Angle-resolved Photoemission Experiments Research Articles

Tunable Van Hove Singularity without Structural Instability in Kagome Metal CsTi_{3}Bi_{5}.

In kagome metal CsV_{3}Sb_{5}, multiple intertwined orders are accompanied by both electronic and structural instabilities. These exotic orders have attracted much recent attention, but their origins remain elusive. The newly discovered CsTi_{3}Bi_{5} is a Ti-based kagome metal to parallel CsV_{3}Sb_{5}. Here, we report angle-resolved photoemission experiments and first-principles calculations on pristine and Cs-doped CsTi_{3}Bi_{5} samples. Our results reveal that the van Hove singularity (vHS) in CsTi_{3}Bi_{5} can be tuned in a large energy range without structural instability, different from that in CsV_{3}Sb_{5}. As such, CsTi_{3}Bi_{5} provides a complementary platform to disentangle and investigate the electronic instability with a tunable vHS in kagome metals.

Traces of electron-phonon coupling in one-dimensional cuprates

The appearance of certain spectral features in one-dimensional (1D) cuprate materials has been attributed to a strong, extended attractive coupling between electrons. Here, using time-dependent density matrix renormalization group methods on a Hubbard-extended Holstein model, we show that extended electron-phonon (e–ph) coupling presents an obvious choice to produce such an attractive interaction that reproduces the observed spectral features and doping dependence seen in angle-resolved photoemission experiments: diminished 3kF spectral weight, prominent spectral intensity of a holon-folding branch, and the correct holon band width. While extended e–ph coupling does not qualitatively alter the ground state of the 1D system compared to the Hubbard model, it quantitatively enhances the long-range superconducting correlations and suppresses spin correlations. Such an extended e–ph interaction may be an important missing ingredient in describing the physics of the structurally similar two-dimensional high-temperature superconducting layered cuprates, which may tip the balance between intertwined orders in favor of uniform d-wave superconductivity.

Observation of highly anisotropic bulk dispersion and spin-polarized topological surface states in CoTe2

We present CoTe2 as a new type-II Dirac semimetal supporting Lorentz symmetry violating Dirac fermions in the vicinity of the Fermi energy. By combining first principle ab-initio calculations with experimental angle-resolved photo-emission spectroscopy results, we show the CoTe2 hosts a pair of type-II Dirac fermions around 90 meV above the Fermi energy. In addition to the bulk Dirac fermions, we find several topological band inversions in bulk CoTe2, which gives rise to a ladder of spin-polarized surface states over a wide range of energies. In contrast to the surface states which typically display Rashba-type in-plane spin splitting, we find that CoTe2 hosts novel out-of-plane spin polarization as well. Our work establishes CoTe2 as a potential candidate for the exploration of Dirac fermiology and applications in spintronic devices, infrared plasmonics, and ultrafast optoelectronics.

Charge order and emergent symmetries in cuprate superconductors

In this paper, we present our studies of the phase diagram of the cuprate superconductors performed in recent years. We describe how a few field-theoretical concepts can be used to account for the puzzling properties of these compounds. Starting with a short exposition of recent experimental developments, we then introduce the concept of an emergent SU(2) symmetry, which rotates between the superconducting and charge order parameters. We describe a solvable model for which this symmetry turns out to be exact and derive the corresponding effective field theory in terms of a non-linear σ model. We then turn to the experimental consequences of our model with the presence of skyrmions inside the vortex core and an elegant account of the B−T phase diagram. Next, we move on to the second concept, which is the one of an emergent U(1) gauge field in the theory. Within this more general framework, the pseudogap transition can be viewed as a confining/deconfining transition. Experimental consequences for a few spectroscopic probes like inelastic neutron scattering, Raman spectroscopy, phonon line in x-ray scattering and angle-resolved photoemission experiments are studied. This theory suggests the presence of incoherent bosons at moderate temperatures in the phase diagram of the cuprates, which would undergo a jamming transition at optimal doping (Urbani, 2022). Consequences of the presence of such bosons on the transport properties of these materials are examined.

