Exploration of growth conditions of TaAs Weyl semimetal thin film using pulsed laser deposition

Ta As,the first experimentally discovered Weyl semimetal material,has attracted a lot of attention due to its high carrier mobility,high anisotropy,nonmagnetic properties and strong interaction with light.These make it an ideal candidate for the study of Weyl fermions and applications in quantum computation,thermoelectric devices,and photodetection.For further basic physics studies and potential applications,large-size and high-quality Ta As films are urgently needed.However,it is difficult to grow As-stoichiometry Ta As films due to the volatilization of As during the growth.To solve this problem,we attempted to grow Ta As films on different substrates using targets with different As stoichiometric ratios via pulsed laser deposition(PLD).In this work,we found that partial As ions of the Ga As substrate are likely to diffuse into the Ta As films during growth,which was preliminarily confirmed by structural characterization,surface topography and composition analysis.As a result,the As content in the Ta As film was improved and the Ta As phase was achieved.Our work presents an effective method for the fabrication of Ta As films using PLD,enabling possible use of the Weyl semimetal film for functional devices.

VASP2KP:k·p Models and Landég-Factors from ab initio Calculations

The k·p method is significant in condensed matter physics for the compact and analytical Hamiltonian.In the presence of magnetic field,it is described by the effective Zeeman's coupling Hamiltonian with Landég-factors.Here,we develop an open-source package VASP2KP(including two parts:vasp2mat and mat2kp)to compute k·p parameters and Landég-factors directly from the wavefunctions provided by the density functional theory(DFT)as implemented in Vienna ab initio Simulation Package(VASP).First,we develop a VASP patch vasp2mat to compute matrix representations of the generalized momentum operatorπ=p+1/2mc^(2)[s×■V(r)],spin operator s,time reversal operatorT,and crystalline symmetry operatorsR on the DFT wavefunctions.Second,we develop a python code mat2kp to obtain the unitary transformation U that rotates the degenerate DFT basis towards the standard basis,and then automatically compute the k·p parameters and g-factors.The theory and the methodology behind VASP2KP are described in detail.The matrix elements of the operators are derived comprehensively and computed correctly within the projector augmented wave method.We apply this package to some materials,e.g.,Bi2Se3,Na3Bi,Te,InAs and 1H-TMD monolayers.The obtained effective model's dispersions are in good agreement with the DFT data around the specific wave vector,and the g-factors are consistent with experimental data.The VASP2KP package is available at https://github.com/zjwang11/VASP2KP.

Electronic Structure of the Weak Topological Insulator Candidate Zintl Ba_(3)Cd_(2)Sb_(4)

One of the greatest triumph of condensed matter physics in the past ten years is the classification of materials by the principle of topology.The existence of topological protected dissipationless surface state makes topological insulators great potential for applications and hotly studied.However,compared with the prosperity of strong topological insulators,theoretical predicted candidate materials and experimental confirmation of weak topological insulators(WTIs) are both extremely rare.By combining systematic first-principles calculation and angle-resolved photoemission spectroscopy measurements,we have studied the electronic structure of the dark surface of the WTI candidate Zintl Ba_(3)Cd_(2)Sb_(4)and another related material Ba_(3)Cd_(2)As_(4).The existence of two Dirac surface states on specific side surfaces predicted by theoretical calculations and the observed two band inversions in the Brillouin zone give strong evidence to prove that the Ba_(3)Cd_(2)Sb_(4)is a WTI.The spectroscopic characterization of this Zintl Ba_(3)Cd_(2)N_(4)(N = As and Sb) family materials will facilitate applications of their novel topological properties.

Unusual electronic structure of Dirac material BaMnSb_(2) revealed by angle-resolved photoemission spectroscopy

High resolution angle resolved photoemission measurements and band structure calculations are carried out to study the electronic structure of BaMnSb_(2). All the observed bands are nearly linear that extend to a wide energy range. The measured Fermi surface mainly consists of one hole pocket around Γ and a strong spot at Y which are formed from the crossing points of the linear bands. The measured electronic structure of BaMnSb_(2) is unusual and deviates strongly from the band structure calculations. These results will stimulate further efforts to theoretically understand the electronic structure of BaMnSb_(2) and search for novel properties in this Dirac material.

