Inverse Photoemission Spectra Research Articles

Comment on "Spin- and angle-resolved inverse photoemission setup with spin orientation independent from electron incidence angle" [Rev. Sci. Instrum. 93, 093904 (2022)

The paper under discussion promises a spin- and angle-resolved inverse-photoemission (IPE) setup, where the spin-polarization direction of the electron beam used for excitation "can be tuned to any preferred direction" while "preserving the parallel beam condition." We support the idea to improve IPE setups by introducing a three-dimensional spin-polarization rotator, but we put the presented results to the test by comparing them with the literature results obtained by existing setups. Based on this comparison, we conclude that the presented proof-of-principle experiments miss the target in several aspects. Most importantly, the key experiment of tuning the spin-polarization direction under otherwise allegedly identical experimental conditions causes changes in the IPE spectra that are in conflict with existing experimental results and basic quantum-mechanical considerations. We propose experimental test measurements to identify and overcome the shortcomings.

Photoemission spectral functions from the three-body Green's function

We present an original strategy for the calculation of direct and inverse photo-emission spectra from first principles. The main goal is to go beyond the standard Green's function approaches, such as the $GW$ method, in order to find a good description not only of the quasiparticles but also of the satellite structures, which are of particular importance in strongly correlated materials. To this end we use as a key quantity the three-body Green's function, or, more precisely, its hole-hole-electron and electron-electron-hole parts, and we show how the one-body Green's function, and hence the corresponding spectral function, can be retrieved from it. We show that, contrary to the one-body Green's function, information about satellites is already present in the non-interacting three-body Green's function. Therefore, simple approximations to the three-body self-energy, which is defined by the Dyson equation for the three-body Green's function and which contains many-body effects, can still yield accurate spectral functions. In particular, the self-energy can be chosen to be static which could simplify a self-consistent solution of the Dyson equation. We give a proof of principle of our strategy by applying it to the Hubbard dimer, for which the exact self-energy is available.

On the energy resolution of a GaAs-based electron source for spin-resolved inverse photoemission

The spin resolution in inverse photoemission spectroscopy is achieved by injecting spin-polarized electrons, usually produced by GaAs-based cold cathodes that replace hot-filament electron guns of spin-integrated setups. The overall energy resolution of the system can be enhanced by adjusting either the optical bandpass of the photon detector or the energy distribution of the electron beam. Here we discuss the influence of the photocurrent and the photocathode temperature on the energy broadening of the electron beam through the inverse photoemission spectra of the spin-splitted Shockley surface state of Au(111). First, we find that cooling down the GaAs photocathode to 77 K increases the band gap and reduces the number of allowed vertical transitions, monochromatizing the electron beam with an enhancement of about 30 meV for the energy resolution. Second, we observe a correlation between the generated photocurrent at the electron source, and the space-charge effects at the sample as a reduction of lifetime and spin asymmetry of a polarized bulk state. These observations allow defining a threshold of current density for the optimum acquisition in the measurements of spin-resolved inverse photoemission in Au.

Electronic properties of carbon nanotubes as detected by photoemission and inverse photoemission

The relation between morphology and energy level alignment in carbon nanotubes (CNT) is a crucial information for the optimization of applications in nanoelectronics, optics, mechanics and (bio)chemistry. Here we present a study of the relation between the electronic properties and the morphology of single wall CNT (SWCNT), aligned multi wall CNT (MWCNT) and unaligned MWCNT. The CNT were synthesized via catalytic chemical vapor deposition in ultra-high vacuum conditions. Combined ultraviolet photoemission and inverse photoemission (IPES) spectra reveal a high sensitivity to the nanotube morphology. In the case of unaligned SWCNT the distinctive unoccupied Van Hove singularities (vHs) features are observed in the high resolution IPES spectra. Those features are assigned to semiconducting and metallic SWCNT states, according to calculated vHs DOS. The two MWCNT samples are similar in the diameter of the tube (about 15 nm) and present similar filled and empty electronic states, although the measured features in the aligned MWCNT are more defined. Noteworthy, interlayer states are also revealed. Their intensities are directly related to the MWCNT alignment. Focussing and geometrical effects associated to the MWCNT alignment are discussed to account the spectral differences.

