Experimental Realization of an Intrinsic Magnetic Topological Insulator

An intrinsic magnetic topological insulator(TI) is a stoichiometric magnetic compound possessing both inherent magnetic order and topological electronic states. Such a material can provide a shortcut to various novel topological quantum effects but remained elusive experimentally for a long time. Here we report the experimental realization of thin films of an intrinsic magnetic TI, MnBi2Te4, by alternate growth of a Bi2Te3 quintuple layer and a MnTe bilayer with molecular beam epitaxy. The material shows the archetypical Dirac surface states in angle-resolved photoemission spectroscopy and is demonstrated to be an antiferromagnetic topological insulator with ferromagnetic surfaces by magnetic and transport measurements as well as first-principles calculations. The unique magnetic and topological electronic structures and their interplays enable the material to embody rich quantum phases such as quantum anomalous Hall insulators and axion insulators at higher temperature and in a well-controlled way.

Electronic structure of molecular beam epitaxy grown 1T’-MoTe2 film and strain effect

Atomically thin transition metal dichalcogenide films with distorted trigonal(1T') phase have been predicted to be candidates for realizing quantum spin Hall effect. Growth of 1T' film and experimental investigation of its electronic structure are critical. Here we report the electronic structure of 1T'-MoTe2 films grown by molecular beam epitaxy(MBE).Growth of the 1T'-MoTe2 film depends critically on the substrate temperature, and successful growth of the film is indicated by streaky stripes in the reflection high energy electron diffraction(RHEED) and sharp diffraction spots in the low energy electron diffraction(LEED). Angle-resolved photoemission spectroscopy(ARPES) measurements reveal a metallic behavior in the as-grown film with an overlap between the conduction and valence bands. First principles calculation suggests that a suitable tensile strain along the a-axis direction is needed to induce a gap to make it an insulator. Our work not only reports the electronic structure of MBE grown 1T'-MoTe2 films, but also provides insights for strain engineering to make it possible for quantum spin Hall effect.

Influences of chemical reactions on polysulfide reduction reaction process on promotor surface in Li-S batteries

Polar promotors have been proven effective in catalyzing the polysulfide(PS)reduction reaction(PSRR)process in lithium-sulfur(Li-S)batteries.However,the promotor surface tends to be poisoned due to the accumulation of insoluble discharging products of lithium disulfide(Li_(2)S_(2))and lithium sulfide(Li_(2)S)during Li-S battery operation.Herein,we investigate the detailed PSRR mechanism on the surface of manganese sulfides(MnS)as a representative promoter by performing in-situ Raman mapping measurements.The catalytic ability of MnS enables thorough electrochemical reduction of PSs to Li_(2)S_(2) and Li_(2)S on the MnS surface.The generated Li_(2)S_(2) and Li_(2)S then adsorb the dissolved PSs via chemical reactions among sulfur species during the subsequent PSRR process.This phenomenon mitigates promotor poisoning and continuously improves the reversible capacity.Consequently,the assembled Li-S cell demonstrates excellent electrochemical performance after introducing a conductive interlayer containing a thin piece of carbon nanotube film and MnS promotors.

Universal materials model of deep-learning density functional theory Hamiltonian

Realizing large materials models has emerged as a critical endeavor for materials research in the new era of artificial intelligence,but how to achieve this fantastic and challenging objective remains elusive.Here,we propose a feasible pathway to address this paramount pursuit by developing universal materials models of deep-learning density functional theory Hamiltonian(Deep H),enabling computational modeling of the complicated structure-property relationship of materials in general.By constructing a large materials database and substantially improving the Deep H method,we obtain a universal materials model of Deep H capable of handling diverse elemental compositions and material structures,achieving remarkable accuracy in predicting material properties.We further showcase a promising application of fine-tuning universal materials models for enhancing specific materials models.This work not only demonstrates the concept of Deep H's universal materials model but also lays the groundwork for developing large materials models,opening up significant opportunities for advancing artificial intelligencedriven materials discovery.

Engineering strong spin-frustration at the surface of three-dimensional graphene

Since the first successful fabrication in 2004[1],graphene has received tremendous attention due to its extremely simple atomic structure and alluring physical properties.For example,its massless low energy excitations have a linear dispersion and thus its transport property is governed by Dirac equation instead of Schr?dinger equation.These special electronic structures suppress the intra-valley and inter-valley backscatterings,leading to the half-integer and fractional quantum Hall effect[2]under magnetic field and the relativistic quantum tunneling described by the Klein paradox[3].

