Theory Of Photoemission Research Articles

Surface Doping and Dual Nature of the Band Gap in Excitonic Insulator Ta2NiSe5.

Excitons in semiconductors and molecules are widely utilized in photovoltaics and optoelectronics, and high-temperature coherent quantum states of excitons can be realized in artificial electron-hole bilayers and an exotic material of an excitonic insulator (EI). Here, we investigate the band gap evolution of a putative high-temperature EI Ta2NiSe5 upon surface doing by alkali adsorbates with angle-resolved photoemission and density functional theory (DFT) calculations. The conduction band of Ta2NiSe5 is filled by the charge transfer from alkali adsorbates, and the band gap decreases drastically upon the increase of metallic electron density. Our DFT calculation, however, reveals that there exist both structural and excitonic contributions to the band gap tuned. While electron doping reduces the band gap substantially, it alone is not enough to close the band gap. In contrast, the structural distortion induced by the alkali adsorbate plays a critical role in the gap closure. This work indicates a combined electronic and structural nature for the EI phase of the present system and the complexity of surface doping beyond charge transfer.

Chemical Trends of the Bulk and Surface Termination-Dependent Electronic Structure of Metal-Intercalated Transition Metal Dichalcogenides.

The addition of metal intercalants into the van der Waals gaps of transition metal dichalcogenides has shown great promise as a method for controlling their functional properties. For example, chiral helimagnetic states, current-induced magnetization switching, and a giant valley-Zeeman effect have all been demonstrated, generating significant renewed interest in this materials family. Here, we present a combined photoemission and density-functional theory study of three such compounds: , , and , to investigate chemical trends of the intercalant species on their bulk and surface electronic structure. Our resonant photoemission measurements indicate increased hybridization with the itinerant NbS2-derived conduction states with increasing atomic number of the intercalant, leading to pronounced mixing of the nominally localized intercalant states at the Fermi level. Using spatially and angle-resolved photoemission spectroscopy, we show how this impacts surface-termination-dependent charge transfers and leads to the formation of new dispersive states of mixed intercalant-Nb character at the Fermi level for the intercalant-terminated surfaces. This provides an explanation for the origin of anomalous states previously reported in this family of compounds and paves the way for tuning the nature of the magnetic interactions in these systems via control of the hybridization of the magnetic ions with the itinerant states.

Origin of discrete donor–acceptor pair transitions in 2D Ruddlesden–Popper perovskites

Two-dimensional (2D) van der Waals nanomaterials have attracted considerable attention for potential use in photonic and light–matter applications at the nanoscale. Thanks to their excitonic properties, 2D perovskites are also promising active materials to be included in devices working at room temperature. In this work, we study the presence of very narrow and spatially localized optical transitions in 2D lead halide perovskites by μ-photoluminescence and time-decay measurements. These discrete optical transitions are characterized by sub-millielectronvolt linewidths (≃120μeV) and long decay times (5–8 ns). X-ray photoemission and density-functional theory calculations have been employed to investigate the chemical origin of electronic states responsible of these transitions. The association of phenethylammonium with methylammonium cations into 2D Ruddlesden–Popper perovskites, (PEA)2(MA)n−1PbnI3n+1, particularly in phases with n≥2, has been identified as a mechanism of donor–acceptor pair (DAP) formation, corresponding to the displacement of lead atoms and their replacement by methylammonium. Ionized DAP recombination is identified as the most likely physical source of the observed discrete optical emission lines. The analysis of the experimental data with a simple model, which evaluates the Coulombic interaction between ionized acceptors and donors, returns a donor in Bohr radius of the order of ≃10 nm. The analysis of the spectral and electronic characteristics of these single donor–acceptor states in 2D perovskites is of particular importance both from the point of view of fundamental research, as well as to be able to link the emission of these states with new optoelectronic applications that require long-range optically controllable interactions.

