Resonant Photoemission Spectroscopy Research Articles

Electronic structures of skyrmionic polycrystalline MnSi thin film studied by resonance photoemission and x-ray near edge spectroscopy

Magnetic nanometric skyrmions are small complex vortex-like topological defects, mainly found in non-centrosymmetric crystals such as MnSi. They have potential applications for future spintronic devices. In this article, the structural, electronic, and magnetic states of the Mn atoms in a polycrystalline MnSi thin film facing a c-sapphire substrate were studied using x-ray diffraction, x-ray photo-emission spectroscopy, resonance photoemission spectroscopy (RPES), and extended x-ray absorption fine structure (EXAFS). The valence band spectra indicate the metallic nature of the film. The RPES study reveals the presence of major itinerant Mn 3d states near EF and also the mixed Mn 3d and Si 3s–3p states from 5.3 to 11.3 eV. The EXAFS spectrum does not show the existence of oxygen vacancies in the system, and the obtained magnetic moment in the non-stoichiometric MnSi thin film is a combination of the partially itinerant and partially localized Mn 3d states.

Epitaxial growth of AgCrSe2 thin films by molecular beam epitaxy

AgCrS e 2 exhibits remarkably high ionic conduction, an inversion symmetry-breaking structural transition, and is host to complex non-colinear magnetic orders. Despite its attractive physical and chemical properties and its potential for technological applications, studies of this compound to date are focused almost exclusively on bulk samples. Here, we report the growth of AgCrSe2 thin films via molecular beam epitaxy. Single-orientated epitaxial growth was confirmed by x-ray diffraction, while resonant photoemission spectroscopy measurements indicate a consistent electronic structure as compared to bulk single crystals. We further demonstrate significant flexibility of the grain morphology and cation stoichiometry of this compound via control of the growth parameters, paving the way for the targeted engineering of the electronic and chemical properties of AgCrSe2 in thin-film form.

Analysis of Electrode Materials for Li-Ion Batteries by Synchrotron Soft X-Ray Microspectroscopy

The development of clean energy devices such as Li-ion batteries is attracting attention in order to realize a low-carbon society. In addition to development of materials for Li-ion batteries, advanced analytical methods are also being developed to obtain the guideline of new materials and improve battery performance. We have been working on electronic structure analysis of transition metals and oxygen using synchrotron soft X-ray absorption and emission spectroscopy including ex-situ measurements and operando measurements. [1-3] We have also reported on synchrotron radiation micro-photoemission spectroscopy of all-solid-state Li-ion batteries using an operando measurement system. [4,5]In this presentation, we report analysis of each facet of a single crystalline LiCoO2 particle by resonant photoemission microspectroscopy. Resonant photoemission spectroscopy is element-selective method for obtaining valence band spectra using excitation energies around the absorption edge of target elements. Co 2p-3d absorption (L 2,3 absorption edge) of LiCoO2 was used to study the Co 3d orbitals near the Fermi level in detail. [6] Next, we report on the absorption spectroscopy of LiCoO2 using scanning transmission X-ray microscopy (STXM). The spatial resolution of about 100 nm can be used to analyze the chemical state distribution for each particle or within each particle. [7]References Electrochem. Commun. 50 (2015) 93-96.Phys. Chem. Chem. Phys. 21 (2019) 26351–26357.Phys. Chem. Chem. Phys. 24 (2022) 19177-19183.J. Electron Spectrsc. Relat. Phenom., 233 (2019) 64-68.Sci. Rep., 9 (2019) 12452.CrystEngComm, 25 (2023)183-188.Sci. Rep., 13 (2023) 4639.