Electron removal spectral function of a polaron coupled to dispersive optical phonons

We explore the ground state and thermodynamic properties of the polaron coupled to quantum dispersive optical phonons in one spatial dimension. Calculations are performed using the finite--temperature Lanczos method augmented by a highly efficient construction of the variational Hilbert space. We focus on the electron removal spectral function as relevant for the angle-resolved photoemission experiments. We show that photoemission spectroscopy can be used to measure the phonon dispersion relation in a dilute system of polarons. The spectral weight of observed phonon bands is proportional to the phonon contribution to the wavefunction at finite phonon momentum. In addition, we demonstrate that when removing an electron from a polaron ground state, the polaron band does not appear in the spectral function. The latter becomes observable only at elevated temperatures.

Large Distortion of Fused Aromatics on Dielectric Interlayers Quantified by Photoemission Orbital Tomography.

Polycyclic aromatic compounds with fused benzene rings offer an extraordinary versatility as next-generation organic semiconducting materials for nanoelectronics and optoelectronics due to their tunable characteristics, including charge-carrier mobility and optical absorption. Nonplanarity can be an additional parameter to customize their electronic and optical properties without changing the aromatic core. In this work, we report a combined experimental and theoretical study in which we directly observe large, geometry-induced modifications in the frontier orbitals of a prototypical dye molecule when adsorbed on an atomically thin dielectric interlayer on a metallic substrate. Experimentally, we employ angle-resolved photoemission experiments, interpreted in the framework of the photoemission orbital tomography technique. We demonstrate its sensitivity to detect geometrical bends in adsorbed molecules and highlight the role of the photon energy used in experiment for detecting such geometrical distortions. Theoretically, we conduct density functional calculations to determine the geometric and electronic structure of the adsorbed molecule and simulate the photoemission angular distribution patterns. While we found an overall good agreement between experimental and theoretical data, our results also unveil limitations in current van der Waals corrected density functional approaches for such organic/dielectric interfaces. Hence, photoemission orbital tomography provides a vital experimental benchmark for such systems. By comparison with the state of the same molecule on a metallic substrate, we also offer an explanation why the adsorption on the dielectric induces such large bends in the molecule.

A sample-position-autocorrection system with precision better than 1 µm in angle-resolved photoemission experiments

We present the development of a high-precision sample-position-autocorrection system for photoemission experiments. A binocular vision method based on image pattern matching calculations was realized to track the sample position with an accuracy better than 1 µm, which was much smaller than the spot size of the incident laser. We illustrate the performance of the sample-position-autocorrection system with representative photoemission data on the topological insulator Bi2Se3 and an optimally doped cuprate superconductor Bi2Sr2CaCu2O8+δ. Our method provides new possibilities for studying the temperature-dependent electronic structures in quantum materials using laser-based or spatially resolved photoemission systems with high precision and efficiency.

Band alignment and interlayer hybridisation in transition metal dichalcogenide/hexagonal boron nitride heterostructures

In van der Waals heterostructures, the relative alignment of bands between layers, and the resulting band hybridisation, are key factors in determining a range of electronic properties. This work examines these effects for heterostructures of transition metal dichalcogenides (TMDs) and hexagonal boron nitride (hBN), an ubiquitous combination given the role of hBN as an encapsulating material. By comparing results of density functional calculations with experimental angle-resolved photoemission spectroscopy (ARPES) results, we explore the hybridisation between the valence states of the TMD and hBN layers, and show that it introduces avoided crossings between the TMD and hBN bands, with umklapp processes opening ‘ghost’ avoided crossings in individual bands. Comparison between density functional theory (DFT) and ARPES spectra for the MoSe2/hBN heterostructure shows that the valence bands of MoSe2 and hBN are significantly further separated in energy in experiment as compared to DFT. We then show that a novel scissor operator can be applied to the hBN valence states in the DFT calculations, to correct the band alignment and enable quantitative comparison to ARPES, explaining avoided crossings and other features of band visibility in the ARPES spectra.