Flat-band hybridization between f and d states near the Fermi energy of SmCoIn5

We present high-quality angle-resolved photoemission (ARPES) and density functional theory calculations (DFT+U) of SmCoIn5. We find broad agreement with previously published studies of LaCoIn5 and CeCoIn51,2, confirming that the Sm 4f electrons are mostly localized. Nevertheless, our model is consistent with an additional delocalized Sm component, stemming from hybridization between the 4f electrons and the metallic bands at “hot spot” positions in the Brillouin zone. The dominant hot spot, called γZ, is similar to a source of delocalized f states found in previous experimental and theoretical studies of CeCoIn51,3. In this work, we identify and focus on the role of the Co d states in exploring the relationship between heavy quasiparticles and the magnetic interactions in SmCoIn5, which lead to a magnetically ordered ground state from within an intermediate valence scenario4–6. Specifically, we find a globally flat band consisting of Co d states near E = − 0.7 eV, indicating the possibility of enhanced electronic and magnetic interactions in the “115” family of materials through localization in the Co layer, and we discuss a possible origin in geometric frustration. We also show that the delocalized Sm 4f states can hybridize directly with the Co 3dxz/3dyz orbitals, which occurs in our model at the Brillouin zone boundary point R in a band that is locally flat and touches the Fermi level from above. Our work identifies microscopic ingredients for additional magnetic interactions in the “115” materials beyond the RKKY mechanism, and strongly suggests that the Co d bands are an important ingredient in the formation of both magnetic and superconducting ground states.

Enhanced vacuum ultraviolet photoemission from graphene nanoribbons

Photon-energy dependence of photoemission from seven-atoms-wide armchair graphene nanoribbons (GNRs) is studied experimentally and theoretically up to eV. A strong photon energy dependence of the normal emission from the valence band maximum (VB1) is observed, sharply peaked at eV. The detailed analysis of the light-polarization dependence of the photoemission from VB1 unambiguously characterizes the symmetry of the state. The experimental observations are analyzed based on ab initio one-step theory of photoemission. Off-normal emission is studied in detail and its relation to the standing-wave character of the valence band states is discussed. Excellent agreement with the earlier experiment (Senkovskiy et al 2018 2D Mater. 5 035007) is obtained. Rapid variations of the intensity with the ribbon-transverse photoelectron momentum are predicted from the ab initio theory, which are at variance with the prediction of the tight-binding rigid-wall model. These findings can help interpret angle-resolved photoemission measurements of similar systems. Moreover, the strong enhancement of the photoyield could trigger the GNR application as narrow-band photodetectors and contribute to the design of novel photocathodes for vacuum ultraviolet photodetection.

Angle-resolved pair photoemission theory for correlated electrons

Angle-resolved pair photoemission theory for correlated electrons

Erratum: “Theory of photoemission from cathodes with disordered surfaces” [J. Appl. Phys. 133, 053102 (2023)

Erratum: “Theory of photoemission from cathodes with disordered surfaces” [J. Appl. Phys. 133, 053102 (2023)

Theory of photoemission from cathodes with disordered surfaces

Linear-accelerator-based applications like x-ray free electron lasers, ultrafast electron diffraction, electron beam cooling, and energy recovery linacs use photoemission-based cathodes in photoinjectors for electron sources. Most of these photocathodes are typically grown as polycrystalline materials with disordered surfaces. In order to understand the mechanism of photoemission from such cathodes and completely exploit their photoemissive properties, it is important to develop a photoemission formalism that properly describes the subtleties of these cathodes. The Dowell–Schmerge (D–S) model often used to describe the properties of such cathodes gives the correct trends for photoemission properties like the quantum efficiency (QE) and the mean transverse energy (MTE) for metals; however, it is based on several unphysical assumptions. In the present work, we use Spicer’s three-step photoemission formalism to develop a photoemission model that results in the same trends for QE and MTE as the D–S model without the need for any unphysical assumptions and is applicable to defective thin-film semiconductor cathodes along with metal cathodes. As an example, we apply our model to Cs3Sb thin films and show that their near-threshold QE and MTE performance is largely explained by the exponentially decaying defect density of states near the valence band maximum.

