Charge-Density-Wave-Induced Peak-Dip-Hump Structure and the Multiband Superconductivity in a Kagome Superconductor CsV_{3}Sb_{5}.

The entanglement of charge density wave (CDW), superconductivity, and topologically nontrivial electronic structure has recently been discovered in the kagome metal AV_{3}Sb_{5} (A=K, Rb, Cs) family. With high-resolution angle-resolved photoemission spectroscopy, we study the electronic properties of CDW and superconductivity in CsV_{3}Sb_{5}. The spectra around K[over ¯] is found to exhibit a peak-dip-hump structure associated with two separate branches of dispersion, demonstrating the isotropic CDW gap opening below E_{F}. The peak-dip-hump line shape is contributed by linearly dispersive Dirac bands in the lower branch and a dispersionless flat band close to E_{F} in the upper branch. The electronic instability via Fermi surface nesting could play a role in determining these CDW-related features. The superconducting gap of ∼0.4 meV is observed on both the electron band around Γ[over ¯] and the flat band around K[over ¯], implying the multiband superconductivity. The finite density of states at E_{F} in the CDW phase is most likely in favor of the emergence of multiband superconductivity, particularly the enhanced density of states associated with the flat band. Our results not only shed light on the controversial origin of the CDW, but also offer insights into the relationship between CDW and superconductivity.

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.

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.

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.

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.

Monolayer PtSe2, a New Semiconducting Transition-Metal-Dichalcogenide, Epitaxially Grown by Direct Selenization of Pt.

Single-layer transition-metal dichalcogenides (TMDs) receive significant attention due to their intriguing physical properties for both fundamental research and potential applications in electronics, optoelectronics, spintronics, catalysis, and so on. Here, we demonstrate the epitaxial growth of high-quality single-crystal, monolayer platinum diselenide (PtSe2), a new member of the layered TMDs family, by a single step of direct selenization of a Pt(111) substrate. A combination of atomic-resolution experimental characterizations and first-principle theoretic calculations reveals the atomic structure of the monolayer PtSe2/Pt(111). Angle-resolved photoemission spectroscopy measurements confirm for the first time the semiconducting electronic structure of monolayer PtSe2 (in contrast to its semimetallic bulk counterpart). The photocatalytic activity of monolayer PtSe2 film is evaluated by a methylene-blue photodegradation experiment, demonstrating its practical application as a promising photocatalyst. Moreover, circular polarization calculations predict that monolayer PtSe2 has also potential applications in valleytronics.

Pressure-induced transitions inRCo5(R=Y, La) studied by x-ray emission spectroscopy, x-ray diffraction and density functional theory.

The hydrostatic pressure dependent evolution of the electronic and magnetic structure of LaCo5and YCo5was investigated by means of x-ray emission spectroscopy, x-ray diffraction, and spin-polarized density functional theory (DFT) calculations. Using experimental lattice parameters the DFT correctly predicts the pressure of the magnetic transition in both compounds to be 26 GPa (La) and 22-23 GPa (Y). The transition was experimentally resolved in the changes of the electronic structure via the integrated absolute difference of the CoKßemission spectra. Comparison of theory and experiment confirm for the first time a common feature in both LaCo5and YCo5to be the source of the transition; the Fermi-level crossing of an up-spin polarized flat band driving the systems into a low spin configuration via a Lifshitz type transition of the Fermi surface. Another phase transition observed around 12 GPa in LaCo5was clarified to be caused by the change in the down-spin density of states at the Fermi level.

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.

Anomalous Hall effect in Cu doped Bi2Te3 topological insulator.

The angle resolved photo-emission spectroscopy (ARPES) study and magneto-transport properties of Bi2Cu x Te3-x have been investigated. ARPES study indicates the clear existence of surface states in the as-prepared samples. The estimated bandgap from ARPES is found to be ∼5 meV and 16 meV respectively for x = 0.03 and x = 0.15 samples. Presence of larger Cu concentration (x = 0.15) introduces magnetic ordering. Observed non-linearity in the Hall data is due to the existence of anomalous Hall effect which can be attributed to the 2D transport. The observed magneto-transport features might be related to the surface carriers which is confirmed by ARPES study.

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.

Investigation of multi-mode spin-phonon coupling and local B-site disorder in Pr2CoFeO6 by Raman spectroscopy and correlation with its electronic structure by XPS and XAS studies.

Electronic structure of Pr2CoFeO6 (at 300 K) was investigated by x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy techniques. All three cations, i.e. Pr, Co and Fe were found to be trivalent in nature. XPS valance band analysis suggested the system to be insulating in nature. The analysis suggested that Co3+ ions exist in low spin state in the system. Moreover, Raman spectroscopy study indicated the random distribution of the B-site ions (Co/Fe) triggered by same charge states. In temperature-dependent Raman study, the relative heights of the two observed phonon modes exhibited anomalous behaviour near magnetic transition temperature T N ~ 270 K, thus indicating towards interplay between spin and phonon degrees of freedom in the system. Furthermore, clear anomalous softening was observed below T N which confirmed the existence of strong spin-phonon coupling occurring for at least two phonon modes of the system. The line width analysis of the phonon modes essentially ruled out the role of magnetostriction effect in the observed phonon anomaly. The investigation of the lattice parameter variation across T N (obtained from the temperature-dependent neutron diffraction measurements) further confirmed the existence of the spin-phonon coupling.

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.

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.

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.

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.

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.

The symmetry-resolved electronic structure of 2H-WSe2(0 0 0 1).

The orbital symmetry of the band structure of 2H-WSe2(0 0 0 1) has been investigated by means of angle-resolved photoelectron spectroscopy (ARPES) and density functional theory (DFT). The WSe2(0 0 0 1) experimental band structure is found, by ARPES, to be significantly different for states of even and odd reflection parities along both the [Formula: see text]-[Formula: see text] and [Formula: see text]-[Formula: see text] lines, in good agreement with results obtained from DFT. The light polarization dependence of the photoemission intensities from the top of the valence band for bulk WSe2(0 0 0 1) is explained by the dominance of W 5[Formula: see text] states around the [Formula: see text]-point and W 5d xy states around the [Formula: see text]-point, thus dominated, respectively, by states of even and odd symmetry, with respect to the [Formula: see text]-[Formula: see text] line. The splitting of the topmost valence band at [Formula: see text], due to spin-orbit coupling, is measured to be 0.49 ± 0.01 eV, in agreement with the 0.48 eV value from DFT, and prior measurements for the bulk single crystal WSe2(0 0 0 1), albeit slightly smaller than the 0.513 ± 0.01 eV observed for monolayer WSe2.

Symmetry-resolved surface-derived electronic structure of MoS2(0 0 0 1).

We find a wave vector dependence of the band symmetries for MoS(2)(0 0 0 1) in angle-resolved photoemission. The band structures are found to be significantly different for states of even and odd reflection parities, despite the absence of true mirror plane symmetry away from Γ, the Brillouin zone center, along the line to the K point, at the Brillouin zone edge. Our measurements agree with density functional theory (DFT) calculations for each band symmetry, with the notable exception of the Mo 4d(x(2)-y(2)) contributions to the valence band structure of MoS(2)(0 0 0 1). The band structure is indicative of strong S 3p and Mo 4d hybridization. In particular, the top of the valence band is predominantly composed of Mo 4d(3z(2)-r(2)) derived states near Γ, whereas near K Mo 4d(x(2)-y(2)) as well as Mo 4d(xy) dominate. In contrast, the bottom of the valence band is dominated by Mo 5s and S 3p(z) contributions.