High-resolution Angle-resolved Photoemission Spectroscopy Research Articles

Orbital-dependent electron correlation in double-layer nickelate La3Ni2O7

The latest discovery of high temperature superconductivity near 80 K in La3Ni2O7 under high pressure has attracted much attention. Many proposals are put forth to understand the origin of superconductivity. The determination of electronic structures is a prerequisite to establish theories to understand superconductivity in nickelates but is still lacking. Here we report our direct measurement of the electronic structures of La3Ni2O7 by high-resolution angle-resolved photoemission spectroscopy. The Fermi surface and band structures of La3Ni2O7 are observed and compared with the band structure calculations. Strong electron correlations are revealed which are orbital- and momentum-dependent. A flat band is formed from the Ni-3dz2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}_{{z}^{2}}$$\\end{document} orbitals around the zone corner which is ~ 50 meV below the Fermi level and exhibits the strongest electron correlation. In many theoretical proposals, this band is expected to play the dominant role in generating superconductivity in La3Ni2O7. Our observations provide key experimental information to understand the electronic structure and origin of high temperature superconductivity in La3Ni2O7.

Electronic structure and effective mass of pristine and Cl-doped CsPbBr3

Organic–inorganic lead halide perovskites (LHPs) have attracted great interest owing to their outstanding optoelectronic properties. Typically, the underlying electronic structure would determinate the physical properties of materials. But as for now, limited studies have been done to reveal the underlying electronic structure of this material system, comparing to the huge amount of investigations on the material synthesis. The effective mass of the valance band is one of the most important physical parameters which plays a dominant role in charge transport and photovoltaic phenomena. In pristine CsPbBr3, the Fröhlich polarons associated with the Pb–Br stretching modes are proposed to be responsible for the effective mass renormalization. In this regard, it would be very interesting to explore the electronic structure in doped LHPs. Here, we report high-resolution angle-resolved photoemission spectroscopy (ARPES) studies on both pristine and Cl-doped CsPbBr3. The experimental band dispersions are extracted from ARPES spectra along both Γ¯ – M¯ – Γ¯ and X¯ – M¯ – X¯ high symmetry directions. DFT calculations are performed and directly compared with the ARPES data. Our results have revealed the band structure of Cl-doped CsPbBr3 for the first time, which have also unveiled the effective mass renormalization in the Cl-doped CsPbBr3 compound. Doping dependent measurements indicate that the chlorine doping could moderately tune the renormalization strength. These results will help understand the physical properties of LHPs as a function of doping.

Unveiling diverse coordination-defined electronic structures of reconstructed anatase TiO2(001)-(1 × 4) surface

Transition metal oxides (TMOs) exhibit fascinating physicochemical properties, which originate from the diverse coordination structures between the transition metal and oxygen atoms. Accurate determination of such structure-property relationships of TMOs requires to correlate structural and electronic properties by capturing the global parameters with high resolution in energy, real, and momentum spaces, but it is still challenging. Herein, we report the determination of characteristic electronic structures from diverse coordination environments on the prototypical anatase-TiO2(001) with (1 × 4) reconstruction, using high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/atomic force microscopy, in combination with density functional theory calculation. We unveil that the shifted positions of O 2s and 2p levels and the gap-state Ti 3p levels can sensitively characterize the O and Ti coordination environments in the (1 × 4) reconstructed surface, which show distinguishable features from those in bulk. Our findings provide a paradigm to interrogate the intricate reconstruction-relevant properties in many other TMO surfaces.

Phonon promoted charge density wave in topological kagome metal ScV6Sn6

Charge density wave (CDW) orders in vanadium-based kagome metals have recently received tremendous attention, yet their origin remains a topic of debate. The discovery of ScV6Sn6, a bilayer kagome metal featuring an intriguing 3×3×3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{3}\ imes\\sqrt{3}\ imes3$$\\end{document} CDW order, offers a novel platform to explore the underlying mechanism behind the unconventional CDW. Here, we combine high-resolution angle-resolved photoemission spectroscopy, Raman scattering and density functional theory to investigate the electronic structure and phonon modes of ScV6Sn6. We identify topologically nontrivial surface states and multiple van Hove singularities (VHSs) in the vicinity of the Fermi level, with one VHS aligning with the in-plane component of the CDW vector near the K¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\bar{K}$$\\end{document} point. Additionally, Raman measurements indicate a strong electron-phonon coupling, as evidenced by a two-phonon mode and new emergent modes. Our findings highlight the fundamental role of lattice degrees of freedom in promoting the CDW in ScV6Sn6.

