In spite of extensive studies on NiO and their accomplishments, the rich physics still raises unsolved physical problems. In particular, the nature of the two-peak structure in the valence band photoemission spectra is still controversial. By using \textit{ab initio} LQSGW+DMFT, the two-peak structure is shown to be driven by the concerted effect of antiferromagnetic ordering and intersite electron hopping. Magnetic ordering in the Ni-$e_{g}$ orbitals splits majority- and minority-spin Ni-$t_{2g}$ levels due to local Hund's coupling. Strong hybridization between O-$p$ and Ni-$e_g$, a signature of the Zhang-Rice bound state formation, boosts oxygen-mediated intersite Ni-$e_g$ orbital hopping, resulting in the enhancement of majority-spin Ni $t_{2g}$-$e_{g}$ splitting. Interestingly, these two splittings of distinct physical origins match and give rise to the observed two-peak structure in NiO. Our new understanding should be useful in designing advanced devices based on the NiO for the hole transport.
We have re-examined the valence-band (VB) and core-level electronic structure of NiO by means of hard and soft x-ray photoemission spectroscopy (PES). The spectral weight of the lowest energy state found to be enhanced in the bulk sensitive Ni 2p core-level PES. A configuration-interaction model including the bound state screening has shown significant agreement with the core-level spectra, and the off and on-resonance VB spectra. These results identify the lowest energy state in core-level and VB-PES as the Zhang-Rice doublet bound state, consistent with the spin-fermion model and recent ab i…
We have performed a depth-profile analysis of an all-oxide p-n junction diode n-ZnO/p-NiO using photoemission spectroscopy combined with Ar-ion sputtering. Systematic core-level shifts were observed during the gradual removal of the ZnO overlayer, and were interpreted using a simple model based on charge conservation. Spatial profile of the potential around the interface was deduced, including the charge-depletion width of 2.3 nm extending on the ZnO side and the built-in potential of 0.54 eV.
We have studied the chemical potential shift in La$_{2-x}$Sr$_x$NiO$_4$ and the charge ordering transition in La$_{1.67}$Sr$_{0.33}$NiO$_4$ by photoemission spectroscopy. The result shows a large ($\sim$ 1 eV/hole) downward shift of the chemical potential with hole doping in the high-doping regime ($δ\gtrsim$ 0.33) while the shift is suppressed in the low-doping regime ($δ\lesssim$ 0.33). This suppression is attributed to a segregation of doped holes on a microscopic scale when the hole concentration is lower than $δ\simeq 1/3$. In the $δ= 1/3$ sample, the photoemission intensity at the chemic…
We use hard x-ray photoemission spectroscopy (HAXPES) to investigate the electronic structure of YbAl2, for which the Yb valence has not been consistently reported to date. The bulk sensitivity and the analytical simplicity provided by the Yb 3d core-level HAXPES allow a reliable determination of the mean valence of Yb ions. For YbAl2, it is evaluated to be +2.20, which remains nearly unchanged below 300 K. The Kondo resonance peak with an extremely high Kondo temperature (above 2000 K) is clearly identified in the valence-band spectra. The results indicate that a coherent Kondo state can be r…
Hard x-ray photoemission and optical spectroscopies have been performed on YbS and Yb metal to determine the precise $f$-electron occupation. A comparison of the photoemission spectra with the energy loss functions in bulk and surface, obtained from optical reflectivity, enables us to distinguish between the energy loss satellite of Yb$^{2+}$ peak and Yb$^{3+}$ multiplet. The results clearly indicate a purely divalent Yb state except for the surface of YbS. We demonstrate that the present method is highly reliable in identifying the electronic structure and the mean valence in $f$-electron sys…
The accurate ab-initio simulation of periodic solids with strong correlations is one of the grand challenges of condensed matter. While mature methods exist for weakly correlated solids, the ab-initio description of strongly correlated systems is an active field of research. In this work, we show results for the single particle spectral function of the two correlated $d$-electron solids NiO and MnO from self-energy embedding theory. Unlike earlier work, the theory does not use any adjustable parameters and is fully ab-initio, while being able to treat both the strong correlation and the non-lo…
We report on a comparative study of the valence band electronic structure of NiO as bulk material and of NiO as impurity in MgO. From the impurity we have been able to determine reliably the parameters which describe the local correlations, thereby establishing the compensated-spin character of the first ionization state or the state created by hole doping. Using bulk-sensitive x-ray photoemission we identify pronounced satellite features in the valence band of bulk NiO which cannot be explained by single-site many body approaches nor by mean field calculations. We infer the presence of screen…
