Growth and characterization of Bi(110)/CrTe_(2) heterostructures:Exploring interplay between magnetism and topology

The interplay between topology and magnetism is vital for realizing exotic quantum phenomena,significant examples including quantum anomalous Hall effect,axion insulators,and high-order topological states.These states host great potential for future applications in high-speed and low-consumption electronic devices.Despite being extensively investigated,practical platforms are still scarce.In this work,with molecular beam epitaxy(MBE),we provide the first experimental report on high-quality Bi(110)/CrTe_(2) magnetic heterostructure.By employing in-situ high-resolution scanning tunneling microscopy,we are able to examine the interaction between magnetism and topology.There is a potential edge state at an energy level above the Fermi level,but no edge states observed near the Fermi level The absence of high-order topological corner states near EF highlights the importance of lattice matching and interface engineering in designing high-order topological states.Our study provides key insights into the interplay between two-dimensional magnetic and topological materials and offers an important dimension for engineering magnetic topological states.

Distinct behavior of electronic structure under uniaxial strain in BaFe_(2)As_(2)

We report a study of the electronic structure of BaFe_(2)As_(2) under uniaxial strains using angle-resolved photoemission spectroscopy and transport measurements. Two electron bands at the MY point, with an energy splitting of 50 meV in the strain-free sample, shift downward and merge into each other under a large uniaxial strain, while three hole bands at theГ point shift downward together. However, we also observed an enhancement of the resistance anisotropy under uniaxial strains by electrical transport measurements, implying that the applied strains strengthen the electronic nematic order in BaFe_(2)As_(2). These observations suggest that the splitting of these two electron bands at the MY point is not caused by the nematic order in BaFe_(2)As_(2).

Surface doping manipulation of the insulating ground states in Ta_(2)Pd_(3)Te_(5) and Ta_(2)Ni_(3)Te_(5)

Manipulating emergent quantum phenomena is a key issue for understanding the underlying physics and contributing to possible applications.Here we study the evolution of insulating ground states of Ta_(2)Pu_(3)Te_(5) and Ta_(2)Ni_(3)Te_(5) under in-situ surface potassium deposition via angle-resolved photoemission spectroscopy.Our results confirm the excitonic insulator character of Ta_(2)d_(3)Te_(5).Upon surface doping,the size of its global gap decreases obviously.After a deposition time of more than 7 min,the potassium atoms induce a metal-insulator phase transition and make the system recover to a normal state.In contrast,our results show that the isostructural compound Ta_(2)Ni_(3)Te_(5) is a conventional insulator.The size of its global gap decreases upon surface doping,but persists positive throughout the doping process.Our results not only confirm the excitonic origin of the band gap in Ta_(2)Pd_(3)Te_(5),but also offer an effective method for designing functional quantum devices in the future.

飞秒激光致双液滴光学击穿和等离子体分布研究

为了研究超短激光脉冲与双液滴相互作用过程中的光学击穿和等离子体分布,基于麦克斯韦方程组和电离速率方程,构建了飞秒激光与双液滴的瞬态耦合模型,使用有限元分析方法,对飞秒激光辐照微米量级双液滴的自由电子密度和光场分布进行了计算,得到了双液滴结构对液滴光学击穿和等离子体变化的影响。结果表明,第2个液滴的击穿阈值约为同等条件下单液滴击穿阈值的35%;等离子体的形态和击穿点的位置随双液滴间距发生变化,且在聚焦区域产生纳米等离子体射流;第2个液滴对激光能量的吸收随着双液滴间距的增加而减少;当分别使用满足击穿阈值的光强入射,双液滴吸收的能量约为单液滴的3%;第2个液滴对激光能量的吸收随光强增大而增大,能量吸收比例最终趋于0.01,仅为单液滴的1.5%。该研究为激光诱导水击穿和激光在大气中的传输提供了一定的参考。

锰位Cu离子掺杂对层状锰氧化物La_(1.2)Ca_(1.8)Mn_2O_7电磁性能的影响

研究了Cu离子掺杂对层状钙钛矿结构锰氧化物La1 .2 Ca1 .8Mn2 O7电磁性能的影响。观察到低含量Cu掺杂样品铁磁性减弱 ,出现金属 -绝缘转变与团簇玻璃态行为 ;高含量Cu掺杂样品为绝缘体 ,出现顺磁与自旋玻璃态行为。相对小极化子跃迁模型或热激活半导体带隙模型而言 ,可变程跃迁模型更好地描述了载流子在顺磁状态的电输运机制。

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.