Anomalies in the Dirac bands in the proximity of correlated electrons.

Dirac fermions, particles with zero rest mass, are observed in topological materials and are believed to play a key role in the exotic phenomena in fundamental science and the advancement of quantum technology. Most of the topological systems studied so far are weakly correlated systems and the study of their properties in the presence of electron correlation is an interesting emerging area of research, where the electron correlation is expected to enhance the effective mass of the particles. Here, we studied the properties of Dirac bands in a non-symmorphic layered Kondo lattice system, CeAgSb2, employing high-resolution angle-resolved photoemission spectroscopy and first-principles calculations. In addition to the Dirac cones due to non-symmorphic symmetry, this material hosts Dirac fermions in the squarenet layer in the proximity of a strongly correlated Ce layer exhibiting Kondo behavior. Experimental results reveal crossings of the highly dispersive linear bands at the Brillouin zone boundary due to non-symmorphic symmetry. In addition, there are anisotropic Dirac cones constituted by the squarenet Sb 5p states forming a diamond-shaped nodal line. These Dirac bands are linear in a wide energy range with an unusually high slope. Interestingly, near the local Ce 4f bands, these bands exhibit a change in the slope akin to the formation of a 'kink' observed in other materials due to electron-phonon coupling. The emergence of such exotic properties in proximity to strongly correlated electronic states has significant implications in the study of complex quantum materials including unconventional superconductors.

Topological electronic structure and spin texture of quasi-one-dimensional higher-order topological insulator Bi4Br4

The notion of topological insulators (TIs), characterized by an insulating bulk and conducting topological surface states, can be extended to higher-order topological insulators (HOTIs) hosting gapless modes localized at the boundaries of two or more dimensions lower than the insulating bulk. In this work, by performing high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements with submicron spatial and spin resolution, we systematically investigate the electronic structure and spin texture of quasi-one-dimensional (1D) HOTI candidate Bi4Br4. In contrast to the bulk-state-dominant spectra on the (001) surface, we observe gapped surface states on the (100) surface, whose dispersion and spin-polarization agree well with our ab-initio calculations. Moreover, we reveal in-gap states connecting the surface valence and conduction bands, which is a signature of the hinge states inside the (100) surface gap. Our findings provide compelling evidence for the HOTI phase of Bi4Br4. The identification of the higher-order topological phase promises applications based on 1D spin-momentum locked current in electronic and spintronic devices.

Electronic structure in a transition metal dipnictide TaAs2

The family of transition-metal dipnictides has been of theoretical and experimental interest because this family hosts topological states and extremely large magnetoresistance (MR). Recently, , a member of this family, has been predicted to support a topological crystalline insulating state. Here, by using high-resolution angle-resolved photoemission spectroscopy (ARPES), we reveal both closed and open pockets in the metallic Fermi surface (FS) and linearly dispersive bands on the () surface, along with the presence of extreme MR observed from magneto-transport measurements. A comparison of the ARPES results with first-principles computations shows that the linearly dispersive bands on the measured surface of are trivial bulk bands. The absence of symmetry-protected surface state on the () surface indicates its topologically dark nature. The presence of open FS features suggests that the open-orbit fermiology could contribute to the extremely large MR of .

Observation of momentum-dependent charge density wave gap in a layered antiferromagnet {textrm{Gd}}{textrm{Te}}_{3}

Charge density wave (CDW) ordering has been an important topic of study for a long time owing to its connection with other exotic phases such as superconductivity and magnetism. The R{textrm{Te}}_{3} (R = rare-earth elements) family of materials provides a fertile ground to study the dynamics of CDW in van der Waals layered materials, and the presence of magnetism in these materials allows to explore the interplay among CDW and long range magnetic ordering. Here, we have carried out a high-resolution angle-resolved photoemission spectroscopy (ARPES) study of a CDW material {textrm{Gd}}{textrm{Te}}_{3}, which is antiferromagnetic below sim mathrm {12~K}, along with thermodynamic, electrical transport, magnetic, and Raman measurements. Our ARPES data show a two-fold symmetric Fermi surface with both gapped and ungapped regions indicative of the partial nesting. The gap is momentum dependent, maximum along {overline{Gamma }}-mathrm{overline{Z}} and gradually decreases going towards {overline{Gamma }}-mathrm{overline{X}}. Our study provides a platform to study the dynamics of CDW and its interaction with other physical orders in two- and three-dimensions.