Quantum Oscillations and Electronic Structure in the Large-Chern-Number Topological Chiral Semimetal PtGa

We report the magnetoresistance(MR),de Haas-van Alphen(dHvA) oscillations and the electronic structures of single-crystal PtGa.The large unsaturated MR is observed with the magnetic field B ‖ [111].Evident dHvA oscillations with the B ‖ [001] configuration are observed,from which twelve fundamental frequencies are extracted and the spin-orbit coupling(SOC) induced band splitting is revealed.The light cyclotron effective masses are extracted from the fitting by the thermal damping term of the Lifshitz-Kosevich formula.Combining with the calculated frequencies from the first-principles calculations,the dHvA frequencies F1/F3 and F11/F12are confirmed to originate from the electron pockets at F and R,respectively.The first-principles calculations also reveal the existence of spin-3/2 Rarita-Schwinger-Weyl fermions and time-reversal doubling of the spin-1 excitation at Γ and R with large Chern numbers of ±4 when SOC is included.

Type-II Dirac Semimetal State in a Superconductor Tantalum Carbide

The exploration of topological Dirac semimetals with intrinsic superconductivity can be a most plausible way to discover topological superconductors.We propose that type-II Dirac semimetal states exist in the band structure of TaC,a well-known s-wave superconductor,by using the first-principles calculations and theκ· p effective model.The tilted gapless Dirac cones,which are composed of Ta d and C p orbitals and are protected by C4v symmetry,are found to be below the Fermi level.The bands from Ta d orbitals are greatly coupled with the acoustic modes around the zone boundary,indicating their significant contribution to the superconductivity.The relatively high transition temperature^10.5 K is estimated to be consistent with the experimental data.To bring the type-II Dirac points close to chemical potential,hole doping is needed.This seems to decrease the transition temperature a lot,making the realization of topological superconductivity impossible.

Anisotropic magnetism and band evolution induced by ferromagnetic phase transition in titanium-based kagome ferromagnet SmTi_3Bi_4

Kagome magnets with diverse topological quantum responses are crucial for next-generation topological engineering.The anisotropic magnetism and band evolution induced by ferromagnetic phase transition(FMPT)is reported in a newly discovered titanium-based kagome ferromagnet Sm Ti3Bi4,which features a distorted Ti kagome lattice and Sm atomic zig-zag chains.Temperature-dependent resistivity,heat capacity,and magnetic susceptibility reveal a ferromagnetic ordering temperature Tc of23.2 K.A large magnetic anisotropy,observed by applying the magnetic field along three crystallographic axes,identifies the b axis as the easy axis.Angle-resolved photoemission spectroscopy with first-principles calculations unveils the characteristic kagome motif,including the Dirac point at the Fermi level and multiple van Hove singularities.Notably,a band splitting and gap closing attributed to FMPT is observed,originating from the exchange coupling between Sm 4 f local moments and itinerant electrons of the kagome Ti atoms,as well as the time-reversal symmetry breaking induced by the long-range ferromagnetic order.Considering the large in-plane magnetization and the evolution of electronic structure under the influence of ferromagnetic ordering,such materials promise to be a new platform for exploring the intricate electronic properties and magnetic phases based on the kagome lattice.

手性晶体CoSi中拓扑费米弧的电荷不稳定性

拓扑边界态出现在拓扑非平庸态和平庸态之间的边界上,通常是无间隙的或被称为金属态.例如,拓扑绝缘体的表面态是无间隙的狄拉克态.这些金属拓扑边界态往往可以被近自由费米子很好的描述.如果拓扑边界态具有显著的电子-电子相互作用的行为,无间隙的边界态会转变为有间隙的有序态,例如密度波态或超导态,这在理论上是非常有趣的,但目前还缺少相关的实验证据.本文报道了在Co Si(001)表面观察到了电子-电子相互作用驱动的、在拓扑边界态上形成的非公度电荷密度波(CDW). CDW的波矢随温度的变化而变化,这与拓扑表面费米弧随温度的演化相吻合. CDW相的取向由费米弧的手性决定,这暗示了CDW与费米弧之间存在直接的联系.该发现将促进在拓扑材料边界上寻找更多相互作用驱动的有序态,例如超导和磁性.

