Evolution of Mn1-xGexBi2Te4 Electronic Structure under Variation of Ge Content.

One of the approaches to manipulate MnBi2Te4 properties is the magnetic dilution, which inevitably affects the interplay of magnetism and band topology in the system. In this work, we carried out angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations for analysing changes in the electronic structure of Mn1-xGexBi2Te4 that occur under parameter x variation. We consider two ways of Mn/Ge substitution: (i) bulk doping of the whole system; (ii) surface doping of the first septuple layer. For the case (i), the experimental results reveal a decrease in the value of the bulk band gap, which should be reversed by an increase when the Ge concentration reaches a certain value. Ab-initio calculations show that at Ge concentrations above 50%, there is an absence of the bulk band inversion of the Te pz and Bi pz contributions at the Γ-point with significant spatial redistribution of the states at the band gap edges into the bulk, suggesting topological phase transition in the system. For case (ii) of the vertical heterostructure Mn1-xGexBi2Te4/MnBi2Te4, it was shown that an increase of Ge concentration in the first septuple layer leads to effective modulation of the Dirac gap in the absence of significant topological surface states of spatial redistribution. The results obtained indicate that surface doping compares favorably compared to bulk doping as a method for the Dirac gap value modulation.

Observation of Topological Flat Bands in the Kagome Semiconductor Nb3Cl8.

The destructive interference of wavefunctions in a kagome lattice can give rise to topological flat bands (TFBs) with a highly degenerate state of electrons. Recently, TFBs have been observed in several kagome metals, including Fe3Sn2, FeSn, CoSn, and YMn6Sn6. Nonetheless, kagome materials that are both exfoliable and semiconducting are lacking, which seriously hinders their device applications. Herein, we show that Nb3Cl8, which hosts a breathing kagome lattice, is gapped out because of the absence of inversion symmetry, while the TFBs survive because of the protection of the mirror reflection symmetry. By angle-resolved photoemission spectroscopy measurements and first-principles calculations, we directly observe the TFBs and a moderate band gap in Nb3Cl8. By mechanical exfoliation, we successfully obtain monolayer Nb3Cl8, which is stable under ambient conditions. In addition, our calculations show that monolayer Nb3Cl8 has a magnetic ground state, thus providing opportunities to study the interplay among geometry, topology, and magnetism.

Observation of One-Dimensional Dirac Fermions in Silicon Nanoribbons.

Dirac materials, which feature Dirac cones in the reciprocal space, have been one of the hottest topics in condensed matter physics in the past decade. To date, 2D and 3D Dirac Fermions have been extensively studied, while their 1D counterparts are rare. Recently, Si nanoribbons (SiNRs), which are composed of alternating pentagonal Si rings, have attracted intensive attention. However, the electronic structure and topological properties of SiNRs are still elusive. Here, by angle-resolved photoemission spectroscopy, scanning tunneling microscopy/spectroscopy measurements, first-principles calculations, and tight-binding model analysis, we demonstrate the existence of 1D Dirac Fermions in SiNRs. Our theoretical analysis shows that the Dirac cones derive from the armchairlike Si chain in the center of the nanoribbon and can be described by the Su-Schrieffer-Heeger model. These results establish SiNRs as a platform for studying the novel physical properties in 1D Dirac materials.

Electronic band structure of iridates.

In this review, an attempt has been made to compare the electronic structures of various 5d iridates (iridium oxides), with an effort to note the common features and differences. Both experimental studies, especially angle-resolved photoemission spectroscopy (ARPES) results, and first-principles band structure calculations have been discussed. This brings to focus the fact that the electronic structures and magnetic properties of the high-Z 5d transition iridates depend on the intricate interplay of strong electron correlation, strong (relativistic) spin-orbit coupling, lattice distortion, and the dimensionality of the system. For example, in the thin film limit, SrIrO3 exhibits a metal-insulator transition that corresponds to the dimensionality crossover, with the band structure resembling that of bulk Sr2IrO4.

Observation of Spin-Momentum-Layer Locking in a Centrosymmetric Crystal.

The spin polarization in nonmagnetic materials is conventionally attributed to the outcome of spin-orbit coupling when the global inversion symmetry is broken. The recently discovered hidden spin polarization indicates that a specific atomic site asymmetry could also induce measurable spin polarization, leading to a paradigm shift in research on centrosymmetric crystals for potential spintronic applications. Here, combining spin- and angle-resolved photoemission spectroscopy and theoretical calculations, we report distinct spin-momentum-layer locking phenomena in a centrosymmetric, layered material, BiOI. The measured spin is highly polarized along the Brillouin zone boundary, while the same effect almost vanishes around the zone center due to its nonsymmorphic crystal structure. Our work demonstrates the existence of momentum-dependent hidden spin polarization and uncovers the microscopic mechanism of spin, momentum, and layer locking to each other, thus shedding light on the design metrics for future spintronic materials.

