和達 大樹

シンクロトロン放射(SR)からのX線は10 ps程度以上の時間幅を持つ。X線自由電子レーザー(XFEL)により、1 ps以下の時間幅を持つX線が得られ、物質のダイナミクス研究が可能となった。ここでは、ポンプ・プローブ型の時間分解X線回折を、アメリカのXFEL(LCLS)とドイツのSR(BESSY)で行った結果を示す。コヒーレントフォノン生成、消磁プロセスの観測などの新しい結果が得られた。<br>

We investigate the electronic structure of a perovskite-type Pauli paramagnet SrMoO3 (t(2g) (2)) thin film using hard x-ray photoemission spectroscopy and compare the results to realistic calculations that combine density functional theory within the local-density approximation (LDA) with dynamical mean-field theory (DMFT). Despite the clear signature of electron correlations in the electronic specific heat, the narrowing of the quasiparticle bands is not observed in the photoemission spectrum. This is explained in terms of the characteristic effect of Hund's rule coupling for partially filled t(2g) bands, which induces strong quasiparticle renormalization already for values of Hubbard interaction which are smaller than the bandwidth. This interpretation is supported by DMFT model calculations including Hund's rule coupling, which show a renormalization of low-energy quasiparticles without affecting the overall bandwidth. The photoemission spectra show additional spectral weight around -2.5 eV that is not present in the LDA+DMFT results, pointing to a source of correlations that is not present in our calculations that include only on-site interactions. We interpret this weight as a plasmon satellite, which is supported by the measured core-level spectra that all show satellites at this energy.

Strongly correlated electron systems often exhibit very strong interactions between structural and electronic degrees of freedom that lead to complex and interesting phase diagrams(1,2). For technological applications of these materials it is important to learn how to drive transitions from one phase to another. A key question here is the ultimate speed of such phase transitions, and to understand how a phase transition evolves in the time domain(3-13). Here we apply time-resolved X-ray diffraction to directly measure the changes in long-range order during ultrafast melting of the charge and orbitally ordered phase in a perovskite manganite. We find that although the actual change in crystal symmetry associated with this transition occurs over different timescales characteristic of the many electronic and vibrational coordinates of the system, the dynamics of the phase transformation can be well described using a single time-dependent 'order parameter' that depends exclusively on the electronic excitation.

The coupled electronic-structural modulations of the ligand states in IrTe2 have been studied by x-ray absorption spectroscopy and resonant elastic x-ray scattering (REXS). Distinctive preedge structures are observed at the TeM4,5 (3d -&gt; 5p) absorption edge, indicating the presence of a Te 5p-Ir 5d covalent state near the Fermi level. An enhancement of the REXS signal near the Te 3d -&gt; 5p resonance at the Q=(1/5,0,-1/5) superlattice reflection is observed below the structural transition temperature Ts similar to 280 K. The analysis of the energy-dependent REXS line shape reveals the key role played by the spatial modulation of the covalent Te 5p-Ir 5d bond density in driving the stripelike order in IrTe2, and uncovers its coupling with the charge and/or orbital order at the Ir sites. The similarity between these findings and the charge-ordering phenomenology recently observed in the high-temperature superconducting cuprates suggests that the iridates may harbor similar exotic phases.

We studied the charge-orbital ordering in the superlattice of charge-ordered insulating Pr0.5Ca0.5MnO3 and ferromagnetic metallic La0.5Sr0.5MnO3 by resonant soft x-ray diffraction (RSXD) and hard x-ray photoemission spectroscopy (HXPES). A temperature-dependent incommensurability is found in the orbital ordering by RSXD. In addition, a large hysteresis is observed that is caused by phase competition between the insulating charge ordered and metallic ferromagnetic states. No magnetic phase transitions are observed in contrast to pure Pr0.5Ca0.5MnO3 thin films, confirming the unique character of the superlattice. Mn 2p HXPES spectra revealed a hysteresis in the metalicity, supporting the picture of phase competition. The deviation from the commensurate orbital order can be directly related to the decrease of ordered-layer thickness that leads to dimensional crossover from three-dimensional to two-dimensional orbital ordering.

The antiferromagnetic insulator La1.5Ca0.5CoO4 has been investigated by Co L-2,L-3-edge and O K-edge X-ray absorption spectroscopy (XAS) measurements and Co L-2,L-3-edge resonant soft X-ray magnetic scattering (RXMS) measurement to determine the Co electronic structures associated with magnetic ordering. Co L-2,L-3-edge linear-dichroic XAS shows that Co2+ takes a high-spin (HS) state and Co3+ takes a low-spin (LS) state. Using Co L-2,L-3-edge RXMS, we directly determined that an antiferromagnetic order is formed with a HS state of Co2+ ions. Moreover, the spin and orbital angular momenta of the Co2+ HS state are quantitatively estimated to be 1.1 +/- 0.1 and 1.0 +/- 0.1, respectively, and to align parallel in the ab plane by utilizing the cluster model calculation. The large orbital angular momentum of the Co2+ HS state originates from the small D-4h-symmetry crystal field splitting of t(2g) levels, which is comparable with the spin-orbit coupling constant of the Co 3d orbital.

