Polarization-dependent Angle-resolved Photoemission Spectroscopy Research Articles

Topology Hierarchy of Transition Metal Dichalcogenides Built from Quantum Spin Hall Layers.

The evolution of the physical properties of two-dimensional material from monolayer limit to the bulk reveals unique consequences from dimension confinement and provides a distinct tuning knob for applications. Monolayer 1T'-phase transition metal dichalcogenides (1T'-TMDs) with ubiquitous quantum spin Hall (QSH) states are ideal two-dimensional building blocks of various three-dimensional topological phases. However, the stacking geometry was previously limited to the bulk 1T'-WTe2 type. Here, we introduce the novel 2M-TMDs consisting of translationally stacked 1T'-monolayers as promising material platforms with tunable inverted bandgaps and interlayer coupling. By performing advanced polarization-dependent angle-resolved photoemission spectroscopy as well as first-principles calculations on the electronic structure of 2M-TMDs, we revealed a topology hierarchy: 2M-WSe2 , MoS2, and MoSe2 are weak topological insulators (WTIs), whereas 2M-WS2 is a strong topological insulator (STI). Further demonstration of topological phase transitions by tunning interlayer distance indicates that band inversion amplitude and interlayer coupling jointly determine different topological states in 2M-TMDs. We propose that 2M-TMDs are parent compounds of various exotic phases including topological superconductors and promise great application potentials in quantum electronics due to their flexibility in patterning with two-dimensional materials. This article is protected by copyright. All rights reserved.

Polarization-dependent electronic structure of Ag quantum well states on the MoS2 (0001) surface using ARPES and DFT studies

The electronic band structure of clean and Ag-induced quantum well states on the ${\mathrm{MoS}}_{2}$(0001) surface has been investigated using polarization-dependent angle-resolved photoemission spectroscopy (ARPES) along with first-principle-based density functional theory (DFT) calculations. Using selective linearly polarized light, the orbital symmetry and $k$ dispersion of the surface localized electronic states have been unambiguously determined. The quantum well states (QWS) originating from the Ag ${p}_{z}$ orbitals are confined within the Ag overlayer, while the quantum well resonance states (QWRS) are hybridized with substrate Mo $4{d}_{{z}^{2}}$ and S $3{p}_{z}$ orbitals. The photon energy dependence of the dispersion characteristics confirms the two-dimensional (2D) nature of these quantum states while their momentum dependence is a consequence of the finite lateral coherence length of the electrons. Our ARPES experiments using incident circularly polarized light on ${\mathrm{MoS}}_{2}$(0001) surface show circular dichroism effects (CD-ARPES), similar to recent studies where it is used to probe the initial state orbital angular momentum or the Berry curvature. While the QWS on the Ag overlayer does not exhibit any intensity asymmetry with the helicity of the incident light, the Ag QWRS show circular dichroism characteristics. The observed circular dichroism from the QWRS is attributed to the hybridization of these quantum states to the substrate states. We compare our ARPES results with the calculated orbital projected band structure to obtain a microscopic insight into the electronic behavior, which is critically important for manipulating wave functions for potential device applications.

Rich nature of Van Hove singularities in Kagome superconductor CsV3Sb5

The recently discovered layered kagome metals AV3Sb5 (A = K, Rb, Cs) exhibit diverse correlated phenomena, which are intertwined with a topological electronic structure with multiple van Hove singularities (VHSs) in the vicinity of the Fermi level. As the VHSs with their large density of states enhance correlation effects, it is of crucial importance to determine their nature and properties. Here, we combine polarization-dependent angle-resolved photoemission spectroscopy with density functional theory to directly reveal the sublattice properties of 3d-orbital VHSs in CsV3Sb5. Four VHSs are identified around the M point and three of them are close to the Fermi level, with two having sublattice-pure and one sublattice-mixed nature. Remarkably, the VHS just below the Fermi level displays an extremely flat dispersion along MK, establishing the experimental discovery of higher-order VHS. The characteristic intensity modulation of Dirac cones around K further demonstrates the sublattice interference embedded in the kagome Fermiology. The crucial insights into the electronic structure, revealed by our work, provide a solid starting point for the understanding of the intriguing correlation phenomena in the kagome metals AV3Sb5.