Tunneling current modulation in atomically precise graphene nanoribbon heterojunctions.

Lateral heterojunctions of atomically precise graphene nanoribbons (GNRs) hold promise for applications in nanotechnology, yet their charge transport and most of the spectroscopic properties have not been investigated. Here, we synthesize a monolayer of multiple aligned heterojunctions consisting of quasi-metallic and wide-bandgap GNRs, and report characterization by scanning tunneling microscopy, angle-resolved photoemission, Raman spectroscopy, and charge transport. Comprehensive transport measurements as a function of bias and gate voltages, channel length, and temperature reveal that charge transport is dictated by tunneling through the potential barriers formed by wide-bandgap GNR segments. The current-voltage characteristics are in agreement with calculations of tunneling conductance through asymmetric barriers. We fabricate a GNR heterojunctions based sensor and demonstrate greatly improved sensitivity to adsorbates compared to graphene based sensors. This is achieved via modulation of the GNR heterojunction tunneling barriers by adsorbates.

Fermi Velocity Reduction of Dirac Fermions around the Brillouin Zone Center in In2 Se3 -Bilayer Graphene Heterostructures.

Emergent phenomena such as unconventional superconductivity, Mott-like insulators, and the peculiar quantum Hall effect in graphene-based heterostructures are proposed to stem from the superlattice-induced renormalization of (moiré) Dirac fermions at the graphene Brillouin zone corners. Understanding the corresponding band structure commonly demands photoemission spectroscopy with both sub-meV resolution and large-momentum coverage, beyond the capability of the current state-of-the-art. Here the realization of moiré Dirac cones around the Brillouin zone center in monolayer In2 Se3 /bilayer graphene heterostructure is reported. The renormalization is evidenced by reduced Fermi velocity (≈23%) of the moiré Dirac cones and the reshaped Dirac point at the Γ point where they intersect. While there have been many theoretical predictions and much indirect experimental evidence, the findings here are the first direct observation of Fermi velocity reduction of the moiré Dirac cones. These features suggest strong In2 Se3 /graphene interlayer coupling, which is comparable with that in twisted bilayer graphene. The strategy expands the choice of materials in the heterostructure design and stimulates subsequent broad investigations of emergent physics at the sub-meV energy scale.

The band shifts in MoS2(0001) and WSe2(0001) induced by palladium adsorption.

The band structures of the transition metal dichalcogenides (TMD's) 2H-MoS2(0001) and 2H-WSe2(0001), before and after palladium adsorption, were investigated through angle-resolved photoemission. Palladium adsorption on 2H-MoS2(0001) is seen to result in very different band shifts than seen for palladium on 2H-WSe2(0001). The angle resolved photoemission results of palladium adsorbed on WSe2(0001) indicate that palladium accepts electron density from substrate. The resulting band shift will lead to a decrease in the barriers to the hole injection. The opposite band shifts occur upon palladium adsorption between 2H-MoS2(0001). The overall trend is consistent with the deposition of other metals deposited on TMD's, except that for palladium adsorption on MoS2(0001), there is an increase in the MoS2(0001) substrate band gap with palladium adsorption, as is evident from the combination of photoemission and inverse photoemission.

Topologically Nontrivial Phase-Change Compound GeSb2Te4.

Chalcogenide phase-change materials show strikingly contrasting optical and electrical properties, which has led to their extensive implementation in various memory devices. By performing spin-, time-, and angle-resolved photoemission spectroscopy combined with the first-principles calculation, we report the experimental results that the crystalline phase of GeSb2Te4 is topologically nontrivial in the vicinity of the Dirac semimetal phase. The resulting linearly dispersive bulk Dirac-like bands that cross the Fermi level and are thus responsible for conductivity in the stable crystalline phase of GeSb2Te4 can be viewed as a 3D analogue of graphene. Our finding provides us with the possibility of realizing inertia-free Dirac currents in phase-change materials.

Electrical Tuning of the Excitonic Insulator Ground State of Ta_{2}NiSe_{5}.