Multiband nonequilibrium GW+EDMFT formalism for correlated insulators

We study the dynamics of charge-transfer insulators after a photo-excitation using the three-band Emery model which is relevant for the description of cuprate superconductors. We provide a detailed derivation of the nonequilibrium extension of the multi-band GW+EDMFT formalism and the corresponding downfolding procedure. The Peierls construction of the electron-light coupling is generalized to the multi-band case resulting in a gauge invariant combination of the Peierls intra-band acceleration and dipolar intra-band transitions. We apply the formalism to the study of momentum-dependent (inverse) photo-emission spectra and optical conductivities. The time-resolved spectral function shows a strong renormalization of the charge-transfer gap and a substantial broadening of some of the bands. While the upper Hubbard band exhibits a momentum-dependent broadening, an almost rigid band shift is observed for the ligand bands. The inverse photo-emission spectrum reveals that the inclusion of the non-local and inter-band charge fluctuations lead to a very fast relaxation of holes into the lower Hubbard band. Consistent with the changes in the spectral function, the optical conductivity shows a renormalization of the charge-transfer gap, which is proportional to the photo-doping. The details of the photo-induced changes strongly depend on the dipolar matrix elements, which calls for an ab-initio determination of these parameters.

GW(Γ) method without the Bethe-Salpeter equation for photoabsorption energies of spin-polarized systems

The one-shot $GW$ method, beginning with the local density approximation (LDA), enables one to calculate photoemission and inverse photoemission spectra. In order to calculate photoabsorption spectra, one had to additionally solve the Bethe-Salpeter equation (BSE) for the two-particle (electron-hole) Green's function, which doubly induces evaluation errors. It has been recently reported that the $GW$+BSE method significantly underestimates the experimental photoabsorption energies (PAEs) of small molecules. In order to avoid these problems, we propose to apply the $GW(\mathrm{\ensuremath{\Gamma}})$ method not to the neutral ground state but to the cationic state to calculate PAEs without solving the BSE, which allows a rigorous one-to-one correspondence between the photoabsorption peak and the ``extended'' quasiparticle level. We applied the self-consistent linearized $GW\mathrm{\ensuremath{\Gamma}}$ method including the vertex correction $\mathrm{\ensuremath{\Gamma}}$ to our method, and found that this method gives the PAEs of B, ${\mathrm{Na}}_{3}$, and ${\mathrm{Li}}_{3}$ to within $0.1\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ accuracy.

Fingerprints of spin-orbital polarons and of their disorder in the photoemission spectra of doped Mott insulators with orbital degeneracy

We explore the effects of disordered charged defects on the electronic excitations observed in the photoemission spectra of doped transition metal oxides in the Mott insulating regime by the example of the $R_{1-x}$Ca$_x$VO$_3$ perovskites, where $R=$La,$\dots$,Lu. A fundamental characteristic of these vanadium $d^2$ compounds with partly filled $t_{2g}$ valence orbitals is the persistence of spin and orbital order up to high doping, in contrast to the loss of magnetic order in high-$T_c$ cuprates at low defect concentration. We demonstrate that the disordered electronic structure of doped Mott-Hubbard insulators can be obtained with high precision within the unrestricted Hartree-Fock approximation. In particular: (i) the atomic multiplet excitations in the inverse photoemission spectra and the various defect-related states and satellites are well reproduced, (ii) a robust Mott gap survives up to large doping, and (iii) we show that the defect states inside the Mott gap develop a soft gap at the Fermi energy. The soft defect states gap can be characterized by a shape and a scale parameter extracted from a Weibull statistical sampling of the density of states near the chemical potential. We demonstrate that charge defects trigger small spin-orbital polarons, with their internal kinetic energy responsible for the opening of the soft defect states gap. The small size of spin-orbital polarons is inferred by an analysis of the inverse participation ratio which explains the origin of the robustness of spin and orbital order. Using realistic parameters for La$_{1-x}$Ca$_x$VO$_3$, we show that its soft gap is well reproduced as well as the marginal doping dependence of the position of the chemical potential relative to the center of the lower Hubbard band.

A simple derivation of the exact quasiparticle theory and its extension to arbitrary initial excited eigenstates.