Potential Lifshitz transition at optimal substitution in nematic pnictide Ba1-xSrxNi2As2.

BaNi2As2 is a structural analog of the pnictide superconductor BaFe2As2, which, like the iron-based superconductors, hosts a variety of ordered phases including charge density waves (CDWs), electronic nematicity, and superconductivity. Upon isovalent Sr substitution on the Ba site, the charge and nematic orders are suppressed, followed by a sixfold enhancement of the superconducting transition temperature (Tc). To understand the mechanisms responsible for enhancement of Tc, we present high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements of the Ba1-xSrxNi2As2 series, which agree well with our density functional theory (DFT) calculations throughout the substitution range. Analysis of our ARPES-validated DFT results indicates a Lifshitz transition and reasonably nested electron and hole Fermi pockets near optimal substitution where Tc is maximum. These nested pockets host Ni dxz/dyz orbital compositions, which we associate with the enhancement of nematic fluctuations, revealing unexpected connections to the iron-pnictide superconductors. This gives credence to a scenario in which nematic fluctuations drive an enhanced Tc.

Quasi van der Waals Epitaxy of Rhombohedral-Stacked Bilayer WSe2 on GaP(111) Heterostructure.

The growth of bilayers of two-dimensional (2D) materials on conventional 3D semiconductors results in 2D/3D hybrid heterostructures, which can provide additional advantages over more established 3D semiconductors while retaining some specificities of 2D materials. Understanding and exploiting these phenomena hinge on knowing the electronic properties and the hybridization of these structures. Here, we demonstrate that a rhombohedral-stacked bilayer (AB stacking) can be obtained by molecular beam epitaxy growth of tungsten diselenide (WSe2) on a gallium phosphide (GaP) substrate. We confirm the presence of 3R-stacking of the WSe2 bilayer structure using scanning transmission electron microscopy (STEM) and micro-Raman spectroscopy. Also, we report high-resolution angle-resolved photoemission spectroscopy (ARPES) on our rhombohedral-stacked WSe2 bilayer grown on a GaP(111)B substrate. Our ARPES measurements confirm the expected valence band structure of WSe2 with the band maximum located at the Γ point of the Brillouin zone. The epitaxial growth of WSe2/GaP(111)B helps to understand the fundamental properties of these 2D/3D heterostructures, toward their implementation in future devices.

Orbital-selective charge-density wave in TaTe4

TaTe4, a metallic charge-density wave (CDW) material discovered decades ago, has attracted renewed attention due to its rich interesting properties, such as pressure-induced superconductivity and candidate nontrivial topological phase. Here, using high-resolution angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic structure of TaTe4. At 26 K, we observe a CDW gap as large as 290 meV, which persists up to 500 K. The CDW-modulated band structure shows a complex reconstruction that closely correlates with the lattice distortion. Inside the CDW gap, there exist highly dispersive energy bands contributing to the remnant Fermi surface and metallic behavior in the CDW state. Interestingly, our ab initio calculation reveals that the large CDW gap mainly opens in the electronic states with out-of-plane orbital components, while the in-gap metallic states originate from in-plane orbitals, suggesting an orbital texture that couples with the CDW order. Our results shed light on the interplay between electron, lattice, and orbital in quasi-one-dimensional CDW materials.

Flat bands, non-trivial band topology and rotation symmetry breaking in layered kagome-lattice RbTi3Bi5

A representative class of kagome materials, AV3Sb5 (A = K, Rb, Cs), hosts several unconventional phases such as superconductivity, {{mathbb{Z}}}_{2} non-trivial topological states, and electronic nematic states. These can often coexist with intertwined charge-density wave states. Recently, the discovery of the isostructural titanium-based single-crystals, ATi3Bi5 (A = K, Rb, Cs), which exhibit similar multiple exotic states but without the concomitant charge-density wave, has opened an opportunity to disentangle these complex states in kagome lattices. Here, we combine high-resolution angle-resolved photoemission spectroscopy and first-principles calculations to investigate the low-lying electronic structure of RbTi3Bi5. We demonstrate the coexistence of flat bands and several non-trivial states, including type-II Dirac nodal lines and {{mathbb{Z}}}_{2} non-trivial topological surface states. Our findings also provide evidence for rotational symmetry breaking in RbTi3Bi5, suggesting a directionality to the electronic structure and the possible emergence of pure electronic nematicity in this family of kagome compounds.