Erratum to:Influences of chemical reactions on polysulfide reduction reaction process on promotor surface in Li-S batteries

Erratum to Nano Research,2024,17(4):2712-2718 https://doi.org/10.1007/s12274-023-6129-5 The affiliation of the author,Wenhui Duan,is“2 Department of Physics,Tsinghua University,Beijing 100084,China”,instead of“1 Tsinghua-Foxconn Nanotechnology Research Center,Tsinghua University,Beijing 100084,China;2 Department of Physics,Tsinghua University,Beijing 100084,China”.And the affiliation of the author,Wenhui Duan,in the online version and the ESM file of this paper has been corrected.

The best thermoelectrics revisited in the quantum limit

The classical problem of best thermoelectrics,which was believed originally solved by Mahan and Sofo[Proc.Natl.Acad.Sci.USA 93,7436(1996)],is revisited and discussed in the quantum limit.We express the thermoelectric figure of merit(zT)as a functional of electronic transmission probability T by the Landauer–Büttiker formalism,which is able to deal with thermoelectric transport ranging from ballistic to diffusive regimes.We also propose to apply the calculus of variations to search for the optimal T giving the maximal zT.Our study reveals that the optimal transmission probability T is a boxcar function instead of a delta function proposed by Mahan and Sofo,leading to zT exceeding the well-known Mahan–Sofo limit.Furthermore,we suggest realizing the optimal T in topological material systems.Our work defines the theoretical upper limit for quantum thermoelectrics,which is of fundamental significance to the future development of thermoelectrics.

Group VB transition metal dichalcogenides for oxygen reduction reaction and strain-enhanced activity governed by p-orbital electrons of chalcogen

Developi ng alter native oxyge n reducti on reactio n (ORR) catalysts to replace precious Pt-based metals with abundant materials is the key challe nge of commercial application of fuel cells. Owing to their various compositi ons and tun able electronic properties, transition metal dichalcogenides (TMDs) have the great potential to realize high-efficiency catalysts for ORR. Here, various 3R-phase dichalcogenides of group VB and VIB transition metals (MX2, M = Nb, Ta, Mo, W;X = S, Se, Te) are investigated for ORR catalysts by using density functional theory calculations. The computed over-potentials of group VB TMDs are much less than those of group VIB TMDs. For group VB TMDs, a volcano-type plot of ORR catalytic activity is established on the adsorption energies of *OH, and NbS2 and TaTe2 exhibit best ORR activity with an oveepotential of 0.54 V. To achieve even better activity, strain engineering is performed to tune ORR catalytic activity, and the minimum over-potential of 0.43 V can be realized. We further dem on strate that the shift of p orbital center of surface chalcoge n elements under strain is responsible for tuning the catalytic activity of TMDs.

Enhancement of superconductivity in organic-inorganic hybrid topological materials

Inducing or enhancing superconductivity in topological materials is an important route toward topological superconductivity.Reducing the thickness of transition metal dichalcogenides(e.g.WTe2 and MoTe2)has provided an important pathway to engineer superconductivity in topological matters.However,such monolayer sample is difficult to obtain,unstable in air,and with extremely low Tc.Here we report an experimentally convenient approach to control the interlayer coupling to achieve tailored topological properties,enhanced superconductivity and good sample stability through organic-cation intercalation of the Weyl semimetals MoTe2 and WTe2.The as-formed organic-inorganic hybrid crystals are weak topological insulators with enhanced Tc of 7.0 K for intercalated MoTe2(0.25 K for pristine crystal)and2.3 K for intercalated WTe2(2.8 times compared to monolayer WTe2).Such organic-cation intercalation method can be readily applied to many other layered crystals,providing a new pathway for manipulating their electronic,topological and superconducting properties.

Crossover from 2D metal to 3D Dirac semimetal in metallic PtTe2 films with local Rashba effect

PtTe2 and PtSe2 with trigonal structure have attracted extensive research interests since the discovery of type-II Dirac fermions in the bulk crystals. The evolution of the electronic structure from bulk 3D topological semimetal to 2D atomic thin films is an important scientific question. While a transition from 3D type-II Dirac semimetal in the bulk to 2D semiconductor in monolayer(ML) film has been reported for PtSe2, so far the evolution of electronic structure of atomically thin PtTe2 films still remains unexplored.Here we report a systematic angle-resolved photoemission spectroscopy(ARPES) study of the electronic structure of high quality PtTe2 films grown by molecular beam epitaxy with thickness from 2 ML to 6 ML.ARPES measurements show that PtTe2 films still remain metallic even down to 2 ML thickness, which is in sharp contrast to the semiconducting property of few layer PtSe2 films. Moreover, a transition from 2D metal to 3D type-II Dirac semimetal occurs at film thickness of 4–6 ML. In addition, Spin-ARPES measurements reveal helical spin textures induced by local Rashba effect in the bulk PtTe2 crystal, suggesting that similar hidden spin is also expected in few monolayer PtTe2 films. Our work reveals the transition from2D metal to 3D topological semimetal and provides new opportunities for investigating metallic 2D films with local Rashba effect.