Perovskite-type BiNiO$_3$ is an insulating antiferromagnet in which a charge disproportionation occurs at the Bi site. La substitution for Bi suppresses the charge disproportionation and makes the system metallic. We have measured the photoemission and x-ray absorption (XAS) spectra of Bi$_{1-x}$La$_{x}$NiO$_{3}$ to investigate how the electronic structure changes with La doping. From Ni $2p$ XAS, we observed an increase of the valence of Ni from 2+ toward 3+. Combined with the core-level photoemission study, it was found that the average valence of Bi remains $\sim 4+$ and that the Ni valence…
We present experimental and theoretical results for the variation of the O 1s intensity from a NiO(001) surface as the excitation energy is varied through the Ni 2p1/2,3/2 absorption resonances, and as the incidence angle of the radiation is varied from grazing to larger values. For grazing incidence, a strong multi-atom resonant photoemission (MARPE) effect is seen on the O 1s intensity as the Ni 2p resonances are crossed, but its magnitude decreases rapidly as the incidence angle is increased. Resonant x-ray optical (RXRO) calculations are found to predict these effects very well, although t…
Using a combination of {\it ab initio} bandstructure methods and dynamical mean-field theory we study the single-particle spectrum of the prototypical charge-transfer insulator NiO. Good agreement with photoemission and inverse-photoemission spectra is obtained for both stoichiometric and hole-doped systems. In spite of a large Ni-$d$ spectral weight at the top of the valence band the doped holes are found to occupy mainly the ligand $p$ orbitals. Moreover, high hole doping leads to a significant reconstruction of the single-particle spectrum accompanied by a filling of the correlation gap.
The correlated-band theory implemented as a combination of the local-density approximation with the dynamical mean-field theory is applied to PuO2. An insulating electronic structure, consistent with the experimental valence-band photoemission spectra, is obtained. The calculations yield a nonmagnetic ground state that is characterized by a noninteger filling of the plutonium 5f shell. The noninteger filling as well as the satellites appearing in the 4f core-level photoemission spectra originate in a sizable hybridization of the 5f shell with the 2p states of oxygen.
We have carried out a bulk-sensitive high-resolution photoemission experiment on Mn5Si3. The measurements are performed for both core level and valence band states. The Mn core level spectra are deconvoluted into two components corresponding to different crystallographic sites. The asymmetry of each component is of noticeable magnitude. In contrast, the Si 2p spectrum shows a simple Lorentzian shape with low asymmetry. The peaks of the valence band spectrum correspond well to the peak positions predicted by the former band calculation.
We perform a comprehensive theoretical study of the pressure-induced evolution of the electronic structure, magnetic state, and phase stability of the late transition metal monoxides MnO, FeO, CoO, and NiO using a fully charge self-consistent DFT+dynamical mean-field theory method. Our results reveal that the pressure-induced Mott insulator-to-metal phase transition in MnO-NiO is accompanied by a simultaneous collapse of local magnetic moments and lattice volume, implying a complex interplay between chemical bonding and electronic correlations. We compute the pressure-induced evolution of rela…
The valence level photoemission spectra in the Anderson impurity model for Ce compounds at zero temperature are studied as a function of the photon energy $ω$. Most of former studies on Ce compounds are based on the sudden approximation, which is valid in high energy region. For the photoemission in the adiabatic limit of low energy region, one should consider the dipole matrix elements and the hole-induced photoelectron scattering potential. We can manage it by combining the time-evolution formalism and the $1/N_f$ scheme in a large $f$-level degeneracy $N_f$. This gives the exact results as …
SmO thin film is a new Kondo system showing a resistivity upturn around 10 K and was theoretically proposed to have a topologically nontrivial band structure. We have performed hard x-ray and soft x-ray photoemission spectroscopy to elucidate the electronic structure of SmO. From the Sm 3$d$ core-level spectra, we have estimated the valence of Sm to be $\sim$2.96, proving that the Sm has a mixed valence. The valence-band photoemission spectra exhibit a clear Fermi edge originating from the Sm 5$d$-derived band. The present finding is consistent with the theory suggesting a possible topological…
