The valence and spin state evolution of Mn and Co on TbMn$_{\rm 1-x}$Co$_{\rm x}$O$_3$ series is precisely determined by means of soft and hard x-ray absorption spectroscopy (XAS) and K$β$ x-ray emission spectroscopy (XES). Our results show the change from Mn$^{3+}$ to Mn$^{4+}$ both high-spin (HS) together with the evolution from Co$^{2+}$ HS to Co$^{3+}$ low-spin (LS) with increasing $\rm x$. In addition, high energy resolution XAS spectra on the K pre-edge region are interpreted in terms of the strong charge transfer and hybridization effects along the series. These results correlate well with the spin values of Mn and Co atoms obtained from the K$β$ XES data. From this study, we determine that Co enters into the transition metal sublattice of TbMnO$_3$ as a divalent ion in HS state, destabilizing the Mn long range magnetic order since very low doping compositions (${\rm x} \le 0.1$). Samples in the intermediate composition range ($0.4 \le {\rm x} \le 0.6$) adopt the crystal structure of a double perovskite with long range ferromagnetic ordering which is due to Mn$^{4+}$-O-Co$^{2+}$ superexchange interactions with both cations in HS configuration. Ferromagnetism vanishes for ${\rm x} \ge 0.7$ due to the structural disorder that collapses the double perovskite structure. The spectroscopic techniques reveal the occurrence of Mn$^{4+}$ HS and a fluctuating valence state Co$^{2+}$ HS/Co$^{3+}$ LS in this composition range. Disorder and competitive interactions lead to a magnetic glassy behaviour in these samples.
The photoinduced valence dynamics of EuNi$_2$(Si$_{0.21}$Ge$_{0.79}$)$_2$ are investigated using time-resolved X-ray absorption spectroscopy for Eu $M_5$-edge. Through the pump-probe technique with synchrotron X-ray and Ti:sapphire laser pulse, a photoinduced valence transition is observed from Eu$^{3+}$ to Eu$^{2+}$. Because the lifetime of a photoinduced state can be up to 3 ns, a metastable state is considered to be realized. By comparing the experimental results with the theoretical calculations, the photoinduced valence transition between Eu 4$f$ and conduction electrons is quantitatively…
We study resonant inelastic x-ray scattering (RIXS) peaks corresponding to low energy particle-hole excited states of metallic FeTe and semi-metallic TiSe$_2$ for photon incident energy tuned near the $L_{3}$ absorption edge of Fe and Ti respectively. We show that the cross section amplitudes are well described within a renormalization group theory where the effect of the core electrons is captured by effective dielectric functions expressed in terms of the the atomic scattering parameters $f_1$ of Fe and Ti. This method can be used to extract the dynamical structure factor from experimental R…
We present an x-ray absorption study of the oxidation states of transition-metal-ions of LiMnO2 and its related materials, widely used as cathodes in Li-ion batteries. The comparison between the obtained spectrum and the configuration-interaction cluster-model calculations showed that the Mn3+ in LiMnO2 is a mixture of the high-spin and low-spin states. We found that Li deficiencies occur in the case of Cr substitution, whereas there are no Li deficiencies in the case of Ni substitution. We conclude that the substitution of charge-transfer-type Ni or Cu is effective for LiMnO2 battery material…
Revealing fingerprints of valence excitons in x-ray absorption spectra with the Bethe-Salpeter equation
Universität Hamburg（汉堡大学）；The Hamburg Centre for Ultrafast Imaging (CUI)（汉堡超快成像中心）；Brandenburg University of Technology Cottbus-Senftenberg（勃兰登堡工业大学科特布斯-森夫滕贝格分校）
X-ray absorption spectroscopy was used to determine the valence state in La$_2$Co$_{1-x}$Mn$_{1+x}$O$_6$ ($x\approx 0.23$) thin films. We found that in spite of the non-stoichiometry, Co is in a divalent state while Mn ions show a mixed valence state. The relation of this finding with the magnetic properties of the films is discussed. X-ray magnetic circular dichroism measurements prove that magnetic anisotropy originates from Co spin-orbit coupling and it is strain-dependent: a strong increase of the angular contribution to the magnetic moment is found when in-plane (out-of-plane) and cell pa…
The Fe K X-ray absorption near edge structure (XANES) of BaFe2-xCoxAs2 superconductors was investigated. No appreciable alteration in shape or energy position of this edge was observed with Co substitution. This result provides experimental support to previous ab initio calculations in which the extra Co electron is concentrated at the substitute site and do not change the electronic occupation of the Fe ions. Superconductivity may emerge due to bonding modifications induced by the substitute atom that weakens the spin-density-wave ground state by reducing the Fe local moments and/or increasin…