Fe1+yTexSe1-x:A Delicate and Tunable Majorana Material

We report the observation for the pz electron band and the band inversion in Fe1+yTexSe1-xwith angleresolved photoemission spectroscopy. Furthermore, we found that excess Fe(y>0) inhibits the topological band inversion in Fe1+yTexSe1-x,which explains the absence of Majorana zero modes in previous reports for Fe1+yTexSe1-xwith excess Fe. Based on our analysis of different amounts of Te doping and excess Fe, we propose a delicate topological phase in this material. Thanks to this delicate phase, one may be able to tune the topological transition via applying lattice strain or carrier doping.

多铁性体系中磁电耦合效应及新材料探索

多铁性材料有两个或两个以上的序参量,通过磁电效应可以实现磁电转换功能,在信息存储、传感器等方面具有广阔的应用前景,引起各国材料科学研究工作者的重视.近年来,磁电效应研究在物性研究和探索新材料方面取得了重大进展.

Epitaxial growth and air-stability of monolayer Cu2Te

A new two-dimensional atomic crystal, monolayer cuprous telluride（Cu2Te） has been fabricated on a grapheneSi C（0001） substrate by molecular beam epitaxy（MBE）. The low-energy electron diffraction（LEED） characterization shows that the monolayer Cu2Te forms ■ superstructure with respect to the graphene substrate. The atomic structure of the monolayer Cu2Te is investigated through a combination of scanning tunneling microscopy（STM） experiments and density functional theory（DFT） calculations. The stoichiometry of the Cu2Te sample is verified by x-ray photoelectron spectroscopy（XPS） measurement. The angle-resolved photoemission spectroscopy（ARPES） data present the electronic band structure of the sample, which is in good agreement with the calculated results. Furthermore, air-exposure experiments reveal the chemical stability of the monolayer Cu2Te. The fabrication of this new 2D material with a particular structure may bring new physical properties for future applications.

High quality PdTe_2 thin films grown by molecular beam epitaxy

PdTe2, a member of layered transition metal dichalcogenides （TMDs）, has aroused significant research interest due to the coexistence of superconductivity and type-II Dirac fermions. It provides a promising platform to explore the inter- play between superconducting quasiparticles and Dirac fermions. Moreover, PdTe2 has also been used as a substrate for monolayer antimonene growth. Here in this paper, we report the epitaxial growth of high quality PdTe2 films on bilayer graphene/SiC（0001） by molecular beam epitaxy （MBE）. Atomically thin films are characterized by scanning tunneling microscopy （STM）, X-ray photoemission spectroscopy （XPS）, low-energy electron diffraction （LEED）, and Raman spec- troscopy. The band structure of 6-layer PdTe2 film is measured by angle-resolved photoemission spectroscopy （ARPES）. Moreover, our air exposure experiments show excellent chemical stability of epitaxial PdTe2 film. High-quality PdTe2 films provide opportunities to build antimonene/PdTe2 heterostructure in ultrahigh vacuum for future applications in electronic and optoelectronic nanodevices.

Experimental Investigation of Electronic Structure of La(O,F)BiSe_2

La（O,F）BiSe2 is a layered superconductor and has the same crystal structure with La（O,F）BiS2. We investigate the electronic structure of La（O,F）BiSe2 using the angle-resolved photoemission spectroscopy. Two electron-like Fermi surfaces around X（π, 0） are observed, corresponding to the electron doping of 0.23 per Bi site. We clearly resolve anisotropic band splitting along both Г-X and M-X due to the cooperative effects of large spin-orbit coupling and interlayer coupling. Moreover, we observe an almost non-dispersive electronic state around -0.2 eV between the electron-like bands. This state vanishes after in-situ K evaporation, indicating that it could be the localized surface state caused by defects on the cleaved surface.

