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.

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.

Hybridization Effects Revealed by Angle-Resolved Photoemission Spectroscopy in Heavy-Fermion Ce2IrIn8

We utilize high-resolution resonant angle-resolved photoemission spectroscopy(ARPES)to study the band structure and hybridization effect of the heavy-fermion compound Ce2 IrIn8.We observe a nearly flat band at the binding energy of 7 meV below the coherent temperature Tcoh^40 K,which characterizes the electrical resistance maximum and indicates the onset temperature of hybridization.However,the Fermi vector and the Fermi surface volume have little change around Tcoh,which challenges the widely believed evolution from a hightemperature small Fermi surface to a low-temperature large Fermi surface.Our experimental results of the band structure fit well with the density functional theory plus dynamic mean-field theory calculations.

Quantization of the band at the surface of charge density wave material 2H-TaSe_(2)

By using angle-resolvea photoemission spectroscopy(ARPES) combined with the first-principies electronic structure calculations,we report the quantized states at the surface of a single crystal 2 H-TaSe_(2).We have observed sub-bands of quantized states at the three-dimensional Brillouin zone center due to a highly dispersive band with light effective mass along k_(z) direction.The quantized sub-bands shift upward towards E_(F) while the bulk band at Γ shifts downward with the decrease of temperature across charge density wave(CDW) formation.The band shifts could be intimately related to the CDW.While neither the two-dimensional Fermi-surface nesting nor purely strong electron-phonon coupling can explain the mechanism of CDW in 2 H-TaSe_(2),our experiment may ignite the interest in understanding the CDW mechanism in this family.

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.

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.

Identifying the convergent reaction path from predesigned assembled structures: Dissymmetrical dehalogenation of Br_(2)Py on Ag(111)

On-surface Ullmann coupling has been intensely utilized for the tailor-made fabrication of conjugated frameworks towards molecular electronics, however, reaction mechanisms are still limitedly understood. Herein, we provide a comprehensive elucidation of the surface Ullmann coupling of 2,7-dibromopyrene (Br2Py) on Ag(111) by scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and density function theory (DFT), and reveal that the Ullmann reaction path is unique regardless of predesigned assembled structures. By manipulating deposition conditions, diverse assembled architectures have been constructed for Br2Py on Ag(111), including the ladder phase, parallel arrangement, hexagonal patterns from monomers or Kagome lattices based on organometallic (OM) dimers. Intriguingly, stepwise annealing leads to an identical reaction diagram for the surface Ullmann coupling from individual assembled structures convergent into the brick-wall-pattern OM dimers first, which is deemed to be a stable phase, and then into elongated OM chains in order and eventually long-range polymers with direct C-C coupling. While the reaction mechanism is demonstrated to be dominated by the metal coordinated and halogen bonding motifs, interestingly, it has also been revealed that surface adatoms and dissociated Br atoms play a crucial role in coupling reactions. In contrast to previous reports demonstrating the manipulation of Ullmann reactions by preassembled strategy, herein, weak intermolecular interaction in assembled nanostructures is immediately suppressed by strong covalent bonding during reactions. Importantly, our report proposes essential insights on fundamental understanding of surface Ullmann coupling towards high-yield surface synthesis.

具有拓扑外尔环的电荷密度波超导体In_(x)TaSe_(2)

新型拓扑材料和拓扑相的探索是近年来凝聚态物理中的热点方向.本文发现铟插层的过渡金属硫族化合物In_(x)TaSe_(2)(112体系)具有两个外尔环,以及电荷密度波(CDW)和超导电性共存的特点.在x=0.58的样品中,发现在116和77 K温度以下具有2×√3和2×2的公度CDW,这可以被电阻率、比热和扫描隧道显微镜等测量证实.角分辨光电子能谱测量发现2×2 CCDW会打开其中一个外尔环的能隙,而另一个外尔环仍然存在,且具有理论预言的"drumhead"型表面态.另外,Tc=1 K的超导电性和低温偏离s波的临界场行为表明这个材料很可能是拓扑超导体,这为探索拓扑超导电性提供了新体系.

Electronic structure of LaIrIn5and f-electron character in its related Ce-115 compounds

LaIrIn5 is a reference compound of the heavy-fermion superconductor LaIrIn5.The lack of f electrons in LaIrIn5 indicates that there should not be any f electron participating in the construction of its Fermi surface.Thus the electronic structure comparison between LaIrIn5 and LaIrIn5 provides a good platform to study the properties of f electrons.Here angle-resolved photoemission spectroscopy(ARPES)measurements and density functional theory(DFT)calculations are performed to study the electronic structures of LaIrIn5 and LaIrIn5.We find the valence band structures of the two materials are similar to each other,except for the absence of f bands in LaIrIn5.By analyzing the Fermi crossings of the three conduction bands of the two materials quantitatively,we find the volumes of the electron pocketsαandβaround the M′point become larger from LaIrIn5 to LaIrIn5,while the hole pocketγaround theΓ′point becomes smaller.Together with the calculation results,we confirm that this is mainly originated from the f-electron contribution,while the lattice-constant difference between LaIrIn5 and LaIrIn5 only has a finite influence.We also give a summary of the f-electron character in its related Ce-115 heavy fermion compounds.Our results may be essential for the complete microscopic understanding of the 115 compounds and the related heavy-fermion systems.

Consecutive topological transitions of helical Fermi arcs at saddle points in CoSi

The CoSi family hosts unconventional topological nodes with nonzero Chern numbers.The nontrivial topology is manifested by conspicuous surface Fermi arcs connecting surface projections of the nodes.Here,using angle-resolved photoemission spectroscopy,we have systematically investigated the(001)surface states of pristine and Ni-doped Co Si.The surface states form saddle-like band structures at/near the time-reversal invariant point near the Fermi level.The Fermi arcs undergo consecutive Lifshitz transitions at the saddle points X,leading to changes of the Fermi arc configuration.As the density of states has a van Hove singularity at the saddle points,exotic many-body physical phenomena may emerge accompanied by the topological transitions of surface Fermi arcs.

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.