Assessing Nontrivial Topology in Weyl Semimetals by Dichroic Photoemission

The electronic structure of Weyl semimetals features Berry flux monopoles in the bulk and Fermi arcs at the surface. While angle-resolved photoelectron spectroscopy (ARPES) is successfully used to map the bulk and surface bands, it remains a challenge to explicitly resolve and pinpoint these topological features. Here we combine state-of-the-art photoemission theory and experiments over a wide range of excitation energies for the Weyl semimetals TaAs and TaP. Our results show that simple surface-band-counting schemes, proposed previously to identify nonzero Chern numbers, are ambiguous due to pronounced momentum-dependent spectral weight variations and the pronounced surface-bulk hybridization. Instead, our findings indicate that dichroic ARPES provides an improved approach to identify Fermi arcs but requires an accurate description of the photoelectron final state.

One-Step Theory View on Photoelectron Diffraction: Application to Graphene.

Diffraction of photoelectrons emitted from the core 1s and valence band of monolayer and bilayer graphene is studied within the one-step theory of photoemission. The energy-dependent angular distribution of the photoelectrons is compared to the simulated electron reflection pattern of a low-energy electron diffraction experiment in the kinetic energy range up to about 55 eV, and the implications for the structure determination are discussed. Constant energy contours due to scattering resonances are well visible in photoelectron diffraction, and their experimental shape is well reproduced. The example of the bilayer graphene is used to reveal the effect of the scattering by the subsurface layer. The photoemission and LEED patterns are shown to contain essentially the same information about the long-range order. The diffraction patterns of C 1s and valence band photoelectrons bear similar anisotropy and are equally suitable for diffraction analysis.

Rashba-split image-potential state and unoccupied surface electronic structure of Re(0001)

The influence of spin-orbit interaction on the unoccupied electronic structure of the Re(0001) surface is investigated by spin- and angle-resolved inverse photoemission and density-functional theory calculations. In the two high-symmetry azimuths $\overline{\mathrm{\ensuremath{\Gamma}}}\phantom{\rule{0.16em}{0ex}}\overline{\text{K}}$ and $\overline{\mathrm{\ensuremath{\Gamma}}}\phantom{\rule{0.16em}{0ex}}\overline{\text{M}}$, we identify transitions into $d$-derived bulk states as well as different types of surface states. The Rashba-type spin-split hole pocket around $\overline{\mathrm{\ensuremath{\Gamma}}}$ finds continuation in empty spin-split surface states for higher ${\mathbf{k}}_{\ensuremath{\parallel}}$, thereby forming $\mathsf{W}$-shaped states whose lower parts are partially occupied. A large energy gap below and above the vacuum energy around $\overline{\mathrm{\ensuremath{\Gamma}}}$ hosts image-potential-induced surface states. The $n=1$ member of the Rydberg-like series exhibits a free-electron-like $E({\mathbf{k}}_{\ensuremath{\parallel}})$ dispersion with an effective mass of ${m}^{*}/{m}_{e}=1.2\ifmmode\pm\else\textpm\fi{}0.1$. Careful spin-resolved measurements for several angles of electron incidence allow us to detect Rashba-type spin-dependent energy splittings of this state with a Rashba parameter of ${\ensuremath{\alpha}}_{\text{R}}=105\ifmmode\pm\else\textpm\fi{}33\phantom{\rule{0.16em}{0ex}}\text{meV}\phantom{\rule{0.16em}{0ex}}\AA{}$.