High-resolution angle-resolved photoemission study of oxygen adsorbed Fe/MgO(001)**Present address: Synchrotron Radiation Research Center, National Institutes for Quantum and Radiological Science and Technology, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan

We have investigated the electronic states of clean Fe(001) and oxygen adsorbed Fe(001)–p(1 × 1)-O films epitaxially grown on MgO(001) substrates by means of polarization-dependent angle-resolved photoemission spectroscopy (ARPES) and extensive density-functional theory (DFT) calculations. The observed Fermi surfaces and band dispersions of pure Fe near the Fermi level were modified upon oxygen adsorption. By the detailed comparison of ARPES and DFT results of the oxygen adsorbed Fe surface, we have clarified the orbital-dependent p–d hybridization in the topmost and second Fe layers. Furthermore, the observed energy levels and Fermi wave numbers for the oxygen adsorbed Fe surface were deviated from the DFT calculations depending on the orbital characters and momentum directions, indicating an anisotropic interplay of the electron correlation and p–d hybridization effects in the surface region.

Direct insight into the band structure of SrNbO3

We present the results of a photon energy and polarization dependent angle-resolved photoemission spectroscopy (ARPES) study on high quality, epitaxial ${\mathrm{SrNbO}}_{3}$ thin films prepared in situ by pulsed laser deposition (PLD). We show that the Fermi surface is composed of three bands mainly due to ${\mathrm{t}}_{2g}$ orbitals of Nb $4d$, in analogy with the $3d$-based perovskite systems. The bulk band dispersion for the conduction and valence states obtained by density functional theory (DFT) is generally consistent with the ARPES data. The small discrepancy in the bandwidth close to the Fermi level seems to result from the interplay of correlation effects and the presence of vacancies. The ARPES results are complemented by soft x-ray photoemission spectroscopy measurements in order to provide indications on the chemical states and the stoichiometry of the material.

Momentum-dependent sign inversion of orbital order in superconducting FeSe

We investigate the electronic reconstruction across the tetragonal-orthorhombic structural transition in FeSe by employing polarization-dependent angle-resolved photoemission spectroscopy (ARPES) on detwinned single crystals. Across the structural transition, the electronic structures around the G and M points are modified from four-fold to two-fold symmetry due to the lifting of degeneracy in dxz/dyz orbitals. The dxz band shifts upward at the G point while it moves downward at the M point, suggesting that the electronic structure of orthorhombic FeSe is characterized by a momentum-dependent sign-changing orbital polarization. The elongated directions of the elliptical Fermi surfaces (FSs) at the G and M points are rotated by 90 degrees with respect to each other, which may be related to the absence of the antiferromagnetic order in FeSe.

Fermi surfaces and orbital polarization in superconductingCeO0.5F0.5BiS2revealed by angle-resolved photoemission spectroscopy

We have investigated the electronic structure of ${\mathrm{BiS}}_{2}$-based ${\mathrm{CeO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_{2}$ superconductor using polarization-dependent angle-resolved photoemission spectroscopy (ARPES), and succeeded in elucidating the orbital characters on the Fermi surfaces. In the rectangular Fermi pockets around the $X$ point, the straight portion parallel to the ${k}_{y}$ direction is dominated by Bi $6{p}_{x}$ character. The orbital polarization indicates the underlying quasi-one-dimensional electronic structure of the ${\mathrm{BiS}}_{2}$ system. Moreover, distortions on tetragonally aligned Bi could give rise to the band Jahn-Teller effect.

Photoemission study of the electronic structure and charge density waves of Na2Ti2Sb2O.

