room temperature for anode-free Li solid-state batteries through the control of grain selection growth. △ Less

Submitted 16 November, 2024; originally announced November 2024.

Quantum Interference and Optical Tuning of Self-Trapped Exciton State in Double Halide Perovskite

Authors: Kai-Xuan Xu, Xin-bao Liu, Simin Pang, Zhe Zhang, Yubin Wang, Jiajun Luo, Jiang Tang, Qihua Xiong, Sheng Meng, Shiwu Gao, Jun Zhang

Abstract: Self-trapped excitons (STEs), renowned for their unique radiative properties, have been harnessed in diverse photonic devices. Yet, a full comprehension and manipulation of STEs remain elusive. In this study, we present novel experimental and theoretical evidence of the hybrid nature and optical tuning of the STEs state in Cs2Ag0.4Na0.6InCl6. The detection of Fano resonance in the laser energy-dep… ▽ More Self-trapped excitons (STEs), renowned for their unique radiative properties, have been harnessed in diverse photonic devices. Yet, a full comprehension and manipulation of STEs remain elusive. In this study, we present novel experimental and theoretical evidence of the hybrid nature and optical tuning of the STEs state in Cs2Ag0.4Na0.6InCl6. The detection of Fano resonance in the laser energy-dependent Raman and photoluminescence spectra indicates the emergence of an exciton-phonon hybrid state, a result of the robust quantum interference between the discrete phonon and continuous exciton states. Moreover, we showcase the ability to continuously adjust this hybrid state with the energy and intensity of the laser field. These significant findings lay the foundation for a comprehensive understanding of the nature of STE and its potential for state control. △ Less

Submitted 27 October, 2024; originally announced October 2024.

Optimizing attosecond pulse generation in solids by modulating electronic dynamics with monochromatic laser field

Authors: Xinyuan Zhang, Shiqi Hu, Mengxue Guan, Sheng Meng

Abstract: A practical approach is proposed for efficiently generating ultrashort attosecond pulses (APs) from realistic solid-state materials, aiming to optimize pulse width effectively. By adjusting the photon energy while maintaining a constant peak electric field, this strategy modulates the peak vector potential and laser field period, thereby controlling the high harmonic cutoff energy and the time-dom… ▽ More A practical approach is proposed for efficiently generating ultrashort attosecond pulses (APs) from realistic solid-state materials, aiming to optimize pulse width effectively. By adjusting the photon energy while maintaining a constant peak electric field, this strategy modulates the peak vector potential and laser field period, thereby controlling the high harmonic cutoff energy and the time-domain emission characteristics of the harmonics. The field-driven electronic dynamics lead to a non-monotonic variation in both the intensity and duration of the generated attosecond pulses. The light field frequency can be adjusted to yield the optimal pulse. Beyond the primary demonstration with hexagonal boron nitride as a prototypical material, significant pulse width optimization has been achieved across a range of different materials. This straightforward and versatile strategy shows promise for application in solid-state materials, offering new pathways to promote high harmonic performance. △ Less

Submitted 23 February, 2025; v1 submitted 22 October, 2024; originally announced October 2024.

Comments: 10 pages, 9 figures

Journal ref: Phys. Rev. A 111, 023521 (2025)

Chemical versus physical pressure effects on the structure transition of bilayer nickelates

Authors: Gang Wang, Ningning Wang, Tenglong Lu, Stuart Calder, Jiaqiang Yan, Lifen Shi, Jun Hou, Liang Ma, Lili Zhang, Jianping Sun, Bosen Wang, Sheng Meng, Miao Liu, Jinguang Cheng

Abstract: The observation of high-$T_c$ superconductivity (HTSC) in concomitant with pressure-induced orthorhombic-tetragonal structural transition in the bilayer La$_{3}$Ni$_2$O$_7$ has sparked hopes of achieving HTSC by stabilizing the tetragonal phase at ambient pressure. To mimic the effect of external physical pressures, the application of chemical pressure via replacing La$^3$$^+$ with smaller rare-ea… ▽ More The observation of high-$T_c$ superconductivity (HTSC) in concomitant with pressure-induced orthorhombic-tetragonal structural transition in the bilayer La$_{3}$Ni$_2$O$_7$ has sparked hopes of achieving HTSC by stabilizing the tetragonal phase at ambient pressure. To mimic the effect of external physical pressures, the application of chemical pressure via replacing La$^3$$^+$ with smaller rare-earth R$^3$$^+$ has been considered as a potential route. Here we clarify the distinct effects of chemical and physical pressures on the structural transition of bilayer nickelates through a combined experimental and theoretical investigation. Contrary to general expectations, we find that substitutions of smaller R$^3$$^+$ for La$^3$$^+$ in La$_{3-x}$R$_x$Ni$_2$O$_{7-δ}$, despite of an overall lattice contraction, produce stronger orthorhombic structural distortions and thus require higher pressures to induce the structural transition. We established a quantitative relationship between the critical pressure $P_c$ for structural transition and the average size of $A$-site cations. A linear extrapolation of $P_c$ versus <$r_A$> yields a putative critical value of <$r_A$>$_c$ ~ 1.23 angstrom for $P_c$ ~ 1 bar. The negative correlation between $P_c$ and <$r_A$> indicates that it is unlikely to reduce $P_c$ to ambient by replacing La$^3$$^+$ with smaller R$^3$$^+$ ions. Instead, partial substitution of La$^3$$^+$ with larger cations such as alkaline-earth Sr$^2$$^+$ or Ba$^2$$^+$ might be a feasible approach. Our results provide valuable guidelines in the quest of ambient-pressure HTSC in bilayer nickelates. △ Less

Submitted 18 August, 2024; originally announced August 2024.