Elucidating the active phases of CoOx films on Au(111) in the CO oxidation reaction

Noble metals supported on reducible oxides, like CoOx and TiOx, exhibit superior activity in many chemical reactions, but the origin of the increased activity is not well understood. To answer this question we studied thin films of CoOx supported on an Au(111) single crystal surface as a model for the CO oxidation reaction. We show that three reaction regimes exist in response to chemical and topographic restructuring of the CoOx catalyst as a function of reactant gas phase CO/O2 stoichiometry and temperature. Under oxygen-lean conditions and moderate temperatures (≤150 °C), partially oxidized films (CoOx<1) containing Co0 were found to be efficient catalysts. In contrast, stoichiometric CoO films containing only Co2+ form carbonates in the presence of CO that poison the reaction below 300 °C. Under oxygen-rich conditions a more oxidized catalyst phase (CoOx>1) forms containing Co3+ species that are effective in a wide temperature range. Resonant photoemission spectroscopy (ResPES) revealed the unique role of Co3+ sites in catalyzing the CO oxidation. Density function theory (DFT) calculations provided deeper insights into the pathway and free energy barriers for the reactions on these oxide phases. These findings in this work highlight the versatility of catalysts and their evolution to form different active phases, both topological and chemically, in response to reaction conditions exposing a new paradigm in the catalyst structure during operation.

Electronic correlations in epitaxial CrN thin film

Chromium nitride (CrN) spurred enormous interest due to its coupled magnetostructural and unique metal-insulator transition. The underneath electronic structure of CrN remains elusive. Herein, the electronic structure of epitaxial CrN thin film has been explored by employing resonant photoemission spectroscopy (RPES) and X-ray absorption near edge spectroscopy study in combination with the first-principles calculations. The RPES study indicates the presence of a charge-transfer screened 3d ^n{underline{L}} (L: hole in the N-2p) and 3d ^{n-1} final-states in the valence band regime. The combined experimental electronic structure along with the orbital resolved electronic density of states from the first-principles calculations reveals the presence of Cr(3d)-N(2p) hybridized (3d ^n{underline{L}}) states between lower Hubbard (3d ^{n-1}) and upper Hubbard (3d ^{n+1}) bands with onsite Coulomb repulsion energy (U) and charge-transfer energy (Delta) estimated as approx 4.5 and 3.6 eV, respectively. It verifies the participation of ligand (N-2p) states in low energy charge fluctuations and provides concrete evidence for the charge-transfer (Delta<U) insulating nature of CrN thin film.

Resonant photoemission spectroscopy of atomic layer Fe2N on Cu(111) with continuous angular rotation of linearly polarized light

Linearly polarized soft x-rays provide information about electronic or magnetic anisotropy through absorption into materials or generation of photoelectrons. In order to change the relative angle between linear polarization and sample crystalline axes, either x-ray polarization or the sample needs to be rotated. Due to difficulties of polarization control in the soft x-ray range, a conventional approach was to rotate the sample. However, this method is not compatible, for example, with operando measurements on non-uniform samples where sample size and rotational motion are severely restricted. At BL07LSU of SPring-8, we developed a new method to rotate the linear polarization angle using a segmented cross undulator. We report an application of this linear polarization rotation to resonant photoemission spectroscopy on an magnetic atomic layer Fe2N on Cu(111) to probe the electronic anisotropy of the 3d states in the vicinity of the Fermi level.

Oxygen on-site Coulomb energy in Pr1.3−xLa0.7CexCuO4 and Bi2Sr2CaCu2O8+δ and its relation with Heisenberg exchange