The electronic structure of Na2Ti2Sb2O single crystal is studied by photon energy and polarization dependent angle-resolved photoemission spectroscopy (ARPES). The obtained band structure and Fermi surface agree well with the band structure calculation of Na2Ti2Sb2O in the non-magnetic state, which indicates that there is no magnetic order in Na2Ti2Sb2O and the electronic correlation is weak. Polarization dependent ARPES results suggest the multi-band and multi-orbital nature of Na2Ti2Sb2O. Photon energy dependent ARPES results suggest that the electronic structure of Na2Ti2Sb2O is rather two-dimensional. Moreover, we find a density wave energy gap forms below the transition temperature and reaches 65 meV at 7 K, indicating that Na2Ti2Sb2O is likely a weakly correlated CDW material in the strong electron-phonon interaction regime.

Significant contribution of As4porbitals to the low-lying electronic structure of the 112-type iron-based superconductorCa0.9La0.1FeAs2

We report a systematic polarization-dependent angle-resolved photoemission spectroscopy study of the three-dimensional electronic structure of the recently discovered 112-type iron-based superconductor Ca1-xLaxFeAs2 (x = 0.1). Besides the commonly reported three hole-like and two electron-like bands in iron-based superconductors, we resolve one additional hole-like band around the zone center and one more fast-dispersing band near the X point in the vicinity of Fermi level. By tuning the polarization and the energy of incident photons,we are able to identify the specific orbital characters and the kz dependence of all bands. Combining with band calculations, we find As 4pz and 4px (4py) orbitals contribute significantly to the additional three-dimensional hole-like band and the narrow band, respectively. Also, there are considerable hybridization between the As 4p zand Fe 3d orbitals in the additional hole-like band, which suggests the strong coupling between the unique arsenic zigzag bond layers and the FeAs layers therein. Our findings provide a comprehensive picture of the orbital characters of the low-lying band structure of 112-type iron-based superconductors, which can be a starting point for the further understanding of their unconventional superconductivity.

High-energy anomaly in the angle-resolved photoemission spectra of Nd(2-x)Ce(x)CuO₄: evidence for a matrix element effect.

We use polarization-dependent angle-resolved photoemission spectroscopy (ARPES) to study the high-energy anomaly (HEA) in the dispersion of Nd(2-x)Ce(x)CuO₄, x=0.123. We find that at particular photon energies the anomalous, waterfall-like dispersion gives way to a broad, continuous band. This suggests that the HEA is a matrix element effect: it arises due to a suppression of the intensity of the broadened quasiparticle band in a narrow momentum range. We confirm this interpretation experimentally, by showing that the HEA appears when the matrix element is suppressed deliberately by changing the light polarization. Calculations of the matrix element using atomic wave functions and simulation of the ARPES intensity with one-step model calculations provide further evidence for this scenario. The possibility to detect the full quasiparticle dispersion further allows us to extract the high-energy self-energy function near the center and at the edge of the Brillouin zone.

Symmetry-protected surface state on Mo(1 1 2)

We report the experimental identification of a symmetry-protected surface state on Mo(1 1 2). The utilization of photon-energy- and polarization-dependent angle-resolved photoemission spectroscopy clearly demonstrates that this true surface state preserves its two-dimensional character despite the fact that it resides within the projection of the bulk bands along the line of the surface Brillouin zone. This surface state on Mo(1 1 2) exists due to the forbidden hybridization between the bulk and the surface states, each of which possesses different state symmetries within the crystal. The experimental identification of such a surface state possibly opens up pathways towards controlled manipulation between true surface states and the surface resonances (i.e. between two-dimensional and quasi-two-dimensional states) by breaking the symmetry of the crystal surface.

Layer-By-Layer Entangled Spin-Orbital Texture of the Topological Surface State inBi2Se3

We study Bi(2)Se(3) by polarization-dependent angle-resolved photoemission spectroscopy and density-functional theory slab calculations. We find that the surface state Dirac fermions are characterized by a layer-dependent entangled spin-orbital texture, which becomes apparent through quantum interference effects. This explains the discrepancy between the spin polarization obtained in spin and angle-resolved photoemission spectroscopy-ranging from 20% to 85%-and the 100% value assumed in phenomenological models. It also suggests a way to probe the intrinsic spin texture of topological insulators, and to continuously manipulate the spin polarization of photoelectrons and photocurrents all the way from 0 to ±100% by an appropriate choice of photon energy, linear polarization, and angle of incidence.