Magnetic-field-induced electronic instability of Weyl-like fermions in compressed black phosphorus

Revealing the role of Coulomb interaction in topological semimetals with Dirac/Weyl-like band dispersion shapes a new frontier in condensed matter physics.Topological node-line semimetals(TNLSMs),anticipated as a fertile ground for exploring electronic correlation effects due to the anisotropy associated with their node-line structure,have recently attracted considerable attention.In this study,we report an experimental observation for correlation effects in TNLSMs realized by black phosphorus(BP)under hydrostatic pressure.By performing a combination of nuclear magnetic resonance measurements and band calculations on compressed BP,a magnetic-field-induced electronic instability of Weyl-like fermions is identified under an external magnetic field parallel to the so-called nodal ring in the reciprocal space.Anomalous spin fluctuations serving as the fingerprint of electronic instability are observed at low temperatures,and they are observed to maximize at approximately 1.0 GPa.This study presents compressed BP as a realistic material platform for exploring the rich physics in strongly coupled Weyl-like fermions.

Highly in-plane anisotropic optical properties of fullerene monolayers

Both the intrinsic anisotropic optical materials and fullerene-assembled 2D materials have attracted much interest in fundamental science and potential applications.The synthesis of a monolayer(ML)fullerene makes the combination of these two features plausible.In this work,using first-principles calculations,we systematically study the electronic structure and optical properties of quasi-hexagonal phase(qHP)ML and quasi-tetragonal phase(qTP)ML fullerenes.The calculations of q HP ML show that it is a semiconductor with small anisotropic optical absorption,which agrees with the recent experimental measurements.However,the results for qTP ML reveal that it is a semimetal with highly in-plane anisotropic absorption.The dichroic ratio,namely the absorption ratio of x-and y-polarized lightα_(xx)/α_(yy),is around 12 at photon energy of 0.29 eV.This anisotropy is much more pronounced when the photon energy is between 0.7 and 1.4 eV,whereα_(xx)becomes nearly zero whileα_(yy)is more than two orders of magnitude larger.This indicates qTP ML as a candidate for long-pursuit lossless metal and a potential material for atomically thin polarizer.We hope this will stimulate further experimental eforts in the study of qTP ML and other fullerene-assembled 2D materials.

Multiple Dirac points including potential spin-orbit Dirac points in nonsymmorphic HfGe0.92Te

The search for new materials with Dirac points has been a fascinating subject of condensed matter physics.Here we first report the growth and band structure of HfGe_(0.92)Te single crystals featuring three different types of Dirac points.HfGe_(0.92)Te crystallizes in a nonsymmorphic tetragonal space group P4/nmm(No.129),having a square Ge-atom plane with vacancies of about 8%.Using angle-resolved photoemission spectroscopy(ARPES),the Dirac nodal line composed of conventional Dirac points vulnerable to spin-orbit coupling(SOC)is observed,accompanied by robust Dirac points protected by the nonsymmorphic symmetry against SOC and vacancies.In particular,spin-orbit Dirac points(SDPs)originating from the surface formed under significant SOC could exist based on ARPES and calculations.Quasi-two-dimensional(quasi-2D)characteristics are confirmed by angular-resolved magnetoresistance.HfGe_(0.92)Te bulk crystals can be easily exfoliated to flakes with a thickness of approximately 5 nm for the quasi-2D nature.Thus,HfGe_(0.92)Te provides a good platform to explore exotic topological phases or topological properties with three different types of Dirac points,which is a potential candidate to achieve novel 2D SDPs.