We demonstrate that the excitonic insulator ground state of Ta_{2}NiSe_{5} can be electrically controlled by electropositive surface adsorbates. Our studies utilizing angle-resolved photoemission spectroscopy reveal intriguing wave-vector-dependent deformations of the characteristic flattop valence band of this material upon potassium adsorption. The observed band deformation indicates a reduction of the single-particle band gap due to the Stark effect near the surface. The present study provides the foundation for the electrical tuning of the many-body quantum states in excitonic insulators.

Rashba-like spin splitting along three momentum directions in trigonal layered PtBi2.

Spin-orbit coupling (SOC) has gained much attention for its rich physical phenomena and highly promising applications in spintronic devices. The Rashba-type SOC in systems with inversion symmetry breaking is particularly attractive for spintronics applications since it allows for flexible manipulation of spin current by external electric fields. Here, we report the discovery of a giant anisotropic Rashba-like spin splitting along three momentum directions (3D Rashba-like spin splitting) with a helical spin polarization around the M points in the Brillouin zone of trigonal layered PtBi2. Due to its inversion asymmetry and reduced symmetry at the M point, Rashba-type as well as Dresselhaus-type SOC cooperatively yield a 3D spin splitting with αR ≈ 4.36 eV Å in PtBi2. The experimental realization of 3D Rashba-like spin splitting not only has fundamental interests but also paves the way to the future exploration of a new class of material with unprecedented functionalities for spintronics applications.

Discovery of Weyl Nodal Lines in a Single-Layer Ferromagnet.

Two-dimensional (2D) materials have attracted great attention and spurred rapid development in both fundamental research and device applications. The search for exotic physical properties, such as magnetic and topological order, in 2D materials could enable the realization of novel quantum devices and is therefore at the forefront of materials science. Here, we report the discovery of twofold degenerate Weyl nodal lines in a 2D ferromagnetic material, a single-layer gadolinium-silver compound, based on combined angle-resolved photoemission spectroscopy measurements and theoretical calculations. These Weyl nodal lines are symmetry protected and thus robust against external perturbations. The coexistence of magnetic and topological order in a 2D material is likely to inform ongoing efforts study the rich physics in 2D topological ferromagnets.

Superstructure-Induced Splitting of Dirac Cones in Silicene.

Atomic scale engineering of two-dimensional materials could create devices with rich physical and chemical properties. External periodic potentials can enable the manipulation of the electronic band structures of materials. A prototypical system is (3×3)-silicene/Ag(111), which has substrate-induced periodic modulations. Recent angle-resolved photoemission spectroscopy measurements revealed six Dirac cone pairs at the Brillouin zone boundary of Ag(111), but their origin remains unclear [Proc. Natl. Acad. Sci. USA 113, 14656 (2016)]. We used linear dichroism angle-resolved photoemission spectroscopy, the tight-binding model, and first-principles calculations to reveal that these Dirac cones mainly derive from the original cones at the K (K^{'}) points of free-standing silicene. The Dirac cones of free-standing silicene are split by external periodic potentials that originate from the substrate-overlayer interaction. Our results not only confirm the origin of the Dirac cones in the (3×3)-silicene/Ag(111) system, but also provide a powerful route to manipulate the electronic structures of two-dimensional materials.

Disorder Quenching of the Charge Density Wave in ZrTe_{3}.

The charge density wave (CDW) in ZrTe_{3} is quenched in samples with a small amount of Te isoelectronically substituted by Se. Using angle-resolved photoemission spectroscopy we observe subtle changes in the electronic band dispersions and Fermi surfaces upon Se substitution. The scattering rates are substantially increased, in particular for the large three-dimensional Fermi surface sheet. The quasi-one-dimensional band is unaffected by the substitution and still shows a gap at low temperature, which starts to open from room temperature. Long-range order is, however, absent in the electronic states as in the periodic lattice distortion. The competition between superconductivity and the CDW is thus linked to the suppression of long-range order of the CDW.

Evidence for a Quasi-One-Dimensional Charge Density Wave in CuTe by Angle-Resolved Photoemission Spectroscopy.

We report the electronic structure of CuTe with a high charge density wave (CDW) transition temperature T_{c}=335 K by angle-resolved photoemission spectroscopy. An anisotropic charge density wave gap with a maximum value of 190 meV is observed in the quasi-one-dimensional band formed by Te p_{x} orbitals. The CDW gap can be filled by increasing the temperature or electron doping through in situ potassium deposition. Combining the experimental results with calculated electron scattering susceptibility and phonon dispersion, we suggest that both Fermi surface nesting and electron-phonon coupling play important roles in the emergence of the CDW.