The quasiparticle (QP) energies, which are minus of the energies required by removing or produced by adding one electron from/to the system, corresponding to the photoemission or inverse photoemission (PE/IPE) spectra, are determined together with the QP wave functions, which are not orthonormal and even not linearly independent but somewhat similar to the normal spin orbitals in the theory of the configuration interaction, by self-consistently solving the QP equation coupled with the equation for the self-energy. The electron density, kinetic, and all interaction energies can be calculated using the QP wave functions. We prove in a simple way that the PE/IPE spectroscopy and therefore this QP theory can be applied to an arbitrary initial excited eigenstate. In this proof, we show that the energy-dependence of the self-energy is not an essential difficulty, and the QP picture holds exactly if there is no relaxation mechanism in the system. The validity of the present theory for some initial excited eigenstates is tested using the one-shot GW approximation for several atoms and molecules.

Electronic Structure of the Kitaev Material α-RuCl3 Probed by Photoemission and Inverse Photoemission Spectroscopies

Recently, α-RuCl3 has attracted much attention as a possible material to realize the honeycomb Kitaev model of a quantum-spin-liquid state. Although the magnetic properties of α-RuCl3 have been extensively studied, its electronic structure, which is strongly related to its Kitaev physics, is poorly understood. Here, the electronic structure of α-RuCl3 was investigated by photoemission (PE) and inverse-photoemission (IPE) spectroscopies. The band gap was directly measured from the PE and IPE spectra and was found to be 1.9 eV, much larger than previously estimated values. Local density approximation (LDA) calculations showed that the on-site Coulomb interaction U could open the band gap without spin-orbit coupling (SOC). However, the SOC should also be incorporated to reproduce the proper gap size, indicating that the interplay between U and SOC plays an essential role. Several features of the PE and IPE spectra could not be explained by the results of LDA calculations. To explain such discrepancies, we performed configuration-interaction calculations for a RuCl63− cluster. The experimental data and calculations demonstrated that the 4d compound α-RuCl3 is a Jeff = 1/2 Mott insulator rather than a quasimolecular-orbital insulator. Our study also provides important physical parameters required for verifying the proposed Kitaev physics in α-RuCl3.

Efficient and low-damage nitrogen doping of graphene via plasma-based methods

We investigate the nitrogen doping of mono- and bi-layer graphene on 6H–SiC(0001) using two plasma-based methods: a lab-scale microwave plasma gun and an industrial-scale plasma-based implanter. As revealed by X-ray photoemission spectroscopy, the thickness of the pristine graphene significantly influences the bonding configuration of the incorporated nitrogen atoms. Using the plasma gun, a high concentration of graphitic-nitrogen (3 at%) is obtained in N-doped bilayer graphene, while only 0.2 at% of pyridinic-nitrogen is incorporated. By contrast, a comparable amount of each nitrogen doping configuration is found with monolayer graphene. The integrity of the bilayer graphene is also better preserved than its monolayer counterpart after nitrogen plasma exposure, as evidenced by their inverse photoemission spectra. It is attributed to a better stability and a lower vacancy creation rate in bilayer graphene during plasma exposure. In addition, the N-doped bilayer graphene shows an efficient n-type doping, with a Dirac point brought 0.45 ± 0.1 eV away from the Fermi energy. In brief, this study reports an efficient method for tailoring the electronic properties of graphene using industry-suited techniques, thereby promoting future graphene-based applications.

Magnetic interactions and electronic structure of Ni–Mn–In

The electronic structure and magnetic properties of a magnetic shape memory alloy Ni–Mn–In have been studied using spin polarized fully relativistic Korringa–Kohn–Rostoker (SPRKKR) method. The total energy calculations with different starting magnetic spin configurations show that the ground state of Ni2Mn1.4In0.6 is ferromagnetic. The spin and orbital magnetic moments of Ni2Mn1.4In0.6 and Ni2MnIn are in good agreement with the magnetization measurements. The exchange coupling parameters of the different sublattice interactions exhibit a strong competition between ferromagnetic and antiferromagnetic configurations, due to the substitution of excess Mn atoms at the In site in Ni2Mn1.4In0.6. The Curie temperature of Ni2MnIn, calculated under a mean field approximation, is found to be in relatively good agreement with the experimental values. While Ni2MnIn does not undergo martensite transition, it is shown that a Jahn–Teller splitting of the Ni 3d eg states plays an important role in driving the martensite transformation in Ni2Mn1.4In0.6. We find that both the calculated ultra-violet photoemission spectra and the inverse photoemission spectra are in good agreement with the existing experimental data.