Relaxation effects in twisted bilayer graphene: A multiscale approach

We present a multiscale density functional theory (DFT) informed molecular dynamics and tight-binding approach to capture the interdependent atomic and electronic structures of twisted bilayer graphene. We calibrate the flat band magic angle to be at ${\ensuremath{\theta}}_{\mathrm{M}}=1.{08}^{\ensuremath{\circ}}$ by rescaling the interlayer tunneling for different atomic structure relaxation models as a way to resolve the indeterminacy of existing atomic and electronic structure models whose predicted magic angles vary widely between $0.{9}^{\ensuremath{\circ}}$ and $1.{3}^{\ensuremath{\circ}}$. The interatomic force fields are built using input from various stacking and interlayer distance-dependent DFT total energies including the exact exchange and random phase approximation ($\mathrm{EXX}+\mathrm{RPA}$). We use a Fermi velocity of ${\ensuremath{\upsilon}}_{\mathrm{F}}\ensuremath{\simeq}{10}^{6}$ m/s for graphene that is enhanced by $\ensuremath{\sim}15%$ over the local density approximation (LDA) values. Based on this atomic and electronic structure model we obtain high-resolution spectral functions comparable with experimental angle-resolved photoemission spectroscopy. Our analysis of the interdependence between the atomic and electronic structures indicates that the intralayer elastic parameters compatible with the DFT-LDA, which are stiffer by $\ensuremath{\sim}30%$ than widely used reactive empirical bond order force fields, can combine with $\mathrm{EXX}+\mathrm{RPA}$ interlayer potentials to yield the magic angle at $\ensuremath{\sim}1.{08}^{\ensuremath{\circ}}$ without further rescaling of the interlayer tunneling.

Electronic correlation in nearly free electron metals with beyond-DFT methods

For more than three decades, nearly free-electron elemental metals have been a topic of debate because the computed bandwidths are significantly wider in the local density approximation to density-functional theory (DFT) than indicated by angle-resolved photoemission (ARPES) experiments. Here, we systematically investigate this using first principles calculations for alkali and alkaline-earth metals using DFT and various beyond-DFT methods such as meta-GGA, G0W0, hybrid functionals (YS-PBE0, B3LYP), and LDA + eDMFT. We find that the static non-local exchange, as partly included in the hybrid functionals, significantly increase the bandwidths even compared to LDA, while the G0W0 bands are only slightly narrower than in LDA. The agreement with the ARPES is best when the local approximation to the self-energy is used in the LDA + eDMFT method. We infer that even moderately correlated systems with partially occupied s orbitals, which were assumed to approximate the uniform electron gas, are very well described in terms of short-range dynamical correlations that are only local to an atom.

Observation and theory of strong circular dichroism in angle-revolved photoemission from graphite

Experimental angle-resolved photoemission spectra (ARPES) and its circular dichroism (CD) from a graphite surface are reported together with calculations in the one-step model of photoemission. For photoelectrons emitted with surface parallel momentum k∥ perpendicular to the incoming light, a strong and complex CD signal is observed and analyzed along the Γ-K and Γ-M lines of the Brillouin zone. Calculations using real-space multiple scattering theory agree well with the data both for the total ARPES and the CD intensity maps. In the independent atomic approximation, i.e. when photoelectron scattering is neglected, the total ARPES intensity map is still well described, but not the CD. We find that photoelectron scattering makes a large contribution to the CD signal and may change its sign. Moreover, the CD of the bands depends strongly on photon energy, while this is much less the case for the total ARPES intensity. The present findings show that an accurate description of the photoemission final state, including photoelectron scattering effects, is crucial for understanding the CD in ARPES from surfaces and two-dimensional materials.

Spectroscopic evidence of highly correlated electrons in VSe2

We present detailed high-resolution angle-resolved photoemission experiments on $\mathrm{V}{\mathrm{Se}}_{2}$ grown under various conditions. The surface electronic structure optimally grown samples can host high-temperature spectral kink, quasiparticle peak, and the Fermi gap. Collective observation of these electronic features refers to the strong electronic correlation and is usually attributed to the characteristics of superconductors. Temperature evolution of the quasiparticle peak and the Fermi gap also follow the trend observed in high-${T}_{c}$ superconductors as progressive quenching of the quasiparticle peak at a lower temperature than the complete Fermi gap closing. As a result, the quasiparticle peak can be observed up to 100 $\ifmmode\pm\else\textpm\fi{}$5 K while the Fermi gap persists up to $\ensuremath{\sim}$150 K. These realizations can guide future studies to induce high-temperature superconductivity in transition metal dichalcogenides as well as understand the physics behind high-${T}_{c}$ superconductors.