Comments: 17 pages and 5 figures

Evidence for Two-dimensional Weyl Fermions in Air-Stable Monolayer PtTe$_{1.75}$

Authors: Zhihao Cai, Haijun Cao, Haohao Sheng, Xuegao Hu, Zhenyu Sun, Qiaoxiao Zhao, Jisong Gao, Shin-ichiro Ideta, Kenya Shimada, Jiawei Huang, Peng Cheng, Lan Chen, Yugui Yao, Sheng Meng, Kehui Wu, Zhijun Wang, Baojie Feng

Abstract: The Weyl semimetals represent a distinct category of topological materials wherein the low-energy excitations appear as the long-sought Weyl fermions. Exotic transport and optical properties are expected because of the chiral anomaly and linear energy-momentum dispersion. While three-dimensional Weyl semimetals have been successfully realized, the quest for their two-dimensional (2D) counterparts… ▽ More The Weyl semimetals represent a distinct category of topological materials wherein the low-energy excitations appear as the long-sought Weyl fermions. Exotic transport and optical properties are expected because of the chiral anomaly and linear energy-momentum dispersion. While three-dimensional Weyl semimetals have been successfully realized, the quest for their two-dimensional (2D) counterparts is ongoing. Here, we report the realization of 2D Weyl fermions in monolayer PtTe$_{1.75}$, which has strong spin-orbit coupling and lacks inversion symmetry, by combined angle-resolved photoemission spectroscopy, scanning tunneling microscopy, second harmonic generation, X-ray photoelectron spectroscopy measurements, and first-principles calculations. The giant Rashba splitting and band inversion lead to the emergence of three pairs of critical Weyl cones. Moreover, monolayer PtTe$_{1.75}$ exhibits excellent chemical stability in ambient conditions, which is critical for future device applications. The discovery of 2D Weyl fermions in monolayer PtTe$_{1.75}$ opens up new possibilities for designing and fabricating novel spintronic devices. △ Less

Submitted 12 December, 2024; v1 submitted 30 July, 2024; originally announced July 2024.

Journal ref: Nano Lett. 24, 10237-10243 (2024)

Unveiling van Hove singularity modulation and fluctuated charge order in kagome superconductor $\rm{CsV_3Sb_5}$ via time-resolved ARPES

Authors: Yigui Zhong, Takeshi Suzuki, Hongxiong Liu, Kecheng Liu, Zhengwei Nie, Youguo Shi, Sheng Meng, Baiqing Lv, Hong Ding, Teruto Kanai, Jiro Itatani, Shik Shin, Kozo Okazaki

Abstract: Kagome superconductor CsV3Sb5, which exhibits intertwined unconventional charge density wave (CDW) and superconductivity, has garnered significant attention recently. Despite extensive static studies, the nature of these exotic electronic orders remains elusive. In this study, we investigate the non-equilibrium electronic structure of CsV3Sb5 via time- and angle-resolved photoemission spectroscopy… ▽ More Kagome superconductor CsV3Sb5, which exhibits intertwined unconventional charge density wave (CDW) and superconductivity, has garnered significant attention recently. Despite extensive static studies, the nature of these exotic electronic orders remains elusive. In this study, we investigate the non-equilibrium electronic structure of CsV3Sb5 via time- and angle-resolved photoemission spectroscopy. Our results reveal that upon laser excitation, the van Hove singularities immediately shift towards the Fermi level and subsequently oscillate in sync with a 1.3 THz coherent phonon mode. By analyzing the coherent intensity oscillations in the energy-momentum (E-k) map, we find that this coherent phonon is strongly coupled with electronic bands from both Sb and V orbitals. While typically observable only in the CDW state, remarkably, we find that the 1.3-THz coherent phonon mode can be persistently excited at temperatures above T_CDW, suggesting the potential existence of fluctuated CDW in CsV3Sb5. These findings enhance our understanding of the unconventional CDW control of kagome superconductivity. △ Less

Submitted 24 July, 2024; originally announced July 2024.

Comments: 11 pages, 5 figures

Journal ref: Physical Review Research 6, 043328 (2024)

Preservation of Topological Surface States in Millimeter-Scale Transferred Membranes

Authors: Chi Ian Jess Ip, Qiang Gao, Khanhy Du Nguyen, Chenhui Yan, Gangbin Yan, Eli Hoenig, Thomas S. Marchese, Minghao Zhang, Woojoo Lee, Hossein Rokni, Ying Shirley Meng, Chong Liu, Shuolong Yang

Abstract: Ultrathin topological insulator membranes are building blocks of exotic quantum matter. However, traditional epitaxy of these materials does not facilitate stacking in arbitrary orders, while mechanical exfoliation from bulk crystals is also challenging due to the non-negligible interlayer coupling therein. Here we liberate millimeter-scale films of topological insulator Bi$_2$Se$_3$, grown by mol… ▽ More Ultrathin topological insulator membranes are building blocks of exotic quantum matter. However, traditional epitaxy of these materials does not facilitate stacking in arbitrary orders, while mechanical exfoliation from bulk crystals is also challenging due to the non-negligible interlayer coupling therein. Here we liberate millimeter-scale films of topological insulator Bi$_2$Se$_3$, grown by molecular beam epitaxy, down to 3 quintuple layers. We characterize the preservation of the topological surface states and quantum well states in transferred Bi$_{2}$Se$_{3}$ films using angle-resolved photoemission spectroscopy. Leveraging the photon-energy-dependent surface sensitivity, the photoemission spectra taken with $6$ eV and $21.2$ eV photons reveal a transfer-induced migration of the topological surface states from the top to the inner layers. By establishing clear electronic structures of the transferred films and unveiling the wavefunction relocation of the topological surface states, our work paves the physics foundation crucial for the future fabrication of artificially stacked topological materials with single-layer precision. △ Less

Submitted 21 May, 2024; originally announced May 2024.