We study the electronic structure of electron-doped Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_{4}$ (PLCCO ; $T_{c}$ = 27 K, x = 0.1) and hole-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212 ; $T_{c}$ = 90 K) cuprate superconductors using x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (Res-PES). From Res-PES across the O K-edge and Cu L-edge, we identify the O 2p and Cu 3d partial density of states (PDOS) and their correlation satellites which originate in two-hole Auger final states. Using the Cini-Sawatzky method, analysis of the experimental O 2p PDOS shows an oxygen on-site Coulomb energy for PLCCO to be $U_{p}$ = 3.3$\pm$0.5 eV and for Bi2212, $U_{p}$ = 5.6$\pm$0.5 eV, while the copper on-site Coulomb correlation energy, $U_{d}$ = 6.5$\pm$0.5 eV for Bi2212. The expression for the Heisenberg exchange interaction $J$ in terms of the electronic parameters $U_{d}$, $U_{p}$, charge-transfer energy $\Delta$ and Cu-O hopping $t_{pd}$ obtained from a simple Cu$_2$O cluster model is used to carry out an optimization analysis consistent with $J$ known from scattering experiments. The analysis also provides the effective one band on-site Coulomb correlation energy $\tilde{U}$ and the effective hopping $\tilde{t}$. PLCCO and Bi2212 are shown to exhibit very similar values of $\tilde{U}$/$\tilde{t}$ $\sim$9-10, confirming the strongly correlated nature of the singlet ground state in the effective one-band model for both the materials.

The ALBA high-stability monochromator for VUV and soft X-rays

LOREA is ALBA’s beamline devoted to the investigation of solids electronic structure by means of Angle Resolved Photo-Emission Spectroscopy (ARPES). The beamline operates in the photon energy range 10-1000 eV with tuneable linear and circular polarizations produced by an APPLE II helical undulator. Thanks to its energy range and the high photon flux, LOREA is suitable for high resolution VUV ARPES investigations in the 10-200 eV range, while it is feasible to extend ARPES measurements to the 200-600 eV energy range (soft X-ray ARPES). Core level photoemission, resonant photoemission and X-ray absorption spectroscopies will be accessible in the whole energy range. The energy selection is obtained by an Hettrick-Underwood monochromator without entrance slit. The optical arrangement, with 3 spherical mirrors (SM) and 4 plane varied line-spaced (VLS) gratings, is able to cover the entire energy range of the beamline. The monochromator includes the motions to select and do the fine adjustments of the mirrors (pitch and height), and to scan the energy and select among the different gratings (grating pitch and grating horizontal translation). The monochromator has been designed by the ALBA team of engineers, and has been fully assembled and commissioned at the facility. Besides the required range and resolution performances, it has been designed to achieve high stability and reproducibility, and optimal performance of the optical surfaces under different heat loads and conditions. The cooling circuits of mirrors and gratings are mechanically decoupled from the optical elements. In the case of the gratings, heat load is removed by flexible copper straps connected from optics to rigid water lines, through temperature controller devices based on Peltier elements. The use of Peltier element allows stabilizing the temperature of the gratings to room temperature also under quickly varying heat loads. The gradients within the grating are well below the one degree, and the thermal equilibrium with the surrounding mechanics contributes the long-term stability of the system. The water circuit and the Peltiers, rest in an independent platform inside the vacuum chamber, that allows them drift freely with no effect on the position of optical elements. In the case of the mirrors, the water tubes and cooling pads are not pressed against the mirrors, but just in contact through a 0.1 mm thick pellicle of eutectic InGa. This allows for a very efficient heat transfer using a minimum contact surface sufficient to evacuate up to 60 W, and without any deformation of the mirrors. The mechanics are also designed so that no flexible loops are required, which contributes to a better vibration stability of the system. Mirrors and gratings can be removed from the monochromator inside their holders and with the cooling scheme installed on it. This is a mandatory goal of design, as it is necessary for a careful installation and control of surface deformations at the optics laboratory. In this contribution we describe the main features of the monochromator that allow reaching the target performances, especially those concerning the cooling scheme. And also, we provide details about the positioning mechanics of the optical elements, the energy scanning mechanism and the vacuum system. The monochromator has been already mounted and installed and it is already in operation. The first results of the He photoionization spectra shows an energy resolution better than 10meV at 60eV, with a strong ionization signal and very low noise.