The orbital characters of low-energy electronic structure in iron-chalcogenide superconductor K x Fe2−y Se2

The newly discovered iron-chalcogenide superconductor K x Fe2−y Se2 exhibits a distinct electronic structure from other iron-based superconductors. Exploiting polarization-dependent angle-resolved photoemission spectroscopy, we have determined the orbital characters of band structure in a K x Fe2−y Se2 superconductor. To a large extent, we find that K x Fe2−y Se2 superconductor shares similar orbital characters with other iron-based superconductors, but with its own characteristics. For example, we have resolved two highly degenerate electron cylinders around the zone corner in the s and p geometries, respectively, indicating negligible interactions between them. Moreover, in contrast to the band calculation results, the small electron pocket around Z is found to be mainly consisted of the d Z orbital. The determined orbital characters would help to construct a realistic model for K x Fe2−y Se2.

Three-dimensional electronic structure and interband nesting in the stoichiometric superconductor LiFeAs

We report the three-dimensional electronic structure of iron pnictide superconductor LiFeAs obtained by polarization-dependent angle-resolved photoemission spectroscopy. The obtained orbital characters of each Fermi surface (FS) as well as the band dispersions were qualitatively consistent with those derived from local density approximation band calculations. It was found that FS nesting appears between a two-dimensional hole band at the zone center and an electron band at the zone corner with the same ${d}_{xy}$ orbital character. A shadow band attributed to $(\ensuremath{\pi},\ensuremath{\pi},\ensuremath{\pi})$ band folding was also observed without the spin-density-wave transition. This result suggests that FS nesting between bands with the same orbital ${d}_{xy}$ character due to spin fluctuation plays an important role in LiFeAs.

Symmetry breaking via orbital-dependent reconstruction of electronic structure in detwinned NaFeAs

The superconductivity discovered in iron pnictides is intimately related to a nematic ground state, where the ${C}_{4}$ rotational symmetry is broken via the structural and magnetic transitions. We here study the nematicity in NaFeAs with polarization-dependent angle-resolved photoemission spectroscopy. A uniaxial pressure was applied on the sample to overcome the twinning effect in the low temperature ${C}_{2}$-symmetric state and obtain a much simpler electronic structure than that of a twinned sample. We found the electronic structure undergoes an orbital-dependent reconstruction in the nematic state, primarily involving the ${d}_{xy}$- and ${d}_{yz}$-dominated bands. These bands strongly hybridize with each other, inducing a band splitting, while the ${d}_{xz}$-dominated bands only exhibit an energy shift without any reconstruction. These findings suggest that the spin fluctuations at high temperatures and their coupling with the orbital degree of freedom could be the dominant force to drive the nematicity, while the ferro-orbital ordering between ${d}_{xz}$ and ${d}_{yz}$ orbitals can only play a minor role here.

Direct measurement of quantum phases in graphene via photoemission spectroscopy

Quantum phases provide us with important information for understanding the fundamental properties of a system. However, the observation of quantum phases, such as Berry's phase and the sign of the matrix element of the Hamiltonian between two nonequivalent localized orbitals in a tight-binding formalism, has been challenged by the presence of other factors, e.g. , dynamic phases and spin or valley degeneracy, and the absence of methodology. Here, we report a way to directly access these quantum phases, through polarization-dependent angle-resolved photoemission spectroscopy (ARPES), using graphene as a prototypical two-dimensional material. We show that the momentum- and polarization-dependent spectral intensity provides direct measurements of (i) the phase of the band wavefunction and (ii) the sign of matrix elements for nonequivalent orbitals. Upon rotating light polarization by $\ensuremath{\pi}/2$, we found that graphene with a Berry's phase of $n\ensuremath{\pi}$ ($n=1$ for single- and $n=2$ for double-layer graphene for Bloch wavefunction in the commonly used form) exhibits the rotation of ARPES intensity by $\ensuremath{\pi}/n$, and that ARPES signals reveal the signs of the matrix elements in both single- and double-layer graphene. The method provides a technique to directly extract fundamental quantum electronic information on a variety of materials.