Comments: 4 figures

Journal ref: Nano Letters 2024

Nonvolatile optical control of interlayer stacking order in 1T-TaS2

Authors: Junde Liu, Pei Liu, Liu Yang, Sung-Hoon Lee, Mojun Pan, Famin Chen, Jierui Huang, Bei Jiang, Mingzhe Hu, Yuchong Zhang, Zhaoyang Xie, Gang Wang, Mengxue Guan, Wei Jiang, Huaixin Yang, Jianqi Li, Chenxia Yun, Zhiwei Wang, Sheng Meng, Yugui Yao, Tian Qian, Xun Shi

Abstract: Nonvolatile optical manipulation of material properties on demand is a highly sought-after feature in the advancement of future optoelectronic applications. While the discovery of such metastable transition in various materials holds good promise for achieving this goal, their practical implementation is still in the nascent stage. Here, we unravel the nature of the ultrafast laser-induced hidden… ▽ More Nonvolatile optical manipulation of material properties on demand is a highly sought-after feature in the advancement of future optoelectronic applications. While the discovery of such metastable transition in various materials holds good promise for achieving this goal, their practical implementation is still in the nascent stage. Here, we unravel the nature of the ultrafast laser-induced hidden state in 1T-TaS2 by systematically characterizing the electronic structure evolution throughout the reversible transition cycle. We identify it as a mixed-stacking state involving two similarly low-energy interlayer orders, which is manifested as the charge density wave phase disruption. Furthermore, our comparative experiments utilizing the single-pulse writing, pulse-train erasing and pulse-pair control explicitly reveal the distinct mechanism of the bidirectional transformations -- the ultrafast formation of the hidden state is initiated by a coherent phonon which triggers a competition of interlayer stacking orders, while its recovery to the initial state is governed by the progressive domain coarsening. Our work highlights the deterministic role of the competing interlayer orders in the nonvolatile phase transition in the layered material 1T-TaS2, and promises the coherent control of the phase transition and switching speed. More importantly, these results establish all-optical engineering of stacking orders in low-dimensional materials as a viable strategy for achieving desirable nonvolatile electronic devices. △ Less

Submitted 5 May, 2024; originally announced May 2024.

Coexistence of interacting charge density waves in a layered semiconductor

Authors: B. Q. Lv, Alfred Zong, Dong Wu, Zhengwei Nie, Yifan Su, Dongsung Choi, Batyr Ilyas, Bryan T. Fichera, Jiarui Li, Edoardo Baldini, Masataka Mogi, Y. -B. Huang, Hoi Chun Po, Sheng Meng, Yao Wang, N. L. Wang, Nuh Gedik

Abstract: Coexisting orders are key features of strongly correlated materials and underlie many intriguing phenomena from unconventional superconductivity to topological orders. Here, we report the coexistence of two interacting charge-density-wave (CDW) orders in EuTe4, a layered crystal that has drawn considerable attention owing to its anomalous thermal hysteresis and a semiconducting CDW state despite t… ▽ More Coexisting orders are key features of strongly correlated materials and underlie many intriguing phenomena from unconventional superconductivity to topological orders. Here, we report the coexistence of two interacting charge-density-wave (CDW) orders in EuTe4, a layered crystal that has drawn considerable attention owing to its anomalous thermal hysteresis and a semiconducting CDW state despite the absence of perfect FS nesting. By accessing unoccupied conduction bands with time- and angle-resolved photoemission measurements, we find that mono- and bi-layers of Te in the unit cell host different CDWs that are associated with distinct energy gaps. The two gaps display dichotomous evolutions following photoexcitation, where the larger bilayer CDW gap exhibits less renormalization and faster recovery. Surprisingly, the CDW in the Te monolayer displays an additional momentum-dependent gap renormalization that cannot be captured by density-functional theory calculations. This phenomenon is attributed to interlayer interactions between the two CDW orders, which account for the semiconducting nature of the equilibrium state. Our findings not only offer microscopic insights into the correlated ground state of EuTe4 but also provide a general non-equilibrium approach to understand coexisting, layer-dependent orders in a complex system. △ Less

Submitted 14 April, 2024; originally announced April 2024.

Comments: To appear in PRL

Journal ref: Physical Review Letters 132, 206401 (2024)

GPTFF: A high-accuracy out-of-the-box universal AI force field for arbitrary inorganic materials

Authors: Fankai Xie, Tenglong Lu, Sheng Meng, Miao Liu

Abstract: This study introduces a novel AI force field, namely graph-based pre-trained transformer force field (GPTFF), which can simulate arbitrary inorganic systems with good precision and generalizability. Harnessing a large trove of the data and the attention mechanism of transformer algorithms, the model can accurately predict energy, atomic forces, and stress with Mean Absolute Error (MAE) values of 3… ▽ More This study introduces a novel AI force field, namely graph-based pre-trained transformer force field (GPTFF), which can simulate arbitrary inorganic systems with good precision and generalizability. Harnessing a large trove of the data and the attention mechanism of transformer algorithms, the model can accurately predict energy, atomic forces, and stress with Mean Absolute Error (MAE) values of 32 meV/atom, 71 meV/Å, and 0.365 GPa, respectively. The dataset used to train the model includes 37.8 million single-point energies, 11.7 billion force pairs, and 340.2 million stresses. We also demonstrated that GPTFF can be universally used to simulate various physical systems, such as crystal structure optimization, phase transition simulations, and mass transport. △ Less

Submitted 29 February, 2024; originally announced February 2024.