Nonmonotonous temperature fluctuations of the orbital moment and spin-orbit coupling in multiferroic gallium ferrite thin films

Temperature dependent x-ray absorption spectroscopy and x-ray magnetic circular dichroism experiments were performed at the Fe ${L}_{2,3}$ edges to unveil the magnetic orbital moment and spin-orbit coupling (SOC) behavior in thin films of the multiferroic ${\mathrm{Ga}}_{0.6}{\mathrm{Fe}}_{1.4}{\mathrm{O}}_{3}$ (GFO) compound. A thorough analysis of the data evidenced a nonmonotonous behavior for both magnetic orbital moment and SOC with a minimum at approximately 120 K, accompanied with indications of disturbances of the magnetic exchange and electric field dipole. Resonant photoemission spectroscopy experiments performed at the Fe ${L}_{2,3}$ edges rendered an important modification of the crystal field configuration around this temperature value, pointing to a temperature-driven structural trigonal distortion. These instabilities can be compared to those observed in other iron oxides, in particular, during transitions such as the Verwey transition, with the difference that on GFO they do not have any impact on the macroscopic magnetic properties, such as the coercive field and the global magnetization. This work demonstrates that the absence of instabilities in magnetism at the macroscopic scale does not guarantee the absence of fluctuations at the atomic scale. Such atomic-scale fluctuations of the SOC are undoubtedly shown here and have an impact on the spin Hall magnetoresistance response of the films. This point is of importance for prospective applications of GFO in spin-orbitronics.

Influence of Excess Charge on Water Adsorption on the BiVO4(010) Surface.

We present a combined computational and experimental study of the adsorption of water on the Mo-doped BiVO4(010) surface, revealing how excess electrons influence the dissociation of water and lead to hydroxyl-induced alterations of the surface electronic structure. By comparing ambient pressure resonant photoemission spectroscopy (AP-ResPES) measurements with the results of first-principles calculations, we show that the dissociation of water on the stoichiometric Mo-doped BiVO4(010) surface stabilizes the formation of a small electron polaron on the VO4 tetrahedral site and leads to an enhanced concentration of localized electronic charge at the surface. Our calculations demonstrate that the dissociated water accounts for the enhanced V4+ signal observed in ambient pressure X-ray photoelectron spectroscopy and the enhanced signal of a small electron polaron inter-band state observed in AP-ResPES measurements. For ternary oxide surfaces, which may contain oxygen vacancies in addition to other electron-donating dopants, our study reveals the importance of defects in altering the surface reactivity toward water and the concomitant water-induced modifications to the electronic structure.

Study on the magnetic origin in p-type ferromagnetic semiconductor (Ga,Fe)Sb: ab initio calculations

The electronic and magnetic properties of Ga 1−x Fe x Sb (x=0.125, 0.25, 0.5) have been investigated using the first-principles calculations based on density functional theory (DFT) and DFT plus on-site repulsion scheme (DFT+U). We found that the Coulomb U parameter plays an important role in predicting the correct stable magnetic order. Using the DFT+U method, the ferromagnetic (FM) ordering is found to be the magnetic ground state for both concentrations 12.5% and 25%, where the DFT failed without on-site repulsion U-term correction. This failure of semi-local DFT functionals is the fingerprint of the correlated nature of Fe-3d electrons. This result is in good agreement with experiment. Increasing Fe concentration up to 50% changing the magnetic ground state from FM to antiferromagnetic (AF) phase. We have used the Heyd-Scuseria-Ernzerhof (HSE) method to calculate the unfolded band structure of supercell for Ga 1−x Fe x Sb for x=0.125, in both FM and AF phases. The location of Fe-3d states in the calculation of the unfolded band structure showed a good accordance with recent resonance photoemission spectroscopy (RPES) experimental results. We have found that hole carriers have an important role at Fermi level E F . The rules of thumb to predict the stability of the ferromagnetism in Fe doped GaSb FMS have been discussed.