Large Spin Hall Conductivity and Excellent Hydrogen Evolution Reaction Activity in Unconventional PtTe1.75 Monolayer

Two-dimensional(2D)materials have gained lots of attention due to the potential applications.In this work,we propose that based on first-principles calculations,the(2×2)patterned PtTe_(2)monolayer with kagome lattice formed by the well-ordered Te vacancy(PtTe_(1.75))hosts large and tunable spin Hall conductivity(SHC)and excellent hydrogen evolution reaction(HER)activity.The unconventional nature relies on the A1@1b band representation of the highest valence band without spin–orbit coupling(SOC).The large SHC comes from the Rashba SOC in the noncentrosymmetric structure induced by the Te vacancy.Even though it has a metallic SOC band structure,theℤ_(2)invariant is well defined because of the existence of the direct bandgap and is computed to be nontrivial.The calculated SHC is as large as 1.25×10^(3)h/e(Ωcm)^(−1)at the Fermi level(EF).By tuning the chemical potential from EF−0.3 to EF+0.3 eV,it varies rapidly and monotonically from−1.2×10^(3)to 3.1×10^(3)h/e(Ωcm)^(−1)In addition,we also find that the Te vacancy in the patterned monolayer can induce excellent HER activity.Our results not only offer a new idea to search 2D materials with large SHC,i.e.,by introducing inversion–symmetry breaking vacancies in large SOC systems,but also provide a feasible system with tunable SHC(by applying gate voltage)and excellent HER activity.

Topological nodal line semimetals predicted from first-principles calculations

Topological semimetals are newly discovered states of quantum matter, which have extended the con- cept of topological states from insulators to metals and attracted great research interest in recent years. In general, there are three kinds of topological semimetals, namely Dirac semimetals, Weyl semimet- als, and nodal line semimetals. Nodal line semimetals can be considered as precursor states for other topological states. For example, starting from such nodal line states, the nodal line structure might evolve into Weyl points, convert into Dirac points, or become a topological insulator by introducing the spin-orbit coupling （SOC） or mass term. In this review paper, we introduce theoretical materials that show the nodal line semimetal state, including the all-carbon Mackay-Terrones crystal （MTC）, anti-perovskite Cu3PdN, pressed black phosphorus, and the CaP3 family of materials, and we present the design principles for obtaining such novel states of matter.

Topological Nodal States in Circuit Lattice

Te search for artifcial structure with tunable topological properties is an interesting research direction of today’s topological physics.Here,we introduce a scheme to realize topological nodal states with a three-dimensional periodic inductor-capacitor(LC)circuit lattice,where the topological nodal line state and Weyl state can be achieved by tuning the parameters of inductors and capacitors.A tight-binding-like model is derived to analyze the topological properties of the LC circuit lattice.Te key characters of the topological states,such as the drumhead-like surface bands for nodal line state and the Fermi arc-like surface bands for Weyl state,are found in these systems.We also show that the Weyl points are stable with the fabrication errors of electric devices.

High-pressure opens new windows for topological materials

Recently Guo et al.[1]have performed a systematic simulation on the high-pressure phases of the first experimentally available Weyl semimetals TaAs family.Through such a cheap but powerful technique,they theoretically found a

Magnetic Weyl semimetal finally confirmed

Recently,a couple of Science papers[1,2]have reported the existence of Fermi arcs on the surface of a ferromagnetic compound,namely Co3Sn2S2,which was first proposed to be a magnetic Weyl semimetal(WSM)according to the giant anomalous Hall effect[3,4].The new evidence is in agreement with this proposal,suggesting that the magnetic WSM has been found after a nearly eight year research[5,6].This finding,along with the discovery of nonmagnetic WSMs of TaAs family in 2015[7,8],has completed the WSM classification.In comparison with the nonmagnetic WSM,magnetic WSM has a long-range magnetic order coupled with Weyl nodes.Furthermore,it offers more ways to control and explore the properties of WSMs.