Journal ref: Science Bulletin, 10.1016/j.scib.2024.08.039

Recent Breakthrough in AI-Driven Materials Science: Tech Giants Introduce Groundbreaking Models

Authors: Miao Liu, Sheng Meng

Abstract: A close look of Google's GNoME inorganic materials dataset [Nature 624, 80 (2023)], and 11 things you would like to know. A close look of Google's GNoME inorganic materials dataset [Nature 624, 80 (2023)], and 11 things you would like to know. △ Less

Submitted 8 February, 2024; originally announced February 2024.

Journal ref: Mater. Futures 3 027501 (2024)

Electrokinetic origin of swirling flow on nanoscale interface

Authors: Shuangshuang Meng, Yu Han, Wei Zhao, Yueqiang Zhu, Chen Zhang, Xiaoqiang Feng, Ce Zhang, Duyang Zang, Guangyin Jing, Kaige Wang

Abstract: The zeta ($ζ$) potential is a pivotal metric for characterizing the electric field topology within an electric double layer - an important phenomenon on phase interface. It underpins critical processes in diverse realms such as chemistry, biomedical engineering, and micro/nanofluidics. Yet, local measurement of $ζ$ potential at the interface has historically presented challenges, leading researche… ▽ More The zeta ($ζ$) potential is a pivotal metric for characterizing the electric field topology within an electric double layer - an important phenomenon on phase interface. It underpins critical processes in diverse realms such as chemistry, biomedical engineering, and micro/nanofluidics. Yet, local measurement of $ζ$ potential at the interface has historically presented challenges, leading researchers to simplify a chemically homogenized surface with a uniform $ζ$ potential. In the current investigation, we present evidence that, within a microchannel, the spatial distribution of $ζ$ potential across a chemically homogeneous solid-liquid interface can become two-dimensional (2D) under an imposed flow regime, as disclosed by a state-of-art fluorescence photobleaching electrochemistry analyzer (FLEA) technique. The $ζ$ potential' s propensity to become increasingly negative downstream, presents an approximately symmetric, V-shaped pattern in the spanwise orientation. Intriguingly, and of notable significance to chemistry and engineering, this 2D $ζ$ potential framework was found to electrokinetically induce swirling flows in tens of nanometers, aligning with the streamwise axis, bearing a remarkable resemblance to the well-documented hairpin vortices in turbulent boundary layers. Our findings gesture towards a novel perspective on the genesis of vortex structures in nanoscale. Additionally, the FLEA technique emerges as a potent tool for discerning $ζ$ potential at a local scale with high resolution, potentially accelerating the evolution and applications of novel surface material. △ Less

Submitted 5 February, 2024; originally announced February 2024.

Sliding ferroelectric memories and synapses

Authors: Xiuzhen Li, Biao Qin, Yaxian Wang, Yue Xi, Zhiheng Huang, Mengze Zhao, Yalin Peng, Zitao Chen, Zitian Pan, Jundong Zhu, Chenyang Cui, Rong Yang, Wei Yang, Sheng Meng, Dongxia Shi, Xuedong Bai, Can Liu, Na Li, Jianshi Tang, Kaihui Liu, Luojun Du, Guangyu Zhang

Abstract: Ferroelectric materials with switchable electric polarization hold great promise for a plethora of emergent applications, such as post-Moore's law nanoelectronics, beyond-Boltzmann transistors, non-volatile memories, and above-bandgap photovoltaic devices. Recent advances have uncovered an exotic sliding ferroelectric mechanism, which endows to design atomically thin ferroelectrics from non-ferroe… ▽ More Ferroelectric materials with switchable electric polarization hold great promise for a plethora of emergent applications, such as post-Moore's law nanoelectronics, beyond-Boltzmann transistors, non-volatile memories, and above-bandgap photovoltaic devices. Recent advances have uncovered an exotic sliding ferroelectric mechanism, which endows to design atomically thin ferroelectrics from non-ferroelectric parent monolayers. Although notable progress has been witnessed in understanding its fundamental properties, functional devices based on sliding ferroelectrics, the key touchstone toward applications, remain elusive. Here, we demonstrate the rewritable, non-volatile memory devices at room-temperature utilizing a two-dimensional (2D) sliding ferroelectric semiconductor of rhombohedral-stacked bilayer molybdenum disulfide. The 2D sliding ferroelectric memories (SFeMs) show superior performances with a large memory window of >8V, a high conductance ratio of above 106, a long retention time of >10 years, and a programming endurance greater than 104 cycles. Remarkably, flexible SFeMs are achieved with state-of-the-art performances competitive to their rigid counterparts and maintain their performances post bending over 103 cycles. Furthermore, synapse-specific Hebbian forms of plasticity and image recognition with a high accuracy of 97.81% are demonstrated based on flexible SFeMs. Our work demonstrates the sliding ferroelectric memories and synaptic plasticity on both rigid and flexible substrates, highlighting the great potential of sliding ferroelectrics for emerging technological applications in brain-inspired in-memory computing, edge intelligence and energy-efficient wearable electronics. △ Less

Submitted 29 January, 2024; originally announced January 2024.