Few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy, performed with optical 500-1000 nm supercontinuum and broadband XUV pulses (30-50 eV), simultaneously probes dynamics of photoexcited carriers in WS$_{2}$ at the W O$_3$ edge (37-45 eV) and carrier-induced modifications of core-exciton absorption at the W N$_{6,7}$ edge (32-37 eV). Access to continuous core-to-conduction band absorption features and discrete core-exciton transitions in the same XUV spectral region in a semiconductor provides a novel means to investigate the effect of carrier excitation on core-exciton…
In2O3 is a prototype wide-band-gap semiconductor that exhibits metallic conductivity when highly doped with Sn while retaining a high degree of transparency to visible light. It is widely used as a transparent window/electrode in solar cells and LEDs. The functionality of In2O3 would be greatly extended if p-type conductivity could also be achieved. Using electronic structure calculations, we show that adding Bi to In2O3, in the form of dilute (In_{1-x}Bi_{x})_2O3} alloys, leads to a new valence band that is sufficiently higher in energy than the original O-2p valence band to allow for p-type …
Multiferroic bismuth ferrite (BiFeO3) thin films were prepared by pulsed laser deposition (PLD) technique. Electronic structures of the film have been studied by in situ photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). Both the Fe 2p PES and XAS spectra show that Fe ion is formally in +3 valence state. The Fe 2p and O K edge XAS spectra indicate that the oxygen octahedral crystal ligand field splits the unoccupied Fe 3d state to t2g and eg states. Valence band Fe 2p-3d resonant photoemission results indicate that hybridization between Fe 3d and O 2p plays important rol…
The electronic structures of multiferroic oxides of Ba(1-x)Bi(x)Ti(0.9)Fe(0.1)O(3) (0 < x < 0.12) have been investigated by employing photoemission spectroscopy and soft x-ray absorption spectroscopy (XAS). The measured Fe and Ti 2p XAS spectra show that Ti ions are in the Ti4+ states for all x and that Fe ions are Fe2+-Fe3+ mixed-valent for x > 0. The valence states of Fe ions are found to be nearly trivalent for x=0, and decreases with increasing x from being nearly trivalent (v(Fe)~ 3) for x=0 to v(Fe)~ 2.6 for x=0.12. The valence states of both Ti and Ba ions do not change for all x < 0.12…
An experimental technique of x-ray fluorescence holography for investigating valence-selective local structures was established by employing the incident x-ray energy at a characteristic one near an x-ray absorption edge, and it was adopted to discriminate environments around Yb2+ and Yb3+ ions in a YbInCu4 valence transition material below and above the transition temperature of 42 K. The reconstructed images of the neighboring atoms around Yb3+ show a fcc structure as observed by diffraction experiments, whereas those around Yb2+ have curious cross (+) features, indicating a positional shift…
We report on the valence fluctuation of Ce in CeMo$_{2}$Si$_{2}$C as studied by means of magnetic susceptibility $χ(T)$, specific heat $C(T)$, electrical resistivity $ρ(T)$ and x-ray absorption spectroscopy. Powder x-ray diffraction revealed that CeMo$_{2}$Si$_{2}$C crystallizes in CeCr$_{2}$Si$_{2}$C-type layered tetragonal crystal structure (space group \textit{P4/mmm}). The unit cell volume of CeMo$_{2}$Si$_{2}$C deviates from the expected lanthanide contraction, indicating non-trivalent state of Ce ions in this compound. The observed weak temperature dependence of the magnetic susceptibili…
The crystal structures and the physical (magnetic, electrical transport and thermodynamic) properties of the ternary compounds CeRhSi2 and Ce2Rh3Si5 (orthorhombic CeNiSi2- and U2Co3Si5-type structures, respectively) were studied in wide ranges of temperature and magnetic field strength. The results revealed that both materials are valence fluctuating systems, in line with previous literature reports. Direct evidence for valence fluctuations was obtained by means of Ce LIII-edge x-ray absorption spectroscopy and Ce 3d core-level x-ray photoelectron spectroscopy. The experimental data were confr…
In order to probe the changes in the valence state and magnetic properties of Eu metal under extreme pressure, x-ray absorption near-edge spectroscopy, x-ray magnetic circular dichroism and synchrotron Mössbauer spectroscopy experiments have been carried out. The Mössbauer isomer shift exhibits an anomalous pressure dependence, passing through a maximum near 20 GPa. Density functional theory has been applied to give insight into the pressure-induced changes in both Eu's electronic structure and Mössbauer isomer shift. Contrary to previous reports, Eu is found to remain nearly divalent to the h…