Development of magnetism in Fe-doped magnetic semiconductors: Resonant photoemission and x-ray magnetic circular dichroism studies of (Ga,Fe)As

Fe-doped III-V ferromagnetic semiconductors (FMSs) such as (In,Fe)As, (Ga,Fe)Sb, (In,Fe)Sb, and (Al,Fe)Sb are promising materials for spintronic device applications because of the availability of both $\mathrm{n}$- and $\mathrm{p}$-type materials and the high Curie temperatures. On the other hand, (Ga,Fe)As, which has the same zinc-blende crystal structure as the Fe-doped III-V FMSs, shows paramagnetism. The origin of the different magnetic properties in the Fe-doped III-V semiconductors remains to be elucidated. To address this issue, we use resonant photoemission spectroscopy (RPES) and x-ray magnetic circular dichroism (XMCD) to investigate the electronic and magnetic properties of the Fe ions in a paramagnetic $({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05})\mathrm{As}$ thin film. The observed Fe $2p\text{\ensuremath{-}}3d$ RPES spectra show that the Fe $3d$ states are similar to those of ferromagnetic (Ga,Fe)Sb. The estimated Fermi level is located in the middle of the band gap in (Ga,Fe)As. The Fe ${L}_{2,3}$ XMCD spectra of $({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05})\mathrm{As}$ show preedge structures, which are not observed in the Fe-doped FMSs, indicating that the minority-spin (\ensuremath{\downarrow}) ${e}_{\ensuremath{\downarrow}}$ states are vacant in $({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05})\mathrm{As}$. The XMCD results suggest that the carrier-induced ferromagnetic interaction in $({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05})\mathrm{As}$ is short ranged and weaker than that in the Fe-doped FMSs. The experimental findings suggest that the electron occupancy of the ${e}_{\ensuremath{\downarrow}}$ states contributes to the appearance of ferromagnetism in the Fe-doped III-V semiconductors, for $\mathrm{p}$-type as well as $\mathrm{n}$-type compounds.

Intervalence charge transfer and charge transport in the spinel ferrite ferromagnetic semiconductor Ru-doped CoFe2O4

Inter-valence charge transfer (IVCT) is electron transfer between two metal $M$ sites differing in oxidation states through a bridging ligand: $M^{n+1} + M'^{m} \rightarrow M^{n} + M'^{m+1}$. It is considered that IVCT is related to the hopping probability of electron (or the electron mobility) in solids. Since controlling the conductivity of ferromagnetic semiconductors (FMSs) is a key subject for the development of spintronic device applications, the manipulation of the conductivity through IVCT may become a new approach of band engineering in FMSs. In Ru-doped cobalt ferrite CoFe$_2$O$_4$ (CFO) that shows ferrimagnetism and semiconducting transport properties, the reduction of the electric resistivity is attributed to both the carrier doping caused by the Ru substitution for Co and the increase of the carrier mobility due to hybridization between the wide Ru $4d$ and the Fe $3d$ orbitals. The latter is the so-called IVCT mechanism that is charge transfer between the mixed valence Fe$^{2+}$/Fe$^{3+}$ states facilitated by bridging Ru $4d$ orbital: Fe$^{2+}$ + Ru$^{4+}$ $\leftrightarrow$ Fe$^{3+}$ + Ru$^{3+}$. To elucidate the emergence of the IVCT state, we have conducted x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RPES) measurements on non-doped CFO and Co$_{0.5}$Ru$_{0.5}$Fe$_2$O$_4$ (CRFO) thin films. The observations of the XAS and RPES spectra indicate that the presence of the mixed valence Fe$^{2+}$/Fe$^{3+}$ state and the hybridization between the Fe $3d$ and Ru $4d$ states in the valence band. These results provide experimental evidence for the IVCT state in CRFO, demonstrating a novel mechanism that controls the electron mobility through hybridization between the $3d$ transition-metal cations with intervening $4d$ states.