Multi-loop node line states in ternary MgSrSi-type crystals

Node line band-touchings protected by mirror symmetry(named as m-NLs),the product of inversion and time reversal symmetry S=PT(named as s-NLs),or nonsymmorphic symmetry are nontrivial topological objects of topological semimetals in the Brillouin Zone.In this work,we screened a family of MgSrSi-type crystals using first principles calculations,and discovered that more than 70 members are node-line semimetals.A new type of multi-loop structure was found in AsRhTi that a s-NL touches robustly with a m-NL at some“nexus point”,and in the meanwhile a second m-NL crosses with the s-NL to form a Hopf-link.Unlike the previously proposed Hopf-link formed by two s-NLs or two m-NLs,a Hopf-link formed by a s-NL and a m-NL requires a minimal three-band model to characterize its essential electronic structure.The associated topological surface states on different surfaces of AsRhTi crystal were also obtained.Even more complicated and exotic multi-loop structure of NLs were predicted in AsFeNb and PNiNb.Our work may shed light on search for exotic multi-loop node-line semimetals in real materials.

Topological electronic states in HfRuP family superconductors

Based on the first-principles calculations and experimental measurements,we report that the hexagonal phase of ternary transition metal pnictides TT’X(T=Zr,Hf;T’=Ru;X=P,As),which are well-known noncentrosymmetric superconductors with relatively high transition temperatures,host nontrivial bulk topology.Before the superconducting phase transition,we find that HfRuP belongs to a Weyl semimetal phase with 12 pairs of type-II Weyl points,while ZrRuAs,ZrRuP and HfRuAs belong to a topological crystalline insulating phase with trivial Fu-Kane Z_(2) indices but nontrivial mirror Chern numbers.High-quality single crystal samples of the noncentrosymmetric superconductors with these two different topological states have been obtained and the superconductivity is verified experimentally.The wide-range band structures of ZrRuAs have been identified by ARPES and reproduced by theoretical calculations.Combined with intrinsic superconductivity,the nontrivial topology of the normal state may generate unconventional superconductivity in both bulk and surfaces.Our findings could largely inspire the experimental searching for possible topological superconductivity in these compounds.

非常规材料:电子分布不在原子上

在拓扑量子化学中,研究人员发展了空间群下完备的基本能带表示,既对应于实空间的局域轨道,又对应于倒空间的一组不可约表示.通过计算实际材料的高对称k点的不可约表示,再利用基本能带表示分解来判断材料的拓扑性质.本研究发现拓扑平庸的材料可以分为电子电荷中心在原子上的常规材料和电荷中心不在原子上的“非常规材料”.这种非常规的电子分布特征在特定晶体对称群下可以方便地由能带表示分解来判断:占据态是基础能带表示的组合,但不是原子轨道对应的能带表示的组合.同时作者引入空位上的(必要)能带表示和实空间不变量来标定这一非常规电子分布特性.利用这一基于对称性本征值的能带表示分解方法,本文通过高通量搜索找到了423个非常规材料,并讨论了这些材料热电、储氢、催化、超导等特殊的化学性质与非常规电荷分布的相关性.

S_(4)对称性体系中外尔半金属的高通量搜索

基于对称性的不可约表示和相容性关系,人们可以将材料分为拓扑(或平庸)绝缘体(满足相容性关系),或是拓扑半金属(不满足相容性关系).但对于外尔半金属的判断通常超出了这种基于对称性方法的能力.换句话说,不论材料中占据态能带满不满足相容性关系,或者对称性指标是否为零,外尔点都可能出现在该材料中.本文针对具有S4对称性的体系提出了一种新型的拓扑不变量χ,并以此来检测体系中是否有外尔点的存在.χ可以通过一维Wilson-loop方法计算出来.将这种方法应用到高通量第一性原理计算中,作者在非磁和磁性体系中预测了大量新的外尔半金属材料.这些新发现的拓扑半金属材料具有许多独特的性质,如磁阻挫效应、超导和自旋玻璃态等.这些材料为外尔费米子和各种奇异物态之间相互作用研究提供了平台.