We use x-ray absorption spectroscopy to investigate the local structure and electronic properties of bulk La$_{1-y}$Sr$_{y}$Co$_{1-x}$Nb$_x$O$_3$ ($y=$ 2$x$ as LSCNO and $y=$ 0 as LCNO) samples. The x-ray absorption near-edge spectra (XANES) of LSCNO at Co K-edge affirm the valence state of Co in 3+. However, in the case of the LCNO, a subtle variation in the valence state of Co ions from 3+ to 2+ is evident with Nb substitution. The detailed analysis of the Fourier transform (FT) of the extended x-ray absorption fine structure (EXAFS) for the LSCNO samples exhibit the two groups of the bond-l…
We have investigated the evolution of the electronic properties of La1-xSrxCrO3 (for the full range of x) epitaxial films deposited by molecular beam epitaxy (MBE) using x-ray diffraction, x-ray photoemission spectroscopy, Rutherford backscattering spectrometry, x-ray absorption spectroscopy, electrical transport, and ab initio modeling. LaCrO3 is an antiferromagnetic insulator whereas SrCrO3 is a metal. Substituting Sr2+ for La3+ in LaCrO3 effectively dopes holes into the top of valence band, leading to Cr4+ (3d2) local electron configurations. Core-level and valence-band features monotonical…
Hole-doping-induced changes in the electronic structure of La$_{1-x}$Sr$_x$FeO$_3$ : soft x-ray photoemission and absorption study of epitaxial thin films
We present an x-ray absorption study of the dependence of the V oxidation state on the thickness of LaVO$_3$ (LVO) and capping LaAlO$_3$ (LAO) layers in the multilayer structure of LVO sandwiched between LAO. We found that the change of the valence of V as a function of LAO layer thickness can be qualitatively explained by a transition between electronically reconstructed interfaces and a chemical reconstruction. The change as a function of LVO layer thickness is complicated by the presence of a considerable amount of V$^{4+}$ in the bulk of the thicker LVO layers.
Using circularly polarised radiation and a combination of bulk-sensitive hard X-ray photoelectron spectroscopy and X-ray-absorption spectroscopy (XAS) we studied the electronic and magnetic structure of epitaxial Mn$_{1.8}$Co$_{1.2}$Ga thin films. Spin resolved Bloch spectral functions, density of states as well as charge and magnetisation densities were investigated by a first-principles analysis of full potential, fully relativistic Korringa--Kohn--Rostoker calculations of the electronic structure. The valence states were experimentally investigated by using linear dichroism in the angular d…
Time-dependent density functional theory (TDDFT) simulations of transient core-level spectroscopies require a balanced treatment of both valence- and core-electron excitations. To this end, tuned range-separated hybrid exchange-correlation functionals within the generalized Kohn-Sham scheme offer a computationally efficient means of simultaneously improving the accuracy of valence and core excitation energies in TDDFT by mitigating delocalization errors across multiple length-scales. In this work range-separated hybrid functionals are employed in conjunction with the velocity-gauge formulation…
The electronic structure of (Ce,Yb)CoIn5 has been studied by a combination of photoemission, x-ray absorption and bulk property measurements. Previous findings of a Ce valence near 3+ for all x and of an Yb valence near 2.3+ for x>0.3 were confirmed. One new result of this study is that the Yb valence for x<0.2 increases rapidly with decreasing x from 2.3+ toward 3+, which correlates well with de Haas van Alphen results showing a change of Fermi surface around x=0.2. Another new result is the direct observation by angle resolved photoemission Fermi surface maps of about 50% cross sectional are…
We report a novel mechanism, consisting of redistribution of valence electrons near the Fermi level, during interactions of intense femtosecond X-ray pulses with a Co/Pd multilayer. The changes in Co 3d valence shell occupation were directly revealed by fluence-dependent changes of the Co L$_3$ X-ray absorption and magnetic circular dichroism spectra near the excitation threshold. The valence shell redistribution arises from inelastic scattering of high energy Auger electrons and photoelectrons that lead to transient holes below and electrons above the Fermi level on the femtosecond time scale…
X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 and (Pr,Y)0.55Ca0.45CoO3 compositions reveal that the valence of praseodymium ions is stable and essentially +3 (Pr [4f 2]) in the metallic state, but abruptly changes when carriers localize approaching the oxidation state +4 (Pr [4f 1]). This mechanism appears to be the driving force of the metal-insulator transition. The ground insulating state of Pr0.5Ca0.5CoO3 is an homogeneous Co3.5-d state stabilized by a charge transfer from Pr to Co sites: 1/2Pr3+ + Co3.5 \to 1/2Pr3+2d + Co3.5-d, with 2d \approx 0.26 e-.