Growth of atomically thick transition metal sulfide films on graphene/6H-SiC（0001） by molecular beam epitaxy

We report the growth and characterization of atomically thick NbS2, TaS2, and FeS films on a 6H-SiC（0001） substrate terminated with monolayer or bilayer epitaxial graphene. The crystal and electronic structures are studied by scanning tunneling microscopy and reflection high-energy electron diffraction. The NbS2 monolayer is solely in the 2H structure, while the TaS2 monolayer contains both 1T and 2H structures. Charge-density waves are observed in all phases. For the FeS films, the tetragonal structure coexists with the hexagonal one and no superconductivity is observed.

Growth of large scale PtTe,PtTe_(2) and PtSe_(2) films on a wide range of substrates

1T phase of transition metal dichalcogenides(TMDCs)formed by group 10 transition metals(e.g.Pt,Pd)have attracted increasing interests due to their novel properties and potential device applications.Synthesis of large scale thin films with controlled phase is critical especially considering that these materials have relatively strong interlayer interaction and are difficult to exfoliate.Here we report the growth of centimeter-scale PtTe,1T-PtTe2 and 1T-PtSe2 films via direct deposition of Pt metals followed by tellurization or selenization.We find that by controlling the Te flux,a hitherto-unexplored PtTe phase can also be obtained,which can be further tuned into PtTe2 by high temperature annealing under Te flux.These films with different thickness can be grown on a wide range of substrates,including NaCl which can be further dissolved to obtain free-standing PtTe2 or PtSe2 films.Moreover,a systematic thickness dependent resistivity and Hall conductivity measurements show that distinguished from the semiconducting PtSe2 with hole carriers,PtTe2 and PtTe films are metallic.Our work opens new opportunities for investigating the physical properties and potential applications of group 10 TMDC films and the new monochalcogenide PtTe film.

Graphene opens pathways to a carbon-neutral cement industry

Global cement production has increased 30 times since 1950 and nearly 4 times since 1990,becoming the third-largest source of anthropogenic carbon dioxide(CO_(2))emissions after fossil fuels and land-use changes[1].But the cement industry is under scrutiny.In order to achieve the Paris Agreement’s targets and reach net-zero CO_(2)emissions by 2050.

Heterogeneous relational message passing networks for molecular dynamics simulations

With many frameworks based on message passing neural networks proposed to predict molecular and bulk properties,machine learning methods have tremendously shifted the paradigms of computational sciences underpinning physics,material science,chemistry,and biology.While existing machine learning models have yielded superior performances in many occasions,most of them model and process molecular systems in terms of homogeneous graph,which severely limits the expressive power for representing diverse interactions.In practice,graph data with multiple node and edge types is ubiquitous and more appropriate for molecular systems.Thus,we propose the heterogeneous relational message passing network(HermNet),an end-to-end heterogeneous graph neural networks,to efficiently express multiple interactions in a single model with ab initio accuracy.HermNet performs impressively against many top-performing models on both molecular and extended systems.Specifically,HermNet outperforms other tested models in nearly 75%,83%and 69%of tasks on revised Molecular Dynamics 17(rMD17),Quantum Machines 9(QM9)and extended systems datasets,respectively.In addition,molecular dynamics simulations and material property calculations are performed with HermNet to demonstrate its performance.Finally,we elucidate how the design of HermNet is compatible with quantum mechanics from the perspective of the density functional theory.Besides,HermNet is a universal framework,whose sub-networks could be replaced by other advanced models.

Pressure-induced Lifshitz transition in the type Ⅱ Dirac semimetal PtTe_2

Numerous exotic properties have been discovered in Dirac Semimetals(DSMs) and Weyl Semimetals(WSMs). In a given DSM/WSM, the Dirac/Weyl nodes usually coexist with other bulk states, making their respective contribution elusive. In this work, we distinguish the role of bulk states from the tilted Dirac nodes on the transport properties in DSMs. Specifically, we applied pressure to a type-II DSM material, PtTe2, and studied its pressure modified electronic and lattice structure systematically by using in situ transport measurements and X-ray diffraction(XRD). A pressure-induced transition at about 20 GPa is revealed in the transport properties, while the layered lattice structure is robust against pressure as illustrated in XRD measurement results.Density functional theory(DFT) calculations suggest that this is originated from the Lifshitz transition in the bulk states. Our findings provide evidence to identify the bulk states' influence on transport from the topologically-protected DSM states in the DSM material.