Doping evolution of superconductivity, charge order, and band topology in hole-doped topological kagome superconductors Cs(V1−xTix)3Sb5

The newly discovered kagome superconductors $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ ($A=\mathrm{K}$, Rb, Cs) exhibit superconductivity, charge order, and band topology simultaneously. To explore the intricate interplay between the superconducting and charge-density-wave (CDW) orders, we investigate the doping evolution of underlying electronic structures in doped topological kagome superconductors $\mathrm{Cs}{({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x})}_{3}{\mathrm{Sb}}_{5}$ where the Ti dopant introduces hole-like charge carriers. Despite the absence of the CDW phase transition in doped compounds even for the lowest doping level of $x=0.047$, the superconductivity survives in all doped samples with enhanced critical temperatures. The high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements reveal that the Ti dopant in the kagome plane lowers the chemical potential, pushing the van Hove singularity (VHS) at $M$ point above the Fermi level. First-principle simulations corroborate the doping evolution of the band structure observed in ARPES, and affirm that the CDW instability does not occur once the VHS moves above the Fermi level, explaining the absence of the CDW ordering in our doped samples $\mathrm{Cs}{({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x})}_{3}{\mathrm{Sb}}_{5}$. Our results also demonstrate a competition between the CDW and superconducting orders in the kagome-metal superconductor ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$, although the superconductivity is likely inconsequential of the CDW order.

Evidence for Band Renormalizations in Strong-Coupling Superconducting Alkali-Fulleride Films.

There has been a long-standing debate about the mechanism of the unusual superconductivity in alkali-intercalated fullerides. In this Letter, using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structures of superconducting K_{3}C_{60} thin films. We observe a dispersive energy band crossing the Fermi level with the occupied bandwidth of about 130meV. The measured band structure shows prominent quasiparticle kinks and a replica band involving the Jahn-Teller active phonon modes, which reflects strong electron-phonon coupling in the system. The electron-phonon coupling constant is estimated to be about 1.2, which dominates the quasiparticle mass renormalization. Moreover, we observe an isotropic nodeless superconducting gap beyond the mean-field estimation (2Δ/k_{B}T_{c}≈5). Both the large electron-phonon coupling constant and large reduced superconducting gap suggest a strong-coupling superconductivity in K_{3}C_{60}, while the electronic correlation effect is suggested by the observation of a waterfall-like band dispersion and the small bandwidth compared with the effective Coulomb interaction. Our results not only directly visualize the crucial band structure but also provide important insights into the mechanism of the unusual superconductivity of fulleride compounds.

Electronic signatures of successive itinerant, antiferromagnetic transitions in hexagonal La2Ni7

We use high-resolution angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to study the electronic and magnetic properties of La2Ni7, an itinerant magnetic system with a series of three magnetic transition temperatures upon cooling, which end in a weak antiferromagnetic ground state. Our APRES data reveal several electron and hole pockets that have hexagonal symmetry near the Γ point. We observe significant reconstruction of the band structure upon successive magnetic transitions at T 1 ∼ 61 K, T 2 ∼ 57 K and T 3 ∼ 42 K. Several features observed in ARPES data were reasonably well reproduced by DFT calculations, while others were not. In particular, the flat band near E F predicted by DFT in antiferromagnet (AFM) state, was seemingly absent in ARPES data. Our results detail the effects of magnetic ordering on the electronic structure in a Ni-based weak AFM and highlight challenges of current computational methods.

Three-dimensional angle-resolved photoemission study of bulk SiGe single crystals

The composition- and structure-dependent electronic band structure of SiGe alloys facilitates a rich variety of its application in various fields. In this study, we performed high-resolution core-level and angle-resolved photoemission spectroscopy (ARPES) of unstrained SiGe single crystals with the (001) surface. Thermally cleaned surfaces of the single crystals of SiGe alloys exhibited double domain (2 × 1) reconstruction, wherein the asymmetric dimer was composed of Ge atoms, and Si atoms were located below the subsurface region. The compositional dependence of three valence bands and their constant energy contours were clearly resolved by three-dimensional ARPES measurements using the high-intensity synchrotron radiation. The valence bands of unstrained SiGe alloys could be well described by the composition-based interpolation of the band parameters of pristine Si and Ge crystals.