We present a combined experimental and theoretical study of the electronic structure of the intermetallic compound TiGePt by means of photoelectron spectroscopy, x-ray absorption spectroscopy and fullpotential band structure calculations. It was recently shown that TiGePt undergoes a structural phase transition by heating which is accompanied by a large volume contraction and a drastic change of physical properties, in particular a large decrease of the electrical resistivity. The present study revealed substantial differences in the electronic structure for the two TiGePt modifications, altho…
We have investigated the electronic and optical properties of epitaxial La1-xSrxFeO3 for x from 0 to 1 prepared by molecular beam epitaxy. Core-level and valence-band x-ray photoemission features monotonically shift to lower binding energy with increasing x, indicating downward movement of the Fermi level toward to the valence band maximum. Both Fe 2p and O 1s spectra broaden to higher binding energy with increasing x, consistent with delocalization of Sr-induced holes in the Fe 3d/O 2p hybridized valence band. Combining X-ray valence band photoemission and O K-edge x-ray absorption data, we m…
We have studied the electronic structure of Li$_{1+x}$[Mn$_{0.5}$Ni$_{0.5}$]$_{1-x}$O$_2$ ($x$ = 0.00 and 0.05), one of the promising cathode materials for Li ion battery, by means of x-ray photoemission and absorption spectroscopy. The results show that the valences of Mn and Ni are basically 4+ and 2+, respectively. However, the Mn$^{3+}$ component in the $x$ = 0.00 sample gradually increases with the bulk sensitivity of the experiment, indicating that the Jahn-Teller active Mn$^{3+}$ ions are introduced in the bulk due to the site exchange between Li and Ni. The Mn$^{3+}$ component gets neg…
Band gap engineering in SrTiO${}_{3}$ and related titanate perovskites has long been explored due to the intriguing properties of the materials for photocatalysis and photovoltaic applications. A popular approach in the materials chemistry community is to substitutionally dope aliovalent transition metal ions onto the B site in the lattice to alter the valence band. However, in such a scheme there is limited control over the dopant valence, and compensating defects often form. Here we demonstrate a novel technique to controllably synthesize Fe$^{2+}$- and Fe$^{3+}$-doped SrTiO${}_{3}$ thin fil…
X-ray absorption and resonant inelastic x-ray scattering measurements are carried out on lithium nitrate LiNO$_3$. The $σ$ orbitals around the nitrogen atoms exhibit a large lifetime effect. Experimentally, this is manifest as an apparent weakening of the x-ray emission signal from these states, but a closer examination shows that instead it is due to extreme broadening. This echos previous studies on ammonium nitrate, which, despite large differences in the cation and space group, showed a similar effect associated with the nitrate. Using first-principles $GW$ self-energy and Bethe-Salpeter e…
We performed a systematic study of soft X-ray absorption spectroscopy in various manganese oxides and fluorides. Both Mn L-edges and ligand (O and F) K-edges are presented and compared with each other. Despite the distinct crystal structure and covalent/ionic nature in different systems, the Mn-L spectra fingerprint the Mn valence and spin states through spectral lineshape and energy position consistently and evidently. The clear O- and F- K pre-edge features in our high resolution spectra enable a quantitative definition of the molecular orbital diagram with different Mn valence. In addition,…
Temperature dependence of the X-ray absorption near-edge structure (XANES) spectra at the Pr $L_{3}$- and Tb $L_{3}$-edges was measured for the (Pr$_{1-y}$Tb$_{y})_{0.7}$Ca$_{0.3}$CoO$_{3}$ system, in which a metal-insulator (MI) and spin-state (SS) transition took place simultaneously at a critical temperature $T_{\rm MI}$. A small increase in the valence of the terbium ion was found below $T_{\rm MI}$, besides the enhancement of the praseodymium valence; the trivalent states, which are stable at room temperature, change to a 3+/4+ ionic mixture at low temperatures. In particular for the $y$=…
Synchrotron radiation x-ray absorption and emission spectroscopy techniques, complemented by high-resolution transmission electron microscopy methods and density functional theory calculations are employed to investigate the effect of Mn in Al$_{x}$Ga$_{1-x}$N:Mn samples with an Al content up to 100%. The atomic and electronic structure of Mn is established together with its local environment and valence state. A dilute alloy without precipitation is obtained for Al$_{x}$Ga$_{1-x}$N:Mn with Al concentrations up to 82%, and the surfactant role of Mn in the epitaxial process is confirmed.