The orbital characters and kz dispersions of bands in iron-pnictide NaFeAs

The electronic structure and orbital characters of iron-pnictide NaFeAs have been studied by polarization dependent angle-resolved photoemission spectroscopy. Some of the bands are mixed with the orbitals of opposite symmetries, which could be interpreted by the hybridization among the bands. According to the photon energy dependent experiment, the kz dispersions of the bands that cross the Fermi energy are weak in both paramagnetic and spin density wave states. However, a band well below the Fermi level shows a kz dispersion of 41meV, which mainly contains the dz2 orbital.

Orbital-Dependent Modifications of Electronic Structure across the Magnetostructural Transition inBaFe2As2

Laser angle-resolved photoemission spectroscopy (ARPES) is employed to investigate the temperature (T) dependence of the electronic structure in BaFe2As2 across the magnetostructural transition at T{N} approximately 140 K. A drastic transformation in Fermi surface (FS) shape across T{N} is observed, as expected by first-principles band calculations. Polarization-dependent ARPES and band calculations consistently indicate that the observed FSs at k{z} approximately pi in the low-T antiferromagnetic state are dominated by the Fe3d{zx} orbital, leading to the twofold electronic structure. These results indicate that magnetostructural transition in BaFe2As2 accompanies orbital-dependent modifications in the electronic structure.

A versatile system for ultrahigh resolution, low temperature, and polarization dependent Laser-angle-resolved photoemission spectroscopy

We have developed a low temperature ultrahigh resolution system for polarization dependent angle-resolved photoemission spectroscopy (ARPES) using a vacuum ultraviolet (vuv) laser (hnu=6.994 eV) as a photon source. With the aim of addressing low energy physics, we show the system performance with angle-integrated PES at the highest energy resolution of 360 mueV and the lowest temperature of 2.9 K. We describe the importance of a multiple-thermal-shield design for achieving the low temperature, which allows a clear measurement of the superconducting gap of tantalum metal with a T(c)=4.5 K. The unique specifications and quality of the laser source (narrow linewidth of 260 mueV, high photon flux), combined with a half-wave plate, facilitates ultrahigh energy and momentum resolution polarization dependent ARPES. We demonstrate the use of s- and p-polarized laser-ARPESs in studying the superconducting gap on bilayer-split bands of a high T(c) cuprate. The unique features of the quasi-continuous-wave vuv laser and low temperature enables ultrahigh-energy and -momentum resolution studies of the spectral function of a solid with large escape depth. We hope the present work helps in defining polarization dependent laser excited angle-resolved photoemission spectroscopy as a frontier tool for the study of electronic structure and properties of materials at the sub-meV energy scale.

Dispersion of a Hole in a Two-DimensionalCu3O4Plane: A Tale of Two Singlets

Polarization-dependent angle-resolved photoemission spectroscopy measurements of Ba{sub 2}Cu {sub 3}O{sub 4}Cl{sub 2} indicate the presence of two different Zhang-Rice singlets in the two-dimensional Cu{sub 3}O{sub 4} plane of this insulating copper oxychloride. With the aid of model calculations, we show that one singlet is moving in the antiferromagnetically ordered cupratelike Cu{sub A}O {sub 2} subsystem and the other on the effectively paramagnetic sublattice formed by the extra Cu{sub B} atoms. The dispersion of the former is thus determined by the superexchange integral J{sub A} , and that of the latter by the hopping integral t{sub B} . Consequently, as far as the role of the magnetic background is concerned, the dispersion relation of a single hole in this system represents simultaneously both the low and very high doping limits of a cuprate plane in a single structure. {copyright} {ital 1997} {ital The American Physical Society}