Revealing the Polaron State at the MoS2/TiO2 Interface

Interfacial polarons determine the distribution of free charges at the interface and thus play important roles in manipulating the physicochemical properties of hybridized polaronic materials. In this work, we investigated the electronic structures at the atomically flat interface of the single-layer MoS2 (SL-MoS2) on the rutile TiO2 surface using high-resolution angle-resolved photoemission spectroscopy. Our experiments directly visualized both the valence band maximum and the conduction band minimum (CBM) of SL-MoS2 at the K point, which clearly defines a direct bandgap of ∼2.0 eV. Detailed analyses corroborated by density functional theory calculations demonstrated that the CBM of MoS2 is formed by the trapped electrons at the MoS2/TiO2 interface that couple with the longitudinal optical phonons in the TiO2 substrate through an interfacial Fröhlich polaron state. Such an interfacial coupling effect may register a new route for tuning the free charges in the hybridized systems of two-dimensional materials and functional metal oxides.

Flat band in hole-doped transition metal dichalcogenide observed by angle-resolved photoemission spectroscopy

Layered transition metal dichalcogenides (TMDCs) gained widespread attention because of their electron-correlation-related physics, such as charge density wave (CDW), superconductivity, etc. In this paper, we report the high-resolution angle-resolved photoemission spectroscopy (ARPES) studies on the electronic structure of Ti-doped 1T-Ti x Ta1–x S2 with different doping levels. We observe a flat band that originates from the formation of the star of David super-cell at the x = 5% sample at the low temperature. With the increasing Ti doping levels, the flat band vanishes in the x = 8% sample due to the extra hole carrier. We also find the band shift and variation of the CDW gap caused by the Ti-doping. Meanwhile, the band folding positions and the CDW vector q CDW are intact. Our ARPES results suggest that the localized flat band and the correlation effect in the 1T-TMDCs could be tuned by changing the filling factor through the doping electron or hole carriers. The Ti-doped 1T-Ti x Ta1–x S2 provides a platform to fine-tune the electronic structure evolution and a new insight into the strongly correlated physics in the TMDC materials.

Thermal hysteretic behavior and negative magnetoresistance in the charge density wave material EuTe4

EuTe4 is a newly-discovered van der Waals material exhibiting a novel charge-density wave (CDW) with a large thermal hysteresis in the resistivity and CDW gap. In this work, we systematically study the electronic structure and transport properties of EuTe4 using high-resolution angle-resolved photoemission spectroscopy (ARPES), magnetoresistance measurements, and scanning tunneling microscopy (STM). We observe a CDW gap of about 200 meV at low temperatures that persists up to 400 K, suggesting that the CDW transition occurs at a much higher temperature. We observe a large thermal hysteretic behavior of the ARPES intensity near the Fermi level, consistent with the resistivity measurement. The hysteresis in the resistivity measurement does not change under a magnetic field up to 7 T, excluding the thermal magnetic hysteresis mechanism. Instead, the surface topography measured with STM shows surface domains with different CDW trimerization directions, which may be important for the thermal hysteretic behavior of EuTe4. Interestingly, we observe a large negative magnetoresistance at low temperatures that can be associated with the canting of magnetically ordered Eu spins. Our work shed light on the understanding of magnetic, transport, and electronic properties of EuTe4.

Electronic structure study of the charge-density-wave Kondo lattice CeTe3

The behaviors of f electrons are crucial for understanding the rich phase diagrams and ground-state properties of heavy fermion (HF) systems. The complicated interactions between f electrons and conduction electrons largely enrich the basic properties of HF compounds. Here the electronic structure, especially the f-electron character, of the charge-density-wave (CDW) Kondo lattice compound CeTe3 has been studied by high-resolution angle-resolved photoemission spectroscopy. A weakly dispersive quasiparticle band near the Fermi level has been observed directly, indicating hybridization between f electrons and conduction electrons. Temperature-dependent measurements confirm the localized to itinerant transition of f electrons as the temperature decreases. Furthermore, an energy gap formed by one conduction band at low temperature is gradually closed with increasing temperature, which probably originates from the CDW transition at extremely high temperature. Additionally, orbital information of different electrons has also been acquired with different photon energies and polarizations, which indicates the anisotropy and diverse symmetries of the orbitals. Our results may help understand the complicated f-electron behaviors when considering its interaction with other electrons/photons in CeTe3 and other related compounds.