Inverse photoemission spectroscopic studies on phase separated La 0.2 Sr 0.8 MnO 3

Abstract We have studied the temperature evolution of the inverse photoemission spectra of phase separated La 0.2 Sr 0.8 MnO 3 . To identify the features in the room temperature experimental spectra, band structure calculations using Korringa–Kohn–Rostoker Green׳s function method were carried out. We find that the features generated by local moment disorder calculations give a better match with the experimental spectrum. In the insulating phase, we observed unusually an increased intensity at around the Fermi level. This puzzling behaviour is attributed to the shift in the chemical potential towards the conduction band and localisation of disorder induced states closer to the conduction band. The present results clearly show the importance of unoccupied electronic states in better understanding of the phase separated systems.

First-principles investigation of organic photovoltaic materialsC60, C70, [C60]PCBM, and bis-[C60]PCBMusing a many-bodyG0W0-Lanczos approach

We present a first-principles investigation of the excited-state properties of electron acceptors in organic photovoltaics including C$_{60}$, C$_{70}$, [6,6]-phenyl-C$_{61}$-butyric-acid-methyl-ester ([C$_{60}$]PCBM), and bis-[C$_{60}$]PCBM using many-body perturbation theory within the Hedin's $G_0W_0$ approximation and an efficient Lanczos approach. Calculated vertical ionization potentials (VIP) and vertical electron affinities (VEA) of C$_{60}$ and C$_{70}$ agree very well with experimental values measured in gas phase. The density of states of all three molecules is also compared to photoemission and inverse photoemission spectra measured on thin-films, exhibiting a close agreement - a rigid energy-gap renormalization owing to intermolecular interactions in the thin-films. In addition, it is shown that the low-lying unoccupied states of [C$_{60}$]PCBM are all derived from the highest-occupied molecular orbitals and the lowest-unoccupied molecular orbitals of fullerene C$_{60}$. The functional side group in [C$_{60}$]PCBM introduces a slight electron transfer to the fullerene cage, resulting in small decreases of both VIP and VEA. This small change of VEA provides a solid justification for the increase of open-circuit voltage when replacing fullerene C$_{60}$ with [C$_{60}$]PCBM as the electron acceptor in bulk heterojunction polymer solar cells.

Quasiparticle properties of the superconducting state of the two-dimensional Hubbard model

Cluster dynamical mean field methods are used to calculate the normal and anomalous components of the electron self energy of the two dimensional Hubbard model. From these the evolution of the superconducting gap and the momentum dependent photoemission and inverse photoemission spectra across the phase diagram are determined. In the pseudogap regime, decreasing the temperature into the superconducting state leads to a decrease in the energy gap and the formation of a `peak-dip-hump' structure in the electronic density of states. The peak feature disperses very weakly. The calculated spectral functions are in good qualitative agreement with published data. The mathematical origin of the behavior is found to be the effect of the superconductivity on the pole structure giving rise to the normal state pseudogap. In particular the "hump" feature is found to arise from a zero crossing of the real part of the electron self energy rather than from an onset of scattering. The effect of superconductivity on the zone diagonal spectra is presented.

Inverse photoemission and photoemission spectroscopic studies on sputter-annealed Ni–Mn–Sn and Ni–Mn–In surfaces

The electronic structure of nearly stoichiometric Ni–Mn–Sn and Ni–Mn–In surface is investigated by inverse photoemission and photoemission spectroscopy. Comparison of the experimental and calculated inverse photoemission spectra shows that the dominant feature is related to Mn 3d-like states. The overall shape and peak position of the theoretically obtained spectra show good agreement with the experimental ultraviolet photoemission valence band spectra. The changes in the composition dependent ultraviolet photoemission spectra reveal the change in degree of Ni 3d and Mn 3d band hybridization. Both inverse photoemission and ultraviolet photoemission study show a rigid band shift between Ni2MnIn and Ni2MnSn because of band filling, due to increase in the number of 5p electrons from In to Sn. Mn 2p and 3s core-level reveal unambiguous existence of exchange splitting in both the materials.