We report the electronic structure of a prototypical valence fluctuation system, YbAl2, using angle-resolved photoemission spectroscopy. The observed band dispersions and Fermi surfaces are well described in terms of band structure calculations based on local density approximation. Strong hybridization between the conduction and 4f bands is identified on the basis of the periodic Anderson model. The evaluated small mass enhancement factor and the high Kondo temperature qualitatively agree with those obtained from thermodynamic measurements. Such findings suggest that the strong hybridization s…
Fundamental principles of mapping 3-dimensional quasiparticle dispersions in the valence band using angle-resolved photoemission spectroscopy are discussed. Such mapping is intrinsically limited in accuracy owing to damping of the final states, resulting in equivalent broadening in the surface-perpendicular wavevector. Mechanisms of the intrinsic accuracy are discussed in depth based on a physically transparent picture involving interplay of the final- and initial-state spectral functions, and illustrated by photoemission simulations and experimental examples. Other interesting effects of 3-di…
Electric conductivity and structural details of the hole-doped polaron conductor La0.9Sr0.1FeO3 (LSF10) are reported. The conductivity of a single crystal shows an exponential increase with temperature with a maxi-mum of 100 S/cm at 700 K and with activation energy of about 375 meV, and a decrease for higher temperatures that follows a power law. The exponential increase of the electric conductivity for 300 K {\leg} T {\leg} 700 is accompanied by a shift of spectral weight in the photoemission valence band towards the Fermi level, indicative of a strong electron-phonon interaction. The subsequ…
The photoemission spectrum and the specific heat are studied in the Hubbard model of small clusters including orbital degeneracy. We focus on the degeneracy and valence dependence in the limit of a large Coulomb interaction. For integer valence, it is found that the degeneracy increases the width of the photoemission spectrum and it reduces the contribution from the charge degrees of freedom to the specific heat. A deviation from integer valence reduces the width of the photoemission spectrum.
Electronic structures of the quantum critical superconductor beta-YbAlB4 and its polymorph alpha-YbAlB4 are investigated by using bulk-sensitive hard x-ray photoemission spectroscopy. From the Yb 3d core level spectra, the values of the Yb valence are estimated to be ~2.73 and ~2.75 for alpha- and beta-YbAlB4, respectively, thus providing clear evidence for valence fluctuations. The valence band spectra of these compounds also show Yb2+ peaks at the Fermi level. These observations establish an unambiguous case of a strong mixed valence at quantum criticality for the first time among heavy ferm…
Recent experiments indicate that the tendency toward the formation of unidirectional charge density waves ("stripes") is common to various underdoped cuprates. We discuss momentum-resolved spectral properties of valence-bond stripes, comparing the situations of ideal and short-range stripe order, the latter being relevant for weak and/or disorder-pinned stripes. We find clear signatures of ordered stripes, although matrix element effects suppress most shadow band features. With decreasing stripe correlation length, stripe signatures are quickly washed out, the only remaining effect being a bro…
Photoemission experiments involve the motion of an electron near a conducting surface. This necessarily generates heat by ohmic losses from eddy currents. This inelastic scattering of the electrons will result in a downward shift in observed spectra. This effect is most pronounced in poorly conducting metals: in good metals the electron's field is screened out of the material, while insulators are by definition unable to absorb electromagnetic energy at low frequencies. We give a classification of photoemission processes which shows that the effect is an extrinsic proces distinct from final st…
A single crystal monolith of La0.9Sr0.1FeO3 and thin pulsed laser deposited film of La0.8Sr0.2Fe0.8Ni0.2O3 were subject to angle integrated valence band photoemission spectroscopy in ultra high vacuum and conductivity experiments in ambient air at temperatures from 300 K to 800 K. Except for several sputtering and annealing cycles, the specimen were not prepared in-situ.. Peculiar changes in the temperature dependent, bulk representative conductivity profile as a result of reversible phase transitions, and irreversible chemical changes are semi-quantitatively reflected by the intensity variati…
Iron resonant valance band photoemission spectra of Sr substituted LaFe0.75Ni0.25 O3-δ have been recorded across the Fe 2p - 3d absorption threshold to obtain Fe specific spectral information on the 3d projected partial density of states. Comparison with La1-xSrxFeO3 resonant VB PES literature data suggests that substitution of Fe by Ni forms electron holes which are mainly O 2p character. Substitution of La by Sr increases the hole concentration to an extent that the eg structure vanishes. The variation of the eg and t2g structures is paralleled by the changes in the electrical conductivity.