Accurate and efficient data acquisition methods for high-resolution angle-resolved photoemission microscopy.

Angle-resolved photoemission spectroscopy (ARPES) is a powerful experimental technique in materials science, as it can directly probe electronic states inside solids in energy (E) and momentum (k) space. As an advanced technique, spatially-resolved ARPES using a well-focused light source (high-resolution ARPES microscopy) has recently attracted growing interests because of its capability to obtain local electronic information at micro- or nano-metric length scales. However, there exist several technical challenges to guarantee high precision in determining translational and rotational positions in reasonable measurement time. Here we present two methods of obtaining k-space mapping and real-space imaging in high-resolution ARPES microscopy. One method is for k-space mapping measurements that enables us to keep a target position on a sample surface during sample rotation by compensating rotation-induced displacements (tracing acquisition method). Another method is for real-space imaging measurements that significantly reduces total acquisition time (scanning acquisition method). We provide several examples of these methods that clearly indicate higher accuracy in k-space mapping as well as higher efficiency in real-space imaging, and thus improved throughput of high-resolution APRES microscopy.

Experimental realization of two-dimensional Dirac nodal line fermions in monolayer Cu2Si.

Topological nodal line semimetals, a novel quantum state of materials, possess topologically nontrivial valence and conduction bands that touch at a line near the Fermi level. The exotic band structure can lead to various novel properties, such as long-range Coulomb interaction and flat Landau levels. Recently, topological nodal lines have been observed in several bulk materials, such as PtSn4, ZrSiS, TlTaSe2 and PbTaSe2. However, in two-dimensional materials, experimental research on nodal line fermions is still lacking. Here, we report the discovery of two-dimensional Dirac nodal line fermions in monolayer Cu2Si based on combined theoretical calculations and angle-resolved photoemission spectroscopy measurements. The Dirac nodal lines in Cu2Si form two concentric loops centred around the Γ point and are protected by mirror reflection symmetry. Our results establish Cu2Si as a platform to study the novel physical properties in two-dimensional Dirac materials and provide opportunities to realize high-speed low-dissipation devices.

Electronic structure of ferromagnetic heavy fermion, YbPdSi, YbPdGe, and YbPtGe studied by photoelectron spectroscopy, x-ray emission spectroscopy, and DFT + DMFT calculations.

Electronic structures of ferromagnetic heavy fermion Yb compounds of YbPdSi, YbPdGe, and YbPtGe are studied by photoelectron spectroscopy around the Yb 4d-4f resonance, resonant x-ray emission spectroscopy at the Yb L 3 absorption edge, and density functional theory combined with dynamical mean field theory calculations. These compounds all have a temperature-independent intermediate Yb valence with large [Formula: see text] and small [Formula: see text] components. The magnitude of the Yb valence is evaluated to be YbPtGe [Formula: see text] YbPdGe [Formula: see text] YbPdSi, suggesting that YbPtGe is the closest to the quantum critical point among the three Yb compounds. Our results support the scenario of the coexistence of heavy fermion behavior and ferromagnetic ordering which is described by a magnetically-ordered Kondo lattice where the magnitude of the Kondo effect and the RKKY interaction are comparable.

Development of laser-based scanning µ-ARPES system with ultimate energy and momentum resolutions.

We have developed a laser-based scanning angle-resolved photoemission spectroscopy system (µ-ARPES) equipped with a high precision 6-axis control system, realizing not only high-resolution photoemission spectroscopy in energy and momentum, but also spatial resolution of a µm scale. This enables our µ-ARPES system to probe fine details of intrinsic electronic states near the Fermi level such as the superconducting gaps and lifetime broadening.

Adsorbate doping of MoS2 and WSe2: the influence of Na and Co.

We have investigated the influence of metal adsorbates (sodium and cobalt) on the occupied and unoccupied electronic structure of MoS2(0 0 0 1) and WSe2(0 0 0 1), through a combination of both photoemission and inverse photoemission. The electronic structure is rigidly shifted in both the WSe2 and MoS2 systems, with either Na or Co adsorption, generally as predicted by accompanying density functional theory based calculations. Na adsorption is found to behave as an electron donor (n-type) in MoS2, while Co adsorption acts as an electron acceptor (p-type) in WSe2. The n-type transition metal dichalcogenide (MoS2) is easily doped more n-type with Na deposition while the p-type transition metal dichalcogenide (WSe2) is easily doped more p-type with Co deposition. The binding energy shifts have some correlation with the work function differences between the metallic adlayer and the transition metal dichalcogenide substrate.