Realization of low-energy type-Ⅱ Dirac fermions in(Ir_(1-x)Pt_x)Te_2 superconductors

Topological Dirac semimetals(DSMs) present a kind of topologically nontrivial quantum state of matter, which has massless Dirac fermions in the bulk and topologically protected states on certain surfaces. In superconducting DSMs, the effects of their nontrivial topology on superconducting pairing could realize topological superconductivity in the bulk or on the surface. As superconducting pairing takes place at the Fermi level E_F, to make the effects possible, the Dirac points should lie in the vicinity of E_F so that the topological electronic states can participate in the superconducting paring. Here,we show using angle-resolved photoelectron spectroscopy that in a series of(Ir_(1-x)Pt_x)Te_2 compounds, the type-Ⅱ Dirac points reside around E_F in the superconducting region, in which the bulk superconductivity has a maximum T_c of ～ 3 K.The realization of the coexistence of bulk superconductivity and low-energy Dirac fermions in(Ir_(1-x)Pt_x)Te_2 paves the way for studying the effects of the nontrivial topology in DSMs on the superconducting state.

Atomically flat surface preparation for surface-sensitive technologies

Surface-sensitive measurements are crucial to many types of researches in condensed matter physics.However,it is difficult to obtain atomically flat surfaces of many single crystals by the commonly used mechanical cleavage.We demonstrate that the grind-polish-sputter-anneal method can be used to obtain atomically flat surfaces on topological materials.Three types of surface-sensitive measurements are performed on CoSi(001)surface with dramatically improved quality of data.This method extends the research area of surface-sensitive measurements to hard-to-cleave alloys,and can be applied to irregular single crystals with selective crystalline planes.It may become a routine process of preparing atomically flat surfaces for surface-sensitive technologies.

Surface State Bands in Superconducting(PtxIr1-x)Te2

Angle-resolved photoemission spectroscopy is performed to study the bulk and surface electronic structures of non- superconducting IrTe2 and superconducting Pto.oblro.95 Te2. In addition to the bulk electronic bands predicted by the local density approximation calculations, we observe two Dirac cone-like bands at the Brillouin zone center, which are non-dispersive along kz, suggesting that the extra bands are surface state bands. As the experimental results are well consistent with the ab initio calculations of surface states, the parity analysis proves that these surface state bands are topologically trivial and thus exclude （PtxIr1-x）Te2 as a possible topological superconductor candidate.

Growth of High-Quality Superconducting FeSe0.5Te0.5 Thin Films Suitable for Angle-Resolved Photoemission Spectroscopy Measurements via Pulsed Laser Deposition

High-quality superconducting FeSe0.5 Te0.5 films are epitaxiMly grown on different substrates by using the pulsed laser deposition method. By measuring the transport properties and surface morphology of films grown on single- crystal substrates of Al2O3 （0001）, SrTiO3 （001）, and MgO （001）, as well as monitoring the real-time growth process on MgO substrates with reflection high energy electron diffraction, we find the appropriate parameters for epitaxial growth of high-quality FeSe0.5 Te0.5 thin films suitable for angle-resolved photoemission spectroscopy measurements. We further report the angle-resolved photoemission spectroscopy characterization of the super- conducting films. The clearly resolved Fermi surfaces and the band structure suggest a sample quality that is as good as that of high-quality single-crystals, demonstrating that the pulsed laser deposition method can serve as a promising technique for in situ preparation and manipulation of iron-based superconducting thin films, which may bring new prosperity to angle-resolved photoemission spectroscopy research on iron-based superconductors.

Coexistence of Polaronic States and Superconductivity in Iron-Pnictide Compound Ba_2Ti_2Fe_2As_4O

The electronic structure of iron-pnictide compound superconductor Ba_2Ti_2Fe_2As_4O, which has metallic intermediate Ti_2O layers, is studied using angle-resolved photoemission spectroscopy. The Ti-related bands show a‘peak-dip-hump＇ line shape with two branches of dispersion associated with the polaronic states at temperatures below around 120 K. This change in the spectra occurs along with the resistivity anomaly that was not clearly understood in a previous study. Moreover, an energy gap induced by the superconducting proximity effect opens in the polaronic bands at temperatures below T_c（~21 K）. Our study provides the spectroscopic evidence that superconductivity coexists with polarons in the same bands near the Fermi level, which provides a suitable platform to study interactions between charge, lattice and spin freedoms in a correlated system.