Physical properties and electronic structure of single-crystal KCo2As2

We present a method for producing high quality $\mathrm{K}{\mathrm{Co}}_{2}{\mathrm{As}}_{2}$ crystals, stable in air and suitable for a variety of measurements. X-ray diffraction, magnetic susceptibility, electrical transport, and heat capacity measurements confirm the high quality and an absence of long range magnetic order down to at least 2 K. Residual resistivity values approaching 0.25 $\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}$ cm are representative of the high quality and low impurity scattering, and a Sommerfeld coefficient $\ensuremath{\gamma}=7.3 \mathrm{mJ}/\mathrm{mol}$ ${\mathrm{K}}^{2}$ signifies weaker correlations than the Fe-based counterparts. Together with Hall effect measurements, angle-resolved photoemission experiments reveal a Fermi surface consisting of electron pockets at the center and corner of the Brillouin zone, in line with theoretical predictions and in contrast to the mixed carrier types of other pnictides with the $\mathrm{Th}{\mathrm{Cr}}_{2}{\mathrm{Si}}_{2}$ structure. A large, linear magnetoresistance of 200% at 14 T, together with an observed linear and hyperbolic, rather than parabolic, band dispersions are unusual characteristics of this compound.

Resilient Fermi Liquid and Strength of Correlations near an Antiferromagnetic Quantum Critical Point.

Near the antiferromagnetic quantum critical point (QCP) of electron-doped cuprate superconductors, angle-resolved photoemission experiments detect hot spots where the Fermi surface disappears. Here, we demonstrate, using the two-particle self-consistent theory, that in the antinodal region the Fermi liquid remains stable for a broad range of angles on the Fermi surface and for all dopings near the QCP. We show how the quasiparticle weight Z and effective mass m^{*} change and then abruptly become meaningless as the hot spots are approached. We propose a dimensionless number, easily accessible in ARPES experiments, that can be used to gauge the strength of correlations.

Realizing Kagome Band Structure in Two-Dimensional Kagome Surface States of RV_{6}Sn_{6} (R=Gd, Ho).

We report angle resolved photoemission experiments on a newly discovered family of kagome metals RV_{6}Sn_{6} (R=Gd, Ho). Intrinsic bulk states and surface states of the vanadium kagome layer are differentiated from those of other atomic sublattices by the real-space resolution of the measurements with a small beam spot. Characteristic Dirac cone, saddle point, and flat bands of the kagome lattice are observed. Our results establish the two-dimensional (2D) kagome surface states as a new platform to investigate the intrinsic kagome physics.

Temperature-independent localization of Ce 4f electrons in cerium monoarsenide

In order to reveal the valence state of Ce ions and the occupancy number of Ce 4f electrons versus temperature in cerium monoarsenide (CeAs), we perform an ab initio calculation by using a many-body scheme combing density functional theory (DFT) with dynamical mean field theory (DMFT), taking into account the spin-orbit coupling (SOC) interaction and on-site Coulomb repulsion between Ce 4f electrons. Results demonstrate that Ce 4f j=5/2 and j=7/2 manifolds exhibit the metallic and insulating characters, respectively, irrespective of temperature. Ce 4f-4f correlation, Ce 4f-conduction electrons hybridization, SOC interaction and final state effects yield a complicated spectrum function for CeAs. Coupling of 4f atomic configuration transition, hybridization and temperature does not change the localized Ce 4f regime in CeAs. Finally, momentum-resolved electronic spectrum function is also discussed for comparison with experimental angle-resolved photoemission spectrum (ARPES).