We present a method for the calculation of photoemission spectra in terms of reduced density matrices. We start from the spectral representation of the one-body Green's function G, whose imaginary part is related to photoemission spectra, and we introduce a frequency-dependent effective energy that accounts for all the poles of G. Simple approximations to this effective energy give accurate spectra in model systems in the weak as well as strong correlation regime. In real systems reduced density matrices can be obtained from reduced density-matrix functional theory. Here we use this approach t…
We report photoemission studies of polycrystalline samples of Sr-doped GdTiO_{3+delta}, which undergoes a Mott-Hubbard-like metal-insulator transition at a Ti-concentration of about 20%. The Ti 3d-derived valence band near the Fermi level displays a two-peak structure consistent with a Hubbard-model-type interpretation as quasiparticle and lower Hubbard band. Contrary to the theoretical expectation the lower Hubbard band does not change its energy with doping. However, the analysis of the core level spectra and elemental mapping with a scanning electron microscope indicate a strongly inhomogen…
We study the mixed valence transition ($T$$_{v}$ $\sim$80 K) in EuNi$_{2}$(Si$_{0.2}$Ge$_{0.8}$)$_{2}$ using Eu 3$d-4f$ X-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RESPES). The Eu$^{2+}$ and Eu$^{3+}$ main peaks show a giant resonance and the spectral features match very well with atomic multiplet calculations. The spectra show dramatic temperature ($T$)-dependent changes over large energies ($\sim$10 eV) in RESPES and XAS. The observed non-integral mean valencies of $\sim$2.35 $\pm$ 0.03 ($T$ = 120 K) and $\sim$2.70 $\pm$ 0.03 ($T$ = 40 K) indicate homogeneous…
By means of photoemission and x-ray absorption spectroscopy, we have studied the electronic structure of (Ni,Zn,Fe,Ti)$_{3}$O$_{4}$ thin films, which exhibits a cluster glass behavior with a spin-freezing temperature $T_f$ of $\sim 230$ K and photo-induced magnetization (PIM) below $T_f$. The Ni and Zn ions were found to be in the divalent states. Most of the Fe and Ti ions in the thin films were trivalent (Fe$^{3+}$) and tetravalent (Ti$^{4+}$), respectively. While Ti doping did not affect the valence states of the Ni and Zn ions, a small amount of Fe$^{2+}$ ions increased with Ti concentrati…
The experimental valence band photoemission spectrum of semiconductors exhibits multiple satellites that cannot be described by the GW approximation for the self-energy in the framework of many-body perturbation theory. Taking silicon as a prototypical example, we compare experimental high energy photoemission spectra with GW calculations and analyze the origin of the GW failure. We then propose an approximation to the functional differential equation that determines the exact one-body Green's function, whose solution has an exponential form. This yields a calculated spectrum, including cross …
We have investigated the electronic states of the uranium monochalcogenide US, which is a typical ferromagnetic uranium compound, using soft x-ray photoemission spectroscopy (SX-PES). In early ultraviolet photoemission spectroscopy studies, two peak structures of the U 5f states were observed and have been interpreted that one has an itinerant character around the Fermi level (EF) and the other located below EF has a character of localized U 5f electrons. In this study, the intrinsic bulk valence-band spectrum of US was first deduced by estimating the contribution of surface states to the vale…
Time-resolved ARPES gives access to the band structure and ultrafast dynamics of excited electronic states in solids. The orbital character of the bands close to the Fermi level is essential to understand the origin of several exotic phenomena in quantum materials. By performing polarization dependent time- and angle-resolved photoemission spectroscopy and by analyzing the chirality of the photoelectron yield for two different crystal orientations, we identify the orbital character of bands below and above the chemical potential for the Dirac semimetal BaNiS2. Our results illustrate how the co…
The temperature ($T$) dependent metal-insulator transition (MIT) in VO$_2$ is investigated using bulk sensitive hard x-ray ($\sim$ 8 keV) valence band, core level, and V 2$p-3d$ resonant photoemission spectroscopy (PES). The valence band and core level spectra are compared with full-multiplet cluster model calculations including a coherent screening channel. Across the MIT, V 3$d$ spectral weight transfer from the coherent ($d^1\underbar{\it {C}}$ final) states at Fermi level to the incoherent ($d^{0}+d^1\underbar{\it {L}}$ final) states, corresponding to the lower Hubbard band, lead to gap-fo…