Comments: 16 pages, 4 figures

Operando real-space imaging of a structural phase transformation in a high-voltage electrode

Authors: Yifei Sun, Sunny Hy, Nelson Hua, James Wingert, Ross Harder, Ying Shirley Meng, Oleg Shpyrko, Andrej Singer

Abstract: Discontinuous solid-solid phase transformations play a pivotal role in determining properties of rechargeable battery electrodes. By leveraging operando Bragg Coherent Diffractive Imaging (BCDI), we investigate the discontinuous phase transformation in LixNi0.5Mn1.5O4 within a fully operational battery. Throughout Li-intercalation, we directly observe the nucleation and growth of the Li-rich phase… ▽ More Discontinuous solid-solid phase transformations play a pivotal role in determining properties of rechargeable battery electrodes. By leveraging operando Bragg Coherent Diffractive Imaging (BCDI), we investigate the discontinuous phase transformation in LixNi0.5Mn1.5O4 within a fully operational battery. Throughout Li-intercalation, we directly observe the nucleation and growth of the Li-rich phase within the initially charged Li-poor phase in a 500 nm particle. Supported by the microelasticity model, the operando imaging unveils an evolution from a curved coherent to planar semi-coherent interface driven by dislocation dynamics. We hypothesize these dislocations exhibit a glissile motion that facilitates interface migration without diffusion of host ions, leaving the particle defect-free post-transformation. Our data indicates negligible kinetic limitations impacting the transformation kinetics, even at discharge rates as fast as C/2. This study underscores BCDI's capability to provide operando insights into nanoscale phase transformations, offering valuable guidance for electrochemical materials design and optimization. △ Less

Submitted 19 December, 2023; originally announced December 2023.

Ab initio self-trapped excitons

Authors: Yunfei Bai, Yaxian Wang, Sheng Meng

Abstract: We propose a new formalism and an effective computational framework to study self-trapped excitons (STE) in insulators and semiconductors from first principles. Using the many-body Bethe-Salpeter equation in combination with perturbation theory, we are able to obtain the mode- and momentum-resolved exciton-phonon coupling matrix element in a perturbative scheme, and explicitly solve the real space… ▽ More We propose a new formalism and an effective computational framework to study self-trapped excitons (STE) in insulators and semiconductors from first principles. Using the many-body Bethe-Salpeter equation in combination with perturbation theory, we are able to obtain the mode- and momentum-resolved exciton-phonon coupling matrix element in a perturbative scheme, and explicitly solve the real space localization of the electron (hole), as well as the lattice distortion. Further, this method allows to compute the STE potential energy surface and evaluate the STE formation energy and Stokes shift. We demonstrate our approach using two-dimensional magnetic semiconductor chromium trihalides and a wide-gap insulator BeO, the latter of which features dark excitons, and make predictions of their Stokes shift and coherent phonon generation which we hope to spark future experiments such as photoluminescence and transient absorption studies. △ Less

Submitted 21 November, 2023; originally announced November 2023.

Comments: 7 pages; 3 figures

Strong electron-phonon coupling in Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$

Authors: Linxing Song, Jianguo Si, Tom Fennell, Uwe Stuhr, Guochu Deng, Jinchen Wang, Juanjuan Liu, Lijie Hao, Huiqian Luo, Miao Liu, Sheng Meng, Shiliang Li

Abstract: The charge density wave (CDW) or nematicity has been found to coexist with superconductivity in many systems. It is thus interesting that the superconducting transition temperature $T_c$ in the doped BaNi$_2$As$_2$ system can be enhanced up to six times as the CDW or nematicity in the undoped compound is suppressed. Here we show that the transverse acoustic phonons of Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$… ▽ More The charge density wave (CDW) or nematicity has been found to coexist with superconductivity in many systems. It is thus interesting that the superconducting transition temperature $T_c$ in the doped BaNi$_2$As$_2$ system can be enhanced up to six times as the CDW or nematicity in the undoped compound is suppressed. Here we show that the transverse acoustic phonons of Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$ are strongly damped in a wide doping range and over the whole $Q$ range, which excludes its origin from either CDW or nematicity. The damping of TA phonons can be understood as large electron-phonon coupling and possible strong hybridization between acoustic and optical phonons as shown by the first-principle calculations. The superconductivity can be quantitatively reproduced by the change of electron-phonon coupling constant calculated by the McMillan equation in the BCS framework, which suggests that no quantum fluctuations of any order is needed to promote the superconductivity. On the contrary, the change of $T_c$ in this system should be understood as the six-fold suppression of superconductivity in undoped compounds. △ Less

Submitted 24 March, 2024; v1 submitted 6 November, 2023; originally announced November 2023.