Hydrogenation of the buffer-layer graphene on 6H-SiC (0001): A possible route for the engineering of graphene-based devices

The hydrogenation at various temperatures of the (6√3 × 6√3)R30° reconstruction of SiC(0001), the so-called buffer layer graphene (BLG), is investigated. For the BLG, a significant concentration of remaining dangling bonds related to unsaturated Si atoms of the outermost SiC bilayer is evidenced in the inverse photoemission spectra. These dangling bonds give rise to a peak around 1 eV above the Fermi level, associated with the upper single-electron states of a Mott-Hubbard insulator, which vanishes upon hydrogenation. Hydrogen atoms adsorbed at ambient temperature remain covalently bound to BLG (H-BLG) up to temperatures of ∼500 °C. They induce additional C-Si bonds at the BLG/SiC interface that saturate the remaining Si dangling bonds, as evidenced in both IPES and Auger electron spectra. The H-BLG further shows a large energy gap and an excess n-type doping in comparison to the pristine BLG. Upon hydrogen exposure at higher temperature (> 700 °C), hydrogen atoms intercalate at the BLG/SiC interface, inducing the formation of a single layer of quasi-free-standing graphene (QFSG) lying on top of a hydrogenated (√3 × √3)R30° reconstruction as supported by IPES. We suggest that the high-stability and the distinct electronic structure of both BLG-derived structures, H-BLG and QFSG, may open a route for the engineering of graphene-based devices.

First-Principles Investigation on Structural and Optical Properties of M+@C60 (Where M = H, Li, Na, and K)

Structural and optical properties of proton and alkali-metal atom cation encapsulated inside C60 molecule, M+@C60 (where M = H, Li, Na, and K), are investigated in detail by first-principles. Two possible ground state atomic geometries where M+ can exist outside or inside C60 are proposed by the optimization using the B3LYP hybrid functional based on the density functional theory. From the potential energy surface obtained in the same level calculation, we confirm that the energy barrier existing at the surface of C60 plays an important role to confine M+ inside. We also apply an all-electron GW + Bethe–Salpeter method based on the many-body perturbation theory and calculate the (inverse) photoemission spectra and the photoabsorption spectra. The calculated spectra are in reasonable agreement with available experimental results, while minor deviation exists, suggesting a non-negligible environment effect.

The surface states of lithium tetraborate

The different low index surface terminations of lithium tetraborate, Li2B4O7, are dominated by electronic states that fall within the band gap of the projection of the bulk states. As a pyroelectric material, lithium tetraborate, Li2B4O7, is a wide band gap dielectric, yet the (110) surface has a much smaller band gap because of occupied surface states that fall at binding energies between the valence band maximum and the Fermi level. The (100) surface is dominated, however, by unoccupied surface states that also fall in the gap between the conduction band minimum and the valence band maximum, but at binding energies just below the conduction band minimum. These very different types of surface states are consistent with the assignment of occupied surface states to states that are observed near the Fermi level in photoemission studies of Li2B4O7(110). The unoccupied surface states are also seen to be consistent with the inverse photoemission spectra of Li2B4O7(100). Clearly, the different surface terminations of lithium tetraborate yield very different types of surface electronic states.

Interplay of geometric and electronic structure in thin films of diindenoperylene on Ag(1 1 1)

Using the example of Diindenoperylene (DIP) a strong dependence of ionization potential, electron affinity and transport gap on the growth conditions of an organic molecular thin film is demonstrated. DIP single crystals show a remarkable polymorphism as single crystals as well as crystalline films on weakly interacting substrates like Au or SiO2 surfaces. We have investigated DIP thin films on Ag(111) substrates and found a strong dependence of the photoemission and inverse photoemission spectra as well as of the low energy electron diffraction (LEED) patterns on the substrate temperature. Three different temperature regions could be identified with distinct reproducible signatures in the structural data as well as in the valence level spectra. Peak positions, line widths, and relative intensities directly correlate with the degree of order and with the molecular orientation. Moreover, we identified a systematic change of the structural quality ranging from high mosaicity at low temperature (<150K) via small ordered domains (150–250K) to a high degree of order at elevated temperature (>350K).

Combined GW and dynamical mean-field theory: Dynamical screening effects in transition metal oxides

We present the first dynamical implementation of the combined GW and dynamical mean-field scheme (“GW + DMFT”) for first-principles calculations of the electronic properties of correlated materials. The application to the ternary transition metal oxide SrVO3 demonstrates that this scheme inherits the virtues of its two parent theories: a good description of the local low-energy correlation physics encoded in a renormalized quasi-particle band structure, spectral weight transfer to Hubbard bands, and the physics of screening driven by long-range Coulomb interactions. Our data is in good agreement with available photoemission and inverse photoemission spectra; our analysis leads to a reinterpretation of the commonly accepted “three-peak structure” as originating from orbital effects rather than from the electron addition peak within the t2g manifold.