Charge excitations across an electronic band gap play an important role in opto-electronics and light harvesting. In contrast to conventional semiconductors, studies of above-band-gap photoexcitations in strongly correlated materials are still in their infancy. Here we reveal the ultrafast dynamics controlled by Hund's physics in strongly correlated photo-excited NiO. By combining time-resolved two-photon photoemission experiments with state-of-the-art numerical calculations, an ultrafast ($\lesssim$ 10\,fs) relaxation due to Hund excitations and related photo-induced in-gap states are identif…
The strain effect from a substrate is an important experimental route to control electronic and magnetic properties in transition-metal oxide (TMO) thin films. Using hard x-ray photoemission spectroscopy, we investigate the strain dependence of the valence states in LaNiO$_{3}$ thin films, strongly correlated perovskite TMO, grown on four substrates: LaAlO$_{3}$, (LaAlO$_{3}$)$_{0.3}$(SrAl$_{0.5}$Ta$_{0.5}$O$_{3}$)$_{0.7}$, SrTiO$_{3}$, and DyScO$_{3}$. A Madelung potential analysis of core-level spectra suggests that the point-charge description is valid for the La ions while it breaks down f…
We have revisited the valence band electronic structure of NiO by means of hard x-ray photoemission spectroscopy (HAXPES) together with theoretical calculations using both the GW method and the local density approximation + dynamical mean-field theory (LDA+DMFT) approaches. The effective impurity problem in DMFT is solved through the exact diagonalization (ED) method. We show that the LDA+DMFT method alone cannot explain all the observed structures in the HAXPES spectra. GW corrections are required for the O bands and Ni-s and p derived states to properly position their binding energies. Our r…
We present a comprehensive study of the photon energy dependence of the valence band photoemission yield in the prototype Mott-Hubbard oxide V2O3. The analysis of our experimental results, covering an extended photon energy range (20-6000 eV) and combined with GW calculations, allow us to identify the nature of the orbitals contributing to the total spectral weight at different binding energies, and in particular to locate the V 4s at about 8 eV binding energy. From this comparative analysis we can conclude that the intensity of the quasiparticle photoemission peak, observed close to the Fermi…
The electronic structure of pulsed laser deposited Mo1-xFexO2 (x=0, 0.02 and 0.05) thin films has been investigated using resonant photoemission spectroscopy near Mo-4p absorption edge. In all the samples a broad Fano- like resonance peak at ~46 eV is observed in the whole area of the valence band which indicates the contribution of the Mo-4d states in the entire valence band region. The doping of Fe in these films leads to decrease in Mo 4d states contributing to electronic states at lower binding energy region. In addition to this, we also observe a shoulder at 4.9 eV in the valence band spe…
Samarium hexaboride (SmB$_6$), a Kondo insulator with mixed valence, has recently attracted much attention as a possible host for correlated topological surface states. Here, we use a combination of x-ray absorption and reflectometry techniques, backed up with a theoretical model for the resonant $M_{4,5}$ absorption edge of Sm and photoemission data, to establish laterally averaged chemical and valence depth profiles at the surface of SmB$_6$. We show that upon cleaving, the highly polar (001) surface of SmB$_6$ undergoes substantial chemical and valence reconstruction, resulting in boron ter…
We present the results of high-resolution valence-band photoemission spectroscopic study of SmB6 which shows evidence for a V-shaped density of states of surface origin within the bulk gap. The spectroscopy data is interpreted in terms of the existence of heavy 4f surface states, which may be useful in resolving the controversy concerning the disparate surface Fermi-surface velocities observed in experiments. Most importantly, we find that the temperature dependence of the valence-band spectrum indicates that a small feature appears at a binding energy of about -9 meV at low temperatures. We a…
The bandstructure of the prototypical charge-transfer insulator NiO is computed by using a combination of an {\it ab initio} bandstructure method and the dynamical mean-field theory with a quantum Monte-Carlo impurity solver. Employing a Hamiltonian which includes both Ni-d and O-p orbitals we find excellent agreement with the energy bands determined from angle-resolved photoemission spectroscopy. This solves a long-standing problem in solid state theory. Most notably we obtain the low-energy Zhang-Rice bands with strongly k-dependent orbital character discussed previously in the context of lo…
Electronic structure of NiS_{2-x}Se_x system has been investigated for various compositions (x) using x-ray photoemission spectroscopy. An analysis of the core level as well as the valence band spectra of NiS_2 in conjunction with many-body cluster calculations provides a quantitative description of the electronic structure of this compound. With increasing Se content, the on-site Coulomb correlation strength (U) does not change, while the band width W of the system increases, driving the system from a covalent insulating state to a pd-metallic state.