Comments: 8 pages, 4 figures

Journal ref: Phys. Rev. B 109, 104518 (2024)

Temperature-Dependent Collective Excitations in a Three-Dimensional Dirac System ZrTe$_{5}$

Authors: Zijian Lin, Cuixiang Wang, Daqiang Chen, Sheng Meng, Youguo Shi, Jiandong Guo, Xuetao Zhu

Abstract: Zirconium pentatelluride (ZrTe$_{5}$), a system with a Dirac linear band across the Fermi level and anomalous transport features, has attracted considerable research interest for it is predicted to be located at the boundary between strong and weak topological insulators separated by a topological semimetal phase. However, the experimental verification of the topological phase transition and the t… ▽ More Zirconium pentatelluride (ZrTe$_{5}$), a system with a Dirac linear band across the Fermi level and anomalous transport features, has attracted considerable research interest for it is predicted to be located at the boundary between strong and weak topological insulators separated by a topological semimetal phase. However, the experimental verification of the topological phase transition and the topological ground state in ZrTe$_{5}$ is full of controversies, mostly due to the difficulty of precisely capturing the small gap evolution with single-particle band structure measurements. Alternatively, the collective excitations of electric charges, known as plasmons, in Dirac systems exhibiting unique behavior, can well reflect the topological nature of the band structure. Here, using reflective high-resolution electron energy loss spectroscopy (HREELS), we investigate the temperature-dependent collective excitations of ZrTe$_{5}$, and discover that the plasmon energy in ZrTe$_{5}$ is proportional to the $1/3$ power of the carrier density $n$, which is a unique feature of plasmons in three-dimensional Dirac systems. Based on this conclusion, the origin of the resistivity anomaly of ZrTe$_{5}$ can be attributed to the temperature-dependent chemical potential shift in extrinsic Dirac semimetals. △ Less

Submitted 5 November, 2023; v1 submitted 11 October, 2023; originally announced October 2023.

Comments: 7 pages, 4 figures

Journal ref: Physical Review B 108, 165146 (2023)

MatChat: A Large Language Model and Application Service Platform for Materials Science

Authors: Ziyi Chen, Fankai Xie, Meng Wan, Yang Yuan, Miao Liu, Zongguo Wang, Sheng Meng, Yangang Wang

Abstract: The prediction of chemical synthesis pathways plays a pivotal role in materials science research. Challenges, such as the complexity of synthesis pathways and the lack of comprehensive datasets, currently hinder our ability to predict these chemical processes accurately. However, recent advancements in generative artificial intelligence (GAI), including automated text generation and question-answe… ▽ More The prediction of chemical synthesis pathways plays a pivotal role in materials science research. Challenges, such as the complexity of synthesis pathways and the lack of comprehensive datasets, currently hinder our ability to predict these chemical processes accurately. However, recent advancements in generative artificial intelligence (GAI), including automated text generation and question-answering systems, coupled with fine-tuning techniques, have facilitated the deployment of large-scale AI models tailored to specific domains. In this study, we harness the power of the LLaMA2-7B model and enhance it through a learning process that incorporates 13,878 pieces of structured material knowledge data. This specialized AI model, named MatChat, focuses on predicting inorganic material synthesis pathways. MatChat exhibits remarkable proficiency in generating and reasoning with knowledge in materials science. Although MatChat requires further refinement to meet the diverse material design needs, this research undeniably highlights its impressive reasoning capabilities and innovative potential in the field of materials science. MatChat is now accessible online and open for use, with both the model and its application framework available as open source. This study establishes a robust foundation for collaborative innovation in the integration of generative AI in materials science. △ Less

Submitted 11 October, 2023; originally announced October 2023.

Journal ref: Chinese Physics B 32, 118104 (2023)

Weberite Na$_2$MM'F$_7$ (M,M'=Redox-Active Metal) as Promising Fluoride-Based Sodium-Ion Battery Cathodes

Authors: Tenglong Lu, Sheng Meng, Miao Liu

Abstract: Sodium-ion batteries are a viable alternative to lithium-ion technology due to the plentiful sodium resources. However, certain commercialization challenges, such as low specific energies and poor cycling performance of current Na-ion cathodes, still need to be addressed. To overcome these hurdles, this study explored the potential of a novel class of fluoride-based materials, specifically trigona… ▽ More Sodium-ion batteries are a viable alternative to lithium-ion technology due to the plentiful sodium resources. However, certain commercialization challenges, such as low specific energies and poor cycling performance of current Na-ion cathodes, still need to be addressed. To overcome these hurdles, this study explored the potential of a novel class of fluoride-based materials, specifically trigonal-type Na$_2$MM'F$_7$ (M and M' are redox-active metals) belonging to the weberite-type compounds, as promising candidates for Na-ion cathodes. Through a comprehensive assessment utilizing ab initio calculations, twelve prospective compounds were identified, demonstrating high thermodynamic stability, large gravimetric capacities (>170 mAh/g), and low net Na-ion migration barriers (<600 meV). Significantly, ten out of the twelve screened compounds exhibit high specific energies exceeding 580 Wh/kg (approximately equals to the specific energy of LiFePO$_4$), indicating their exceptional electrochemical performance. This study will pave the way for further advancements in fluoride-based electrode materials. △ Less

Submitted 6 October, 2023; originally announced October 2023.

Journal ref: J. Mater. Chem. A, 2024,12, 14709

Femtosecond electron diffraction reveals local disorder and local anharmonicity in thermoelectric SnSe

Authors: Jingjun Li, Yingpeng Qi, Qing Yang, Luye Yue, Changyuan Yao, Zijing Chen, Sheng Meng, Dao Xiang, Jianming Cao