We report the study of magnetic and orbital order in Pr0.5Ca0.5MnO3 epitaxial thin films grown on (LaAlO3)(0.3)-(SrAl0.5Ta0.5O3)(0.7). Resonant soft x-ray scattering revealed significant modifications of the magnetic order in the film as compared to the bulk. Namely (i) a different magnetic ordering wave vector, (ii) different spin directions and (iii) an additional magnetic reordering transition. We demonstrate that an analysis of the resonant scattering which is based solely on local symmetries and which does not involve a modeling of energy-dependent lineshapes allows to extract this detailed microscopic information. This approach significantly simplifies the analysis and interpretation of resonant scattering data.

Novel interplay of spin-orbit coupling and electron correlations in complex Ir oxides recently emerged as a new paradigm for correlated electron physics. Because of a large spin-orbit coupling of ~0.5 eV, which is comparable to the transfer energy t and the crystal field splitting $\Delta$ and Coulomb U, a variety of ground states including magnetic insulator, band insulator, semimetal and metal, shows up in a narrow materials phase space. Utilizing such subtle competition of the ground states, we successfully tailor a spin-orbital magnetic insulator out of a semimetal SrIrO$_3$ by controlling dimensionality using superlattice of [(SrIrO$_3$)$_m$, SrTiO$_3$] and show that a magnetic ordering triggers the transition to magnetic insulator. Those results can be described well by a first-principles calculation. This study is an important step towards the design and the realization of topological phases in complex Ir oxides with very strong spin-orbit coupling.

Effects of electron doping on the BiS2-based superconductors Sr1-xLaxFBiS2 (0 <= x <= 0.6) have been investigated using the systematically synthesized polycrystals by means of x-ray diffraction, x-ray absorption spectroscopy, transport and thermodynamic measurements. The pristine compound is a band insulator with the BiS2 layer, which accommodates electron carriers through the La substitution for Sr, as evidenced by the change in x-ray absorption spectra reflecting the occupancy of Bi 6p orbitals. With increasing the carrier density, the resistivity progressively decreases and a bad metallic state appears for x >= 0.45, where bulk superconductivity manifests itself below approximately 3K. The value of T-c gradually increases with decreasing x from 0.6 to 0.45 and immediately decreases down to zero at the critical concentration of x similar to 0.4, resulting in an insulator-superconductor transition highly sensitive to the carrier density. Thermodynamic measurements furthermore have revealed the possible enhancement of the superconducting coupling strength as the insulating phase is approached. The obtained superconducting phase diagram is markedly different from the broad dome-shaped superconducting phase previously reported for RO1-xFxBiS2 (R: rare-earth ion), suggesting a strong influence of the blocking layer on the superconductivity. Instead all these features are similar to those observed in Li-intercalated ZrNCl superconductor, except for the critical electron concentration of as low as 6% in the latter compound. For the present superconductor, notably, the existence of hole-type carriers has been indicated in the normal state from the Hall effect measurements. The Sr1-xLaxFBiS2 system providing the phase diagram for the rigid-band doping in the BiS2 layer would be another prototypical example of superconductivity derived from a doped layered band insulator, hosting both hole-like and electron-like Fermi surfaces.

We report magnetotransport properties for epitaxial thin films of SrFeO3 and SrFe0.99Co0.01O3 with possible skyrmion-related spin textures. Resonant soft x-ray diffraction measurements revealed formation of helical spin structures for both samples (Q//&lt; 111 &gt;). From magnetotransport measurements we found several distinct helimagnetic phases with multiple/single Q vectors. A steep suppression of Hall resistivity is observed above the critical field to reach the high-field conical state, indicating the presence of skyrmionlike topological spin textures at lower fields responsible for the topological Hall effect.

We have studied local structure of LiMnO2, LiMn0.65Cr0.35O2 and LiMn0.5Ni0.5O2 compounds by Mn K-edge extended X-ray absorption fine structure measurements. The local structure of LiMnO2 is found to be consistent with Jahn Teller distorted MnO6 octahedra characterized by two different Mn-O bond distances. The Jahn Teller distortions are suppressed in the Cr and Ni substituted compounds, resulting a single Mn-O distance. However, the Cr atoms tend to occupy a site at a longer distance from Mn in the host lattice (Mn-Cr distance is longer than Mn-Mn distance), unlike the Ni atoms which prefer a site closer to the Mn atoms (Mn-Ni distance is shorter than Mn Mn distance). Incidentally, Mn-O and Mn Mn bonds are substantially stiffer in the Cr and Ni substituted compounds. In addition, the static atomic disorder is confined around Cr atoms in the LiMn0.65Cr0.35O2, that is different from the case of LiMn0.5Ni0.5O2 in which larger static disorder appears in the proximity of the Mn atoms. The results suggest that the differences in the local structure of different compounds should be the likely reason for their differing battery characteristics. (C) 2013 Elsevier B.V. All rights reserved.