Eu2Ir2O7, a candidate Weyl semimetal, shows an insulator-to-metal transition as a function of Bi substitution at the Eu site. In this work, we investigate the (Eu_1-xBi_x)2Ir2O7 series via Hard X-ray Photoemission Spectroscopy (HAXPES), where substitution of larger Bi3+ for Eu3+ is reported to result in an anomalous lattice contraction (20 %) for low Bi doping (3.5 %). Using HAXPES, we confirm that all the cations retain their nominal valence state throughout the series. The asymmetric nature of Bi core-level spectra for compositions in the metallic region indicates that Bi contributes to the …
Angle-resolved valence-band resonant photoemission of nickel metal has been measured close to the 2p core-level thresholds with synchrotron radiation. The well-known 6-eV correlation satellite has an intensity enhancement of about two orders of magnitude at resonance. The angular dependence of the photoemission intensity has been studied as function of photon energy and provides unambiguous evidence for interference effects all the way up to the resonance maximum. The observation of different angular asymmetries, β, for the valence band and the satellite is discussed in connection to the origi…
We have studied the electronic structure of RuP and related Ru pnictides using photoemission spectroscopy. Ru 3d core-level and valence-band spectra of RuP show that the Ru valence is +3 with t_{2g}^5 configuration. The photoemisson spectral weight near the Fermi level is moderately suppressed in the pseudogap phase of RuP, consistent with the pseudogap opening of 2Δ/k_BT_c ~ 3 (gap size Δ~ 50 meV and transition temperature T_c ~ 330 K). The Ru 3d peak remains sharp in the pseudogap phase and the insulating phase of RuP, suggesting that the electronic orderings responsible for the phase transi…
We have investigated the electronic structure of A-site ordered CaCu$_3$Ti$_4$O$_{12}$ as a function of temperature by using angle-integrated and -resolved photoemission spectroscopies. Intrinsic changes of the electronic structure have been successfully observed in the valence band region by the careful consideration of charging effects. The obtained photoemission results have revealed that the intensity of the nearly non-dispersive Cu 3$d$-O 2$p$ hybridized bands at the binding energy of $\sim$2 eV increases with decreasing temperature from 300 to 120 K. This suggests that the density of the…
We have performed x-ray magnetic circular dichroism (XMCD) and valence-band photoemission studies of the diluted ferromagnetic semiconductor Zn$_{1-x}$Cr$_x$Te. XMCD signals due to ferromagnetism were observed at the Cr 2p absorption edge. Comparison with atomic multiplet calculations suggests that the magnetically active component of the Cr ion was divalent under the tetrahedral crystal field with tetragonal distortion along the crystalline a-, b-, and c-axes. In the valence-band spectra, spectral weight near the Fermi level was strongly suppressed, suggesting the importance of Jahn-Teller ef…
We present a method which allows to include narrow-band correlation effects into the description of both valence and core states and we apply it to the prototypical case of nickel. The results of an ab-initio band calculation are used as input mean-field eigenstates for the calculation of self-energy corrections and spectral functions according to a three-body scattering solution of a multi-orbital Hubbard hamiltonian. The calculated quasi-particle spectra show a remarkable agreement with photoemission data in terms of band width, exchange splitting, satellite energy position of valence states…
We report high-resolution hard x-ray photoemission spectroscopy results on (Ga,Mn)As films as a function of Mn doping. Supported by theoretical calculations we identify, over the entire 1% to 13% Mn doping range, the electronic character of the states near the top of the valence band. Magnetization and temperature dependent core-level photoemission spectra reveal how the delocalized character of the Mn states enables the bulk ferromagnetic properties of (Ga,Mn)As.