Abstract: The microscopic arrangement of atoms and molecules is the determining factor in how materials behave and perform. Beyond the long-range periodicity, the local disorder with local structures deviating from the average lattice structure plays a vital role in determining the physical properties of the phonon, electron and spin subsystems in crystalline functional materials. Experimentally characteriz… ▽ More The microscopic arrangement of atoms and molecules is the determining factor in how materials behave and perform. Beyond the long-range periodicity, the local disorder with local structures deviating from the average lattice structure plays a vital role in determining the physical properties of the phonon, electron and spin subsystems in crystalline functional materials. Experimentally characterizing the 3D atomic configuration of such local disorder and correlating it with the advanced functions remain a big challenge. Time-domain evolution of the local disorder, either static or dynamical, is lost due to the characterization at equilibrium state with conventional probing techniques. With the combination of femtosecond electron diffraction, structure factor calculation and TDDFT-MD simulation, we exclusively identify the static local disorder and the local anharmonicity of it in thermoelectric SnSe. The ultrafast structural dynamics in time domain reveal a dominant static off-symmetry displacement of Sn (~0.4 angstrom) and the anharmonicity of this local disorder induces an ultrafast atomic displacement within 100 fs after photoexcitation. The microscopic picture of the local anharmonicity indicates a direct and first signature of the THz Einstein oscillators in real space. Therefore, a glass-like thermal transport channel with the local disorder, the Einstein oscillators and the local anharmonicity, updates the fundamental insight into the long-debated ultralow thermal conductivity in SnSe. The local disorder over one to a few unit cells is pervasive and indispensable in thermoelectric materials, multiferroic materials and correlated electronic materials. Our method of revealing the 3D local disorder and the local correlated interactions by ultrafast structural dynamics will inspire broad interest in construction of the structure-property relationship in material science. △ Less

Submitted 2 October, 2023; originally announced October 2023.

Report number: 2313742

Journal ref: Adv. Mater. 2313742 (2024)

Elucidating Dynamic Conductive State Changes in Amorphous Lithium Lanthanum Titanate for Resistive Switching Devices

Authors: Ryosuke Shimizu, Diyi Cheng, Guomin Zhu, Bing Han, Thomas S. Marchese, Randall Burger, Mingjie Xu, Xiaoqing Pan, Minghao Zhang, Ying Shirley Meng

Abstract: Exploration of novel resistive switching materials attracts attention to replace conventional Si-based transistors and to achieve neuromorphic computing that can surpass the limit of the current Von-Neumann computing for the time of Internet of Things (IoT). Materials priorly used to serve in batteries have demonstrated metal-insulator transitions upon an electrical biasing due to resulting compos… ▽ More Exploration of novel resistive switching materials attracts attention to replace conventional Si-based transistors and to achieve neuromorphic computing that can surpass the limit of the current Von-Neumann computing for the time of Internet of Things (IoT). Materials priorly used to serve in batteries have demonstrated metal-insulator transitions upon an electrical biasing due to resulting compositional change. This property is desirable for future resistive switching devices. Amorphous lithium lanthanum titanate (a-LLTO) was originally developed as a solid-state electrolyte with relatively high lithium ionic conductivity and low electronic conductivity among oxide-type solid electrolytes. However, it has been suggested that electric conductivity of a-LLTO changes depending on oxygen content. In this work, the investigation of switching behavior of a-LLTO was conducted by employing a range of voltage sweep techniques, ultimately establishing a stable and optimal operating condition within the voltage window of -3.5 V to 3.5 V. This voltage range effectively balances the desirable trait of a substantial resistance change by three orders of magnitude with the imperative avoidance of LLTO decomposition. This switching behavior is also confirmed at nanodevice of Ni/LLTO/Ni through in-situ biasing inside focused-ion beam/scanning electron microscope (FIB-SEM). Experiment and computation with different LLTO composition shows that LLTO has two distinct conductivity states due to Ti reduction. The distribution of these two states is discussed using simplified binary model, implying the conductive filament growth during low resistance state. Consequently, our study deepens understanding of LLTO electronic properties and encourages the interdisciplinary application of battery materials for resistive switching devices. △ Less

Submitted 30 September, 2023; originally announced October 2023.

Energy landscape and phase competition of CsV3Sb5-, CsV6Sb6-, and TbMn6Sn6-type Kagome materials

Authors: Guanghui Cai, Yutao Jiang, Hui Zhou, Ze Yu, Kun Jiang, Youguo Shi, Sheng Meng, Miao Liu

Abstract: Finding viable Kagome lattices is vital for materializing novel phenomena in quantum materials. In this work, we performed element substitutions on CsV3Sb5 with space group P6/mmm, TbMn6Sn6 with space group P6/mmm, and CsV6Sb6 with space group R-3 m, respectively, as the parent compounds. A total of 4158 materials were obtained through element substitutions, and these materials were then calculate… ▽ More Finding viable Kagome lattices is vital for materializing novel phenomena in quantum materials. In this work, we performed element substitutions on CsV3Sb5 with space group P6/mmm, TbMn6Sn6 with space group P6/mmm, and CsV6Sb6 with space group R-3 m, respectively, as the parent compounds. A total of 4158 materials were obtained through element substitutions, and these materials were then calculated via density function theory in high-throughput mode. Afterward, 48 materials were identified with high thermodynamic stability (E_hull<5meV/atom). Furthermore, we compared the thermodynamic stability of three different phases with the same elemental composition and predicted some competing phases that may arise during material synthesis. Finally, by calculating the electronic structures of these materials, we attempted to identify patterns in the electronic structure variations as the elements change. This work provides guidance for discovering promising AM3X5/AM6X6 Kagome materials from a vast phase space. △ Less

Submitted 23 August, 2023; originally announced August 2023.