Electronic structure of the intermediate-valence compound EuNi2P2 studied by soft x-ray photoemission spectroscopy

We carried out angle-resolved photoemission (ARPES) experiments using soft x rays to investigate the electronic structure of the intermediate-valence compound ${\mathrm{EuNi}}_{2}{\mathrm{P}}_{2}$. Both the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ components arising from the $4{f}^{6}$ and $4{f}^{5}$ final states were observed in the valence spectra, directly confirming an intermediate-valence character of Eu ions. The three-dimensional band structure was studied by ARPES measurements, and the ARPES results were compared with calculations based on the density-functional theory for the non-4$f$ reference compounds ${\mathrm{SrNi}}_{2}{\mathrm{P}}_{2}$ and ${\mathrm{YNi}}_{2}{\mathrm{P}}_{2}$. We found that the ARPES spectra up to just below the Fermi level are better reproduced by the calculation of ${\mathrm{SrNi}}_{2}{\mathrm{P}}_{2}$ rather than that of ${\mathrm{YNi}}_{2}{\mathrm{P}}_{2}$. The heavy-fermion bands in ${\mathrm{EuNi}}_{2}{\mathrm{P}}_{2}$ are thus considered to be formed through the hybridization between the dispersive valence bands, which resemble those for ${\mathrm{SrNi}}_{2}{\mathrm{P}}_{2}$, and the ${\mathrm{Eu}}^{2+}$ components located at the very vicinity of the Fermi level.

Anomalously strong near-neighbor attraction in doped 1D cuprate chains.

In the cuprates, one-dimensional (1D) chain compounds provide a distinctive opportunity to understand the microscopic physics, owing to the availability of reliable theories. However, progress has been limited by the challenge of controllably doping these materials. We report the synthesis and spectroscopic analysis of the 1D cuprate Ba2-xSrxCuO3+δ over a wide range of hole doping. Our angle-resolved photoemission experiments reveal the doping evolution of the holon and spinon branches. We identify a prominent folding branch whose intensity fails to match predictions of the simple Hubbard model. An additional strong near-neighbor attraction, which may arise from coupling to phonons, quantitatively explains experiments for all accessible doping levels. Considering structural and quantum chemistry similarities among cuprates, this attraction may play a similarly important role in high-temperature cuprate superconductors.

Fermiology of two-dimensional titanium carbide and nitride MXenes

MXenes are a family two-dimensional transition-metal carbide and nitride materials, which often exhibit very good metallic conductivity and are thus of great interest for applications in, e.g., flexible electronics, electrocatalysis, and electromagnetic interference shielding. However, surprisingly little is known about the fermiology of MXenes, i.e., the shape and size of their Fermi surfaces, and its effect on the material properties. One reason for this may be that MXene surfaces are almost always covered by a mixture of functional groups, and studying Fermi surfaces of disordered systems is cumbersome. Here, we study fermiology of four common Ti-based MXenes as a function of the surface functional group composition. We first calculate the effective band structures of systems with explicit mixed surfaces and observe gradual evolution in the filling of the Ti-d band and resulting shift of Fermi level. We then demonstrate that these band structures can be closely approximated by using pseudohydrogenated surfaces, and also compare favorably to the experimental angle-resolved photoemission spectroscopy results. By modifying the pseudohydrogen charge we then proceed to plot Fermi surfaces for all systems and extract their properties, such as the Fermi-surface area and average Fermi velocity. These are in turn used to evaluate the electrical conductivity with the relaxation time fitted to experimentally measured conductivities.

Correlation effect on electronic and lattice properties of cerium oxides: Insights from density functional theory to dynamical mean-field theory

In order to reveal static and dynamic correlation effect in Ce 2 O 3 and CeO 2 with an unified theoretical framework, we perform a first principles calculation on electronic and lattice properties of these two typical correlated systems using density functional theory (DFT) schemes as well as a many-body method merging DFT with dynamical mean field theory (DMFT), taking into account the spin–orbit coupling (SOC) interaction and on-site Coulomb repulsion between Ce 4 f states. Results show that ground state of Ce 2 O 3 is in the localized 4 f 1 atomic configuration, while CeO 2 is a mixed-valence n f = 0.82 correlated system, which are in agreement with recent theoretical calculations and experimental observations. Lattice parameters, bulk modulus and band gap are also consistent with other theoretical calculations and experimental values. In order to compare with experimental angle-resolved photoemission spectrum (ARPES), momentum-resolved electronic spectrum function is also discussed.