We study the electronic structure of Tsai-type cluster-based quasicrystalline approximants, Au$_{64}$Ge$_{22}$Yb$_{14}$ (AGY-I), Au$_{63.5}$Ge$_{20.5}$Yb$_{16}$ (AGY-II), and Zn$_{85.4}$Yb$_{14.6}$ (Zn-Yb), by means of photoemission spectroscopy. In the valence band hard x-ray photoemission spectra of AGY-II and Zn-Yb, we separately observe a fully occupied Yb 4$f$ state and a valence fluctuation derived Kondo resonance peak, reflecting two inequivalent Yb sites, a single Yb atom in the cluster center and its surrounding Yb icosahedron, respectively. The fully occupied 4$f$ signal is absent in…
Three-dimensional band structure of unoccupied and occupied states of the prototype layered material TiTe2 is determined focusing on the GammaA line of the Brillouin zone. Dispersions and lifetimes of the unoccupied states, acting as the final states in the photoemission process, are determined from a very-low-energy electron diffraction experiment supported by first-principles calculations based on a Bloch waves treatment of multiple scattering. The experimental unoccupied states of TiTe2 feature dramatic non-free-electron effects such as multiband composition and non-parabolic dispersions. T…
We studied the electronic structure of the heavy fermion compound Yb(Ru$_{1-x}$Rh$_{x}$)$_2$Ge$_2$ with $x=0$ and nominally $x=0.125$ using ARPES and LDA calculations. We find a valence band structure of Yb corresponding to a non-integer valence close to $3+$. The three observed crystal electric field levels with a splitting of 32 and 75 meV confirm the suggested configuration with a quasi-quartet ground state. The experimentally determined band structure of the conduction electrons with predominantly Ru $4d$ character is well reproduced by our calculations. YbRu$_2$Ge$_2$ undergoes a non-magn…
We have studied the O 2p valence-band structure of Nb-doped SrTiO3, in which a dilute concentration of electrons are doped into the d0 band insulator, by angle-resolved photoemission spectroscopy (ARPES) measurements. We found that ARPES spectra at the valence band maxima at the M [k = (pi/a, pi/a, 0)]and R [k = (pi/a, pi/a, pi/a)] points start from ~ 3.3 eV below the Fermi level (EF), consistent with the indirect band gap of 3.3 eV and the EF position at the bottom of the conduction band. The peak position of the ARPES spectra were, however, shifted toward higher binding energies by ~ 500 meV…
We report a photoemission study at room temperature on BaFe2X3 (X = S and Se) and CsFe2Se3 in which two-leg ladders are formed by the Fe sites. The Fe 2p core-level peaks of BaFe2X3 are broad and exhibit two components, indicating that itinerant and localized Fe 3d sites coexist similar to KxFe2-ySe2. The Fe 2p core-level peak of CsFe2Se3 is rather sharp and is accompanied by a charge-transfer satellite. The insulating ground state of CsFe2Se3 can be viewed as a Fe2+ Mott insulator in spite of the formal valence of +2.5. The itinerant versus localized behaviors can be associated with the stabi…
We present measurements of the thermoelectric power of the plutonium-based unconventional superconductor PuCoGa$_{5}$. The data is interpreted within a phenomenological model for the quasiparticle density of states of intermediate valence systems and the results are compared with results obtained from photoemission spectroscopy. The results are consistent with intermediate valence nature of 5$f$-electrons, furthermore, we propose that measurements of the Seebeck coefficient can be used as a probe of density of states in this material, thereby providing a link between transport measurements and…