Theory of laser-induced photoemission from a metal surface with nanoscale dielectric coating

This paper presents an analytical quantum model for photoemission from metal surfaces coated with an ultrathin dielectric, by solving the 1D time-dependent Schrödinger equation subject to an oscillating double-triangular potential barrier. The model is valid for an arbitrary combination of metal (of any work function and Fermi level), dielectric (of any thickness, relative permittivity, and electron affinity), laser field (strength and wavelength), and dc field. The effects of dielectric properties on photoemission are systematically investigated. It is found that a flat metal surface with dielectric coating can photoemit a larger current density than the uncoated case when the dielectric has smaller relative permittivity and larger electron affinity. Resonant peaks in the photoemission probability and emission current are observed as a function of dielectric thickness or electron affinity due to the quantum interference of electron waves inside the dielectric. Our model is compared with the effective single-barrier quantum model and modified Fowler–Nordheim equation, for both 1D flat cathodes and pyramid-shaped nanoemitters. While the three models show quantitatively good agreement in the optical field tunneling regime, the present model may be used to give a more accurate evaluation of photoemission from coated emitters in the multiphoton absorption regime.

Theory of subcycle time-resolved photoemission: Application to terahertz photodressing in graphene

Motivated by recent experimental progress we revisit the theory of pump–probe time- and angle-resolved photoemission spectroscopy (trARPES), which is one of the most powerful techniques to trace transient pump-driven modifications of the electronic properties. The pump-induced dynamics can be described in different gauges for the light–matter interaction. Standard minimal coupling leads to the velocity gauge, defined by linear coupling to the vector potential. In the context of tight-binding (TB) models, the Peierls substitution is the commonly employed scheme for single-band models. Multi-orbital extensions – including the coupling of the dipole moments to the electric field – have been introduced and tested recently. In this work, we derive the theory of time-resolved photoemission within both gauges from the perspective of nonequilibrium Green’s functions. This approach naturally incorporates the photoelectron continuum, which allows for a direct calculation of the observable photocurrent. Following this route we introduce gauge-invariant expressions for the time-resolved photoemission signal. The theory is applied to graphene pumped with short terahertz pulses, which we treat within a first-principles TB model. We investigate the gauge invariance and discuss typical effects observed in subcycle time-resolved photoemission. Our formalism is an ideal starting point for realistic trARPES simulations including scattering effects.

Theory of Core Level Photoemission in Uranium Intermetallic Compounds Combined with Bayesian Data Analysis Approach

We discuss the Uranium 5f electron states in UPd3 and UAl3 on the basis of the theoretical calculation of the core level photoemission with the use of the impurity Anderson model. In order to suppr...

Theory of multiphoton photoemission disclosing excited states in conduction band of individual TiO2 nanoparticles* *Project supported by the National Natural Science Foundation of China (Grant Nos. 91850109, 11474040, 61605017, and 61775021) and the “111” Project of China (Grant No. D17017).

A theory of multiphoton photoemission is derived to explain the experimentally observed monotonic decrease with the wavelength in the electron yield of TiO2 nanoparticles (NPs) by as large as four orders of magnitude. It is found that the fitting parameter corresponds to the energy position of Ti3d eg and t2g states, and the derived theory is a novel diagnostic of excited states in the conduction band, very importantly, applicable to individual NPs. The difference between four-photon slope NPs and three-photon slope NPs is attributed to the difference in defect density. The success of the theory in solving the puzzling result shows that thermal emission from high-lying levels may dominate over direct multiphoton ionization in solids when the photon number larger than four is required.

Bulk electronic structure of lanthanum hexaboride ( LaB6 ) by hard x-ray angle-resolved photoelectron spectroscopy

In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter physics, and for their applications in advanced technological fields. Among these compounds, LaB$_6$ has a special place, being a traditional d-band metal without additional f- bands. In this paper we investigate the bulk electronic structure of LaB$_6$ using hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. By comparing La 3d core level spectra to cluster model calculations, we identify well-screened peak residing at a lower binding energy compared to the main poorly-screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulk-like band structure of LaB$_6$ determined by hard x-ray angle-resolved photoemission spectroscopy (HARPES). We interpret HARPES experimental results by the free-electron final-state calculations and by the more precise one-step photoemission theory including matrix element and phonon excitation effects. In addition, we consider the nature and the magnitude of phonon excitations in HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that one step theory of photoemission and HARPES experiments provide, at present, the only approach capable of probing true bulk-like electronic band structure of rare-earth hexaborides and strongly correlated materials.