A Mn valence instability on La2/3Ca1/3MnO3 thin films, grown on LaAlO3 (001)substrates is observed by x-ray absorption spectroscopy at the Mn L-edge and O K-edge. As-grown samples, in situ annealed at 800 C in oxygen, exhibit a Curie temperature well below that of the bulk material. Upon air exposure a reduction of the saturation magnetization, MS, of the films is detected. Simultaneously a Mn2+ spectral signature develops, in addition to the expected Mn3+ and Mn4+ contributions, which increases with time. The similarity of the spectral results obtained by total electron yield and fluorescence…
The electronic and magnetic properties of a new diluted magnetic semiconductor (DMS) Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$, which is isostructural to so-called 122-type Fe-based superconductors, are investigated by x-ray absorption spectroscopy (XAS) and resonance photoemission spectroscopy (RPES). Mn $L_{2,3}$-edge XAS indicates that the doped Mn atoms have the valence 2+ and strongly hybridize with the $4p$ orbitals of the tetrahedrally coordinating As ligands. The Mn $3d$ partial density of states (PDOS) obtained by RPES shows a peak around 4 eV and relatively high between 0-2…
We carried out x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy and investigated cation valence states and spin and orbital magnetic moments in the inverse-spinel ferrimagnet Ni$_{1-x}$Co$_{2+y}$O$_{4-z}$ (NCO) epitaxial films with the perpendicular magnetic anisotropy. We show that the oxygen pressure P$_{O2}$ during the film growth by pulsed laser deposition influences not only the cation stoichiometry (site-occupation) but also the cation valence state. Our XAS results show that the Ni in the O$_{h}$-site is in the intermediate valence state betw…
Leibniz-Institut für Kristallzüchtung（莱布尼茨晶体生长研究所）
In this work, we study the absorption properties of AlN in the range of 1.5 - 5.5 eV, as well as the meta-stable change in absorption induced by ultraviolet (UV) irradiation (photochromism), and the restoration of the initial state under the action of irradiation of 2 - 4 eV or elevated temperatures. UV irradiation results in a decrease of the absorption coefficient from 110 cm$^{-1}$ to 55 cm$^{-1}$ at 4.7 eV while in the visible range the absorption coefficient increases from values below 5 cm$^{-1}$ to ~35 cm$^{-1}$. Measurements with two linear polarizations, parallel and perpendicular to …
Investigation of electronic energy levels in a weak ferromagnetic oxygen-deficient BiFeO2.85 thick film using absorption and X ray photoelectron spectroscopic studies
Nickel oxide is a versatile p-type semiconducting oxide with many applications in opto-electronic devices, but high doping concentrations are often required to achieve necessary electrical conductivity. In contrast to many other transparent oxide semiconductors, even moderate levels of doping of NiO can lead to significant optical absorption in the visible spectral range, limiting the application range of the material. This correlation has been reported extensively in literature, but its origin has been unknown until now. This work combines experimental data on optical properties from a variet…
The electronic structure of high-quality van der Waals NiPS$_3$ crystals was studied using near-edge x-ray absorption spectroscopy (NEXAFS) and resonant photoelectron spectroscopy (ResPES) in combination with density functional theory (DFT) approach. The experimental spectroscopic methods, being element specific, allow to discriminate between atomic contributions in the valence and conduction band density of states and give direct comparison with the results of DFT calculations. Analysis of the NEXAFS and ResPES data allows to identify the NiPS$_3$ material as a charge-transfer insulator. Obta…
Principal Hugoniot and K-shell X-ray absorption spectra of warm dense KCl are calculated using the first-principles molecular dynamics method. Evolution of electronic structures as well as the influence of the approximate description of ionization on pressure (caused by the underestimation of the energy gap between conduction bands and valence bands) in the first-principles method are illustrated by the calculation. Pressure ionization and thermal smearing are shown as the major factors to prevent the deviation of pressure from global accumulation along the Hugoniot. In addition, cancellation …