Hidden metal-insulator transition in manganites synthesized via a controllable oxidation

Oxygen usually plays crucial roles in tuning the phase structures and functionalities of complex oxides such as high temperature superconductivity, colossal magnetoresistance, catalysis, etc. Effective and considerable control of the oxygen content in those functional oxides could be highly desired. Here, using perovskite manganite(La0.5Sr0.5)MnO3 as a paradigm, we develop a new pathway to synthesize the epitaxial thin films assisted by an in-situ chemical process, where the oxygen content can be precisely controlled by varying oxidative activity tuned by the atmospheric temperature(Tatm)during the growth. A hidden metal-insulator transition(MIT)emerges due to the phase competition, which is never shown in the phase diagram of this classic manganite. The oxygenmediated interaction between Mn ions together with the change of carrier density might be responsible for this emerging phase, which is compatible with the results of firstprinciple calculations. This work demonstrates that, apart from traditional cation doping, a precise modulation of anion(O2-, S2-, etc.) may provide a new strategy to control phase structures and functionalities of epitaxial compound thin films.

2D materials: Rising star for future applications

Two-dimensional(2D)materials have attracted enormous research interest due to their predominant quantum effects and fascinating materials properties,which potentially lead to various important applications.Meanwhile,the sheer openness of 2D materials makes their interactions with external stimuli particularly efficient.It is much more convenient to modulate the materials properties of 2D systems by mechanical,electronic,optical,and magnetic modulations than in 3D bulks,advantageous for device control.

High areal capacity flexible sulfur cathode based on multi-functionalized super-aligned carbon nanotubes

Rational design of a robust carbon matrix has a profound impact on the performance of flexible/wearable lithium/sulfur batteries.Herein,we demonstrate a freestanding three-dimensional super-aligned carbon nanotube (SACNT) matrix reinforced with a multi-functionalized carbon coating for flexible,high-areal sulfur loading cathode.By employing the sulfur/nitrogen co-doped carbon (SNC)"glue",the joints in the SACNT scaffold are tightly welded together so that the overall mechanical strength of the electrode is significantly enhanced to withstand the repeated bending as well as the volume change during operation.The SNC also shows intriguing catalytic effect that lowers the energy barrier of Li ion transport,propelling a superior redox conversion efficiency.The resulting binder-free and current collector-free sulfur cathode exhibits a high reversible capacity of 1,079 mAh·g^-1 at 1 C,a high-rate capacity of ~ 800 mAh·g^-1 at 5 C,and an average capacity decay rate of 0.037% per cycle at 2 C for 1,500 cycles.Impressively,a large-areal flexible Li/S pouch cell based on such mechanically robust cathode exhibits excellent capacity retention under arbitrary bending conditions.With a high areal sulfur loading of 7 mg·cm^-2,the large-areal flexible cathode delivers an outstanding areal capacity of 6.3 mAh·cm^-2 at 0.5 C (5.86 mA·cm^-2),showing its promise for realizing practical high energy density flexible Li/S batteries.

Symmetry-adapted graph neural networks for constructing molecular dynamics force fields

Molecular dynamics is a powerful simulation tool to explore material properties.Most realistic material systems are too large to be simulated using first-principles molecular dynamics.Classical molecular dynamics has a lower computational cost but requires accurate force fields to achieve chemical accuracy.In this work,we develop a symmetry-adapted graph neural network framework called the molecular dynamics graph neural network(MDGNN)to construct force fields automatically for molecular dynamics simulations for both molecules and crystals.This architecture consistently preserves translation,rotation,and permutation invariance in the simulations.We also propose a new feature engineering method that includes high-order terms of interatomic distances and demonstrate that the MDGNN accurately reproduces the results of both classical and first-principles molecular dynamics.In addition,we demonstrate that force fields constructed by the proposed model have good transferability.The MDGNN is thus an efficient and promising option for performing molecular dynamics simulations of large-scale systems with high accuracy.

Floating forest:A novel breakwater-windbreak structure against wind and wave hazards

A novel floating breakwater-windbreak structure(floating forest)has been designed for the protection of vulnerable coastal areas from extreme wind and wave loadings during storm conditions.The modular arch-shaped concrete structure is positioned perpendicularly to the direction of the prevailing wave and wind.The structure below the water surface acts as a porous breakwater with wave scattering capability.An array of tubular columns on the sloping deck of the breakwater act as an artificial forest-type windbreak.A feasibility study involving hydrodynamic and aerodynamic analyses has been performed,focusing on its capability in reducing wave heights and wind speeds in the lee side.The study shows that the proposed 1 km long floating forest is able to shelter a lee area that stretches up to 600 m,with 40%–60%wave energy reduction and 10%–80%peak wind speed reduction.