X-ray absorption investigation of the site occupancies of the copper element in nominal Cu_(3)Zn(OH)_(6)FBr

With Zn substitution to the three-dimensional antiferromagnetically ordered barlowite Cu_(4)(OH)_(6)FBr,Cu_(3)Zn(OH)_(6)FBr shows no magnetic phase transition down to 50 mK,and the system is suggested to be a two-dimensional kagomé quantum spin liquid[Chin.Phys.Lett.34077502(2017)].A key issue to identify such phase diagram is the exact chemical formula of the substituted compound.With Cu L-edge x-ray absorption spectrum(XAS)combined with the MultiX XAS calculations,we evaluate the Cu concentration in a nominal Cu_(3)Zn(OH)_(6)FBr sample.Our results show that although the Cu concentration is 2.80,close to the expected value,there is 34%residual Cu occupation in intersite layers between kagomé layers.Thus the Zn substitution of the intersite layers is not complete,and likely it intrudes the kagomé layers.

Discovery of Two Families of VSb-Based Compounds with V-Kagome Lattice

We report the structure and physical properties of two newly discovered compounds AV_(8)Sb_(12)and AV_(6)Sb_(6)(A=Cs,Rb),which have C_(2)(space group:Cmmm)and C_(3)(space group:R3 m)symmetry,respectively.The basic Vkagome unit appears in both compounds,but stacking differently.AV_(2)Sb_(2) layer is sandwiched between two V_(3)Sb_(5)layers in AV_(8)Sb_(12),altering the V-kagome lattice and lowering the symmetry of kagome layer from hexagonal to orthorhombic.In AV_(6)Sb_(6),the building block is a more complex slab made up of two half-V_(3)Sb_(5)layers that are intercalated by Cs cations along the c-axis.Transport property measurements demonstrate that both compounds are nonmagnetic metals,with carrier concentrations at around 10^(21)cm^(-3).No superconductivity has been observed in CsV_(8)Sb_(12)above 0.3 K under in situ pressure up to 46 GPa.Compared to CSV_(3)Sb_(5),theoretical calculations and angle-resolved photoemission spectroscopy reveal a quasi-two-dimensional electronic structure in CsV_(8)Sb_(12)with C_(2)symmetry and no van Hove singularities near the Fermi level.Our findings will stimulate more research into V-based kagome quantum materials.

Experimental Synthesis of Strained Monolayer Silver Arsenide on Ag(111)Substrates

Two-dimensional(2 D)materials are playing more and more important roles in both basic sciences and industrial applications.For 2 D materials,strain could tune the properties and enlarge applications.Since the growth of 2 D materials on substrates is often accompanied by strain,the interaction between 2 D materials and substrates is worthy of careful attention.Here we demonstrate the fabrication of strained monolayer silver arsenide(AgAs)on Ag(111)by molecular beam epitaxy,which shows one-dimensional stripe structures arising from uniaxial strain.The atomic geometric structure and electronic band structure are investigated by low energy electron diffraction,scanning tunneling microscopy,x-ray photoelectron spectroscopy,angle-resolved photoemission spectroscopy and first-principle calculations.Monolayer AgAs synthesized on Ag(111)provides a platform to study the physical properties of strained 2 D materials.

Continuously Doping Bi_(2)Sr_(2)CaCu_(2)O_(8+δ)into Electron-Doped Superconductor by CaH_(2)Annealing Method

As a typical hole-doped cuprate superconductor,Bi_(2)Sr_(2)CaCu_(2)O_(8+δ)(Bi2212)carrier doping is mostly determined by its oxygen content.Traditional doping methods can regulate its doping level within the range of hole doping.Here we report the first application of CaH_(2)annealing method in regulating the doping level of Bi2212.By continuously controlling the anneal time,a series of differently doped samples can be obtained.The combined experimental results of x-ray diffraction,scanning transmission electron microscopy,resistance and Hall measurements demonstrate that the CaH_(2)induced topochemical reaction can effectively change the oxygen content of Bi2212 within a very wide range,even switching from hole doping to electron doping.We also found evidence of a low-T_c superconducting phase in the electron doping side.