Journal ref: Chin. Phys. Lett. 2023, 40 (11): 117101

Recycling Silicon Scrap for Spherical Si-C composite as High-Performance Lithium-ion Battery Anodes

Authors: Bhagath Sreenarayanan, Marta Vicencio, Shuang Bai, Bingyu Lu, Ou Mao, Shiva Adireddy, Wurigumula Bao, Ying Shirley Meng

Abstract: The growth of the semiconductor and solar industry has been exponential in the last two decades due to the computing and energy demands of the world. Silicon (Si) is one of the main constituents for both sectors and, thus, is used in large quantities. As a result, a lot of Si waste is generated mainly by these two industries. For a sustainable world, the circular economy is the key; thus, the wast… ▽ More The growth of the semiconductor and solar industry has been exponential in the last two decades due to the computing and energy demands of the world. Silicon (Si) is one of the main constituents for both sectors and, thus, is used in large quantities. As a result, a lot of Si waste is generated mainly by these two industries. For a sustainable world, the circular economy is the key; thus, the waste produced must be upcycled/recycled/reused to complete the circular chain. Herein, we show that an upcycled/recycled Si can be used with carbon as a composite anode material, with high Si content (~40 wt.%) and loading of 3-4 mAh/cm^2 for practical use in lithium-ion batteries. The unique spherical jackfruit-like structure of the Si-C composite can minimize the total lithium inventory loss compared to the conventional Si-C composite and pure Si, resulting in superior electrochemical performance. The superior electrochemical performance of Si-C composites enables the cell energy density of ~325 Wh/kg (with NMC cathode) and ~260 Wh/kg (with LFP cathode), respectively. The results demonstrate that Si-based industrial waste can be upcycled for high-performance Li-ion battery anodes through a controllable, scalable, and energy-efficient route. △ Less

Submitted 17 May, 2023; originally announced May 2023.

Probing Phonon dynamics and Electron-Phonon Coupling by High Harmonic Generation in Solids

Authors: Shi-Qi Hu, Hui Zhao, Xin-Bao Liu, Da-Qiang Chen, Sheng Meng

Abstract: Acting as a highly nonlinear response to the strong laser field, high harmonic generation (HHG) naturally contains the fingerprints of atomic and electronic properties of materials. Electronic properties of a solid such as band structure and topology can thus be probed, while the phonon dynamics during HHG are often neglected. Here we show that by exploiting the effects of phonon deformation on HH… ▽ More Acting as a highly nonlinear response to the strong laser field, high harmonic generation (HHG) naturally contains the fingerprints of atomic and electronic properties of materials. Electronic properties of a solid such as band structure and topology can thus be probed, while the phonon dynamics during HHG are often neglected. Here we show that by exploiting the effects of phonon deformation on HHG, the intrinsic phonon information can be deciphered and direct probing of band- and mode-resolved electron-phonon couplings (EPC) of photoexcited materials is possible. Considering HHG spectroscopy can be vacuum free and unrestricted to electron occupation, this work suggests HHG is promising for all-optical characterization of EPC in solids, especially for gapped quantum states or materials under high pressure. △ Less

Submitted 19 April, 2023; originally announced April 2023.

Elucidating the Role of Prelithiation in Si-based Anodes for Interface Stabilization

Authors: Shuang Bai, Wurigumula Bao, Kun Qian, Bing Han, Weikang Li, Baharak Sayahpour, Bhagath Screenarayanan, Darren H. S. Tan, So-yeon Ham, Ying Shirley Meng

Abstract: Prelithiation as a facile and effective method to compensate the lithium inventory loss in the initial cycle has progressed considerably both on anode and cathode sides. However, much less research has been devoted to the prelithiation effect on the interface stabilization for long-term cycling of Si-based anodes. An in-depth quantitative analysis of the interface that form during the prelithiatio… ▽ More Prelithiation as a facile and effective method to compensate the lithium inventory loss in the initial cycle has progressed considerably both on anode and cathode sides. However, much less research has been devoted to the prelithiation effect on the interface stabilization for long-term cycling of Si-based anodes. An in-depth quantitative analysis of the interface that form during the prelithiation of SiO$_x$ is presented here and the results are compared with prelithiaton of Si anodes. Local structure probe combined with detailed electrochemical analysis reveals that a characteristic mosaic interface is formed on both prelithiated SiO$_x$ and Si anodes. This mosaic interface containing multiple lithium silicates phases, is fundamentally different from the solid electrolyte interface (SEI) formed without prelithiation. The ideal conductivity and mechanical properties of lithium silicates enable improved cycling stability of both prelithiated anodes. With a higher ratio of lithium silicates due to the oxygen participation, prelithiated SiO$_{1.3}$ anode improves the initial coulombic efficiency to 94% in full cell and delivers good cycling retention after hundreds cycles under lean electrolyte conditions. The insights provided in this work could be used to further optimize high Si loading based anode in future high energy density batteries. △ Less

Submitted 13 April, 2023; originally announced April 2023.

Electron-phonon interactions in LuH$_2$, LuH$_3$, and LuN

Authors: Tenglong Lu, Sheng Meng, Miao Liu

Abstract: This paper presents the calculation results of electron-phonon interactions within the LuH$_2$, LuH$_3$, and LuN systems under 0 GPa and 10 GPa via density functional theory at the GGA-PBE level. The purpose of this work is to provide useful data that may be of the interests of the superconducting community as it was reported that the Lu-H-N compound is likely to be a room-temperature superconduct… ▽ More This paper presents the calculation results of electron-phonon interactions within the LuH$_2$, LuH$_3$, and LuN systems under 0 GPa and 10 GPa via density functional theory at the GGA-PBE level. The purpose of this work is to provide useful data that may be of the interests of the superconducting community as it was reported that the Lu-H-N compound is likely to be a room-temperature superconductor under 1 GPa [Nature, 615, 244 (2023)]. △ Less

Submitted 9 April, 2023; originally announced April 2023.