Investigation of Electronic States and Magnetic Domain Structure of La1 – xSmxMn2Si2 (x = 0, 0.25) Layered Intermetallic Compounds by Resonant Photoemission Spectroscopy and Magnetic Force Microscopy

Investigation of Electronic States and Magnetic Domain Structure of La1 – xSmxMn2Si2 (x = 0, 0.25) Layered Intermetallic Compounds by Resonant Photoemission Spectroscopy and Magnetic Force Microscopy

Polarized dependence of soft X-ray absorption near edge structure of ZnO films implanted by Yb

Virgin and Yb-implanted epitaxial ZnO films grown using atomic layer deposition (ALD) were investigated by X-ray absorption spectroscopy (XAS). XAS study revealed a strong polarization dependence of films determined by the orientation of the polarization vector of the synchrotron radiation to the sample surface. It also indicated that the implantation and subsequent annealing have an important influence on the native point defect complexes in the ZnO. Comparison of experimental spectra with the modelled ones, which are computed based on the linear combination of model spectra corresponding to the selected point defects and their complexes, confirmed the presence of donor-acceptor complexes (mVZn - nVO, m = 1,4; n = 1,2) in the samples under study. The mechanism of vacancy complexes formation is unclear as it takes place under non-equilibrium conditions, for which any theoretical method has not been well established. Exploring the 3 d → 4 f absorption, it was found that oxidation state of Yb in ZnO is 3+, which is consistent with the XPS findings and previously conducted Resonant Photoemission Spectroscopy (RPES) investigations. The inversion of the polarization dependence for samples with different Yb fluences visible in Yb M5 spectra can be associated with a tilt of the oxygen pseudo octahedra or/and with their distortion. The analysis of the presented data suggests that the donor-acceptor complexes are present both in as grown and implanted films and may influence their electrical properties. This suggestion was confirmed by previous Hall measurements showing that the resistivity of annealed ZnO:Yb film with a fluence of 5e15 ions/cm2 decreases by about one order compared to the one with a fluence of 5e14 ions/cm2.

Redox-mediated C–C bond scission in alcohols adsorbed on CeO2− x thin films

The decomposition mechanisms of ethanol and ethylene glycol on well-ordered stoichiometric CeO2(111) and partially reduced CeO2−x (111) films were investigated by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, and temperature programmed desorption. Both alcohols partially deprotonate upon adsorption at 150 K and subsequent annealing yielding stable ethoxy and ethylenedioxy species. The C–C bond scission in both ethoxy and ethylenedioxy species on stoichiometric CeO2(111) involves formation of acetaldehyde-like intermediates and yields CO and CO2 accompanied by desorption of acetaldehyde, H2O, and H2. This decomposition pathway leads to the formation of oxygen vacancies. In the presence of oxygen vacancies, C–O bond scission in ethoxy species yields C2H4. In contrast, C–C bond scission in ethylenedioxy species on the partially reduced CeO2−x (111) is favored with respect to C–O bond scission and yields methanol, formaldehyde, and CO accompanied by the desorption of H2O and H2. Still, scission of C–O bonds on both sides of the ethylenedioxy species yields minor amounts of accompanying C2H4 and C2H2. C–O bond scission is coupled with a partial recovery of the lattice oxygen in competition with its removal in the form of water.

Electronic structure of rare-earth semiconducting ErN thin films determined with synchrotron radiation photoemission spectroscopy and first-principles analysis