Hydrogen in tungsten trioxide by membrane photoemission and density functional theory modeling

The measurement of hydrogen-induced changes in the electronic structure of transition metal oxides by x-ray photoelectron spectroscopy is a challenging endeavor, since no photoelectron can be unambiguously assigned to hydrogen. The H-induced electronic structure changes in tungsten trioxide have been known for more than 100 years but are still controversially debated. The controversy stems from the difficulty in disentangling effects due to hydrogenation from the effects of oxygen deficiencies. Using a membrane approach to x-ray photoelectron spectroscopy, in combination with tunable synchrotron radiation, we measure simultaneously core levels and the valence band up to a hydrogen pressure of 1000 mbar. Upon hydrogenation, the intensities of the ${\mathrm{W}}^{5+}$ core level and a state close to the Fermi level increase following the pressure-composition isotherm curve of bulk ${\mathrm{H}}_{x}{\mathrm{WO}}_{3}$. Combining experimental data and density functional theory, the description of the hydrogen-induced coloration by a proton polaron model is corroborated. Although hydrogen is the origin of the electronic structure changes near the Fermi edge, the valence band edge is now dominated by tungsten orbitals instead of oxygen as is the case for the pristine oxide, having wider implications for its use as a (photoelectrochemical) catalyst.

Ab Initio Theory of Photoemission from Graphene.

Angle-resolved photoemission from monolayer and bilayer graphene is studied based on an ab initio one-step theory. The outgoing photoelectron is represented by the time-reversed low energy electron diffraction (LEED) state , which is calculated using a scattering theory formulated in terms of augmented plane waves. A strong enhancement of the emission intensity is found to occur around the scattering resonances. The effect of the photoelectron scattering by the underlying substrate on the polarization dependence of the photocurrent is discussed. The constant initial state spectra are compared to electron transmission spectra of graphene, and the spatial structure of the outgoing waves is analyzed. It turns out that the emission intensity variations do not correlate with the structure of the spectra and are caused by rather subtle interference effects. Earlier experimental observations of the photon energy and polarization dependence of the emission intensity are well reproduced within the dipole approximation, and the Kohn–Sham eigenstates are found to provide a quite reasonable description of the photoemission final states.

Many-body photoemission theory for organic molecular crystals

A many-body photoemission theory based on Keldysh nonequilibrium Green's functions is developed and applied to photoemission from extended states of organic molecules and their crystals, in particular excited by ultraviolet and soft X-ray radiation. In this energy region, electron-phonon interaction and radiation filed screening play some important roles. To incorporate the latter effects, we should use quantum electrodynamics (QED) approach. To discuss the electron-phonon interaction, we apply the Hedin equation including the screened Coulomb interaction mediated by the electron-phonon interaction. This theoretical framework gives us a unified picture of the UPS spectra from organic solids. Furthermore some illustrative numerical calculations are shown to discuss the relative importance of the multiple scatterings, Debye-Waller factors, the peak width of the quasi-particles, and the angular dependence of the phonon satellites. We demonstrate some specific features of ultraviolet photoemission spectroscopy (UPS) spectra from particularly anisotropic organic solids.

Profiling spin and orbital texture of a topological insulator in full momentum space

We investigate the coupled spin and orbital textures of the topological surface state in ${\mathrm{Bi}}_{2}$(Te,${\mathrm{Se})}_{3}$(0001) across full momentum space using spin- and angle-resolved photoelectron spectroscopy and relativistic one-step photoemission theory. For an approximately isotropic Fermi surface in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{2}\mathrm{Se}$, the measured intensity and spin momentum distributions, obtained with linearly polarized light, qualitatively reflect the orbital composition and the orbital-projected in-plane spin polarization, respectively. In ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, the in-plane lattice potential induces a hexagonal anisotropy of the Fermi surface, which manifests in an out-of-plane photoelectron spin polarization with a strong dependence on light polarization, excitation energy, and crystallographic direction.