Erbium nitride (ErN) is an emerging rocksalt rare-earth semiconducting pnictide and has attracted significant interest in recent years for its potential applications in thermoelectric energy conversion, spintronic devices, and for the Gifford-McMahon cryocoolers. Due to the Er intra-$4f$ electronic transition, Er-doped III-nitride semiconductors such as GaN, InGaN, etc. exhibit strong emission in the retina-safe and fiber optical communication wavelength window of 1.54 \ensuremath{\mu}m that is researched extensively for developing solid-state lasers, amplifiers, and light-emitting devices. However, due to ErN's propensity for oxidation in ambient, high-quality ErN thin film growth has been challenging and an in-depth understanding of its electronic structure remains unanswered. In this work, the valence band electronic structure of ErN thin films is measured with normal as well as with resonant synchrotron-radiation photoemission spectroscopy. Photoemission measurements show a valence band maximum and Fermi energy difference of \ensuremath{\sim}2.3 eV in ErN. First-principles density functional theory (DFT) calculations are performed not only to explain the valence band electronic structure but also to determine transport properties such as effective mass and deformation potentials. Strong localized Er-$4f$ states are observed \ensuremath{\sim}6--8 eV below the valence band maxima and the valence band edge is found to exhibit N-$2p$ character. Resonant photoemission data corroborates the DFT calculations. To accurately capture the electronic structure in modeling, beyond generalized gradient approximation (GGA) methods such as (a) Heyd-Scuseria-Ernzerhof hybrid functional and (b) GGA+U Hubbard correction schemes are utilized. Determination of the electronic structure of ErN marks significant progress in developing ErN-based electronic, optoelectronic, and thermoelectric devices.

Electronic structure of superconducting nickelates probed by resonant photoemission spectroscopy

The discovery of infinite-layer nickelate superconductors has spurred enormous interest. While the Ni$^{1+}$ cations possess nominally the same 3$d^9$ configuration as Cu$^{2+}$ in cuprates, the electronic structure variances remain elusive. Here, we present a soft x-ray photoemission spectroscopy study on parent and doped infinite-layer Pr-nickelate thin films with a doped perovskite reference. By identifying the Ni character with resonant photoemission and comparison to density functional theory + U (on-site Coulomb repulsion energy) calculations, we estimate U ~5 eV, smaller than the charge transfer energy $\Delta$ ~8 eV, confirming the Mott-Hubbard electronic structure in contrast to charge-transfer cuprates. Near the Fermi level ($E_F$), we observe a signature of occupied rare-earth states in the parent compound, which is consistent with a self-doping picture. Our results demonstrate a correlation between the superconducting transition temperature and the oxygen 2$p$ hybridization near $E_F$ when comparing hole-doped nickelates and cuprates.

Role of electronic excitation on the anomalous magnetism of elemental copper

The magnetic susceptibility of elemental copper (Cu) shows an anomalous rise at low temperatures superimposed on the expected atypical diamagnetic response. Such temperature-dependent susceptibility, which is also known as the Curie tail, cannot be explained on the basis of the Larmor diamagnetic and Pauli paramagnetic contributions expected in Cu. Using valence band resonant photoemission spectroscopy results and density functional theory calculations, we show the magnetic anomaly appears due to the presence of holes in the Cu $3d$ band, which originates from a thermally excited electronic configuration. Our study therefore highlights that the Curie tail, which is generally overlooked presuming it is either due to paramagnetic impurities or defects, can in fact be intrinsic to a material, and even simple systems such as elemental Cu are susceptible to electronic excitations giving rise to an anomalous magnetic state.

Electronic structure modification in Fe-substituted β-Ga2O3 from resonant photoemission and soft x-ray absorption spectroscopies

Oriented thin films of β-(Ga1−x Fe x )2O3 were deposited by radio frequency magnetron sputtering on c-Al2O3 and GaN substrates. The itinerant character of the Fe 3d states forming the top of the valence band (VB) of the Fe-substituted β-Ga2O3 thin films has been determined from resonant photoelectron spectroscopy. Further, the admixture of the itinerant and localized characters of these Fe 3d states has been obtained for larger binding energies; i.e. deeper in the VB. The bottom of the conduction band (CB) for β-(Ga1−x Fe x )2O3 has been also found to have strongly hybridized states involving Fe 3d and O 2p states compared to that of Ga 4s in pristine β-Ga2O3. This suggests that β-Ga2O3 transforms from a band-like system to a charge-transfer system with Fe substitution. Furthermore, the bandgap red shifts with Fe composition, which has been found to be primarily related to the shift of the CB edge.