We have investigated the electronic and magnetic properties of Co-doped Pr0.8Ca0.2MnO3 thin films using various spectroscopic techniques. X-ray absorption and hard x-ray photoemission spectroscopy revealed that the substituted Co ions are in the divalent state, resulting in hole doping on the Mn atoms. Studies of element-selective magnetic properties by x-ray magnetic circular dichroism found a large orbital magnetic moment for the Co ions. These spectroscopic studies reveal that the substituted Co ions play several roles of hole doping for Mn, ferromagnetic superexchange coupling between the Co2+ and Mn4+ ions, and orbital magnetism of the Co2+ ions. Competition among these complex interactions produces the unique electronic and magnetic behaviors including enhanced coercivity of the Co-doped Pr0.8Ca0.2MnO3.

Photoemission spectroscopy is a powerful experimental technique to study the electronic structures of solids, especially of transition-metal oxides. Recently, hard X-ray photoemission spectroscopy (HXPES) has emerged as a more relevant experimental technique to obtain clear information about bulk states. Here, we describe how HXPES can be conveniently applied to study the interesting subjects on oxide thin films such as the composition dependence and the film thickness dependence of the electronic structures and the interfacial electronic structure of multilayers. © 2013 Elsevier B.V.

We have synthesized BaFeOx thin films by pulsed-laser deposition followed by low temperature (200 degrees C) oxidation to BaFeO3. X-ray diffraction indicates that fully oxidized single crystalline BaFeO3 has been formed with a lattice parameter of 3.97 angstrom, the bulk stoichiometric value. The oxidation state of Fe is confirmed to be 4+ by using hard x-ray photoemission spectroscopy. A ferromagnetic ground state is found with saturation magnetization and Curie temperature of 3.2 mu(B)/formula unit and 115 K, respectively. Unusually, for a uniform cubic ferromagnet, the films are insulating with an optical gap of similar to 1.8 eV. (C) 2013 AIP Publishing LLC.

The fundamental electronic structure of the widely used battery material LixCoO2 still remains a mystery. Soft x-ray absorption spectroscopy of LixCoO2 reveals that holes with strong O 2p character play an essential role in the electronic conductivity of the Co3+/Co4+ mixed valence CoO2 layer. The oxygen holes are bound to the Co4+ sites and the Li-ion vacancy, suggesting that the Li-ion flow can be stabilized by oxygen hole back flow. Such an oxygen hole state of LixCoO2 is unique among the various oxide-based battery materials and is one of the key ingredients to improving their electronic and Li-ion conductivities.

This chapter contains sections titled: Introduction Photoemission Measurements of Interfaces Interfaces between a Mott Insulator and a Band Insulator: LaAlO3/LaVO3 and LaTiO3/SrTiO3 Interfaces between Two Band Insulators: LaAlO3/SrTiO3 Summary References

We report the detailed electronic structure of a hole-doped delafossite oxide CuCr 1-x Mg x O 2 (0≤x≤0.03) studied by photoemission spectroscopy (PES), soft x-ray absorption spectroscopy (XAS), and band-structure calculations within the local-density approximation +U (LDA+U) scheme. Cr/Cu 3p-3d resonant PES reveals that the near-Fermi-level leading structure has primarily the Cr 3d character with a minor contribution from the Cu 3d through Cu 3d-O 2p-Cr 3d hybridization, having good agreement with the band-structure calculations. This indicates that a doped hole will have primarily the Cr 3d character. Cr 2p PES and L-edge XAS spectra exhibit typical Cr3 + features for all x, while the Cu L-edge XAS spectra exhibited a systematic change with x. This indicates now that the Cu valence is monovalent at x=0 and the doped hole should have Cu 3d character. Nevertheless, we surprisingly observed two types of charge-transfer satellites that should be attributed to Cu + (3d10) and Cu2 + (3d9) like initial states in Cu 2p-3d resonant PES spectrum of at x=0, while Cu 2p PES spectra with no doubt shows the Cu + character even for the lightly doped samples. We propose that these contradictory results can be understood by introducing not only the Cu 4s state, but also finite Cu 3d,4s-Cr 3d charge transfer via O 2p states in the ground-state electronic configuration. © 2013 American Physical Society.

We investigated the electronic structure of layered Mn oxide Bi3Mn4O12(NO3) with a Mn honeycomb lattice by x-ray absorption spectroscopy and model calculations. The valence of Mn was determined to be 4+ with a small charge-transfer energy of similar to 1 eV. The values of (J(1), J(2), J(3), J(c), J(c1), and J(c2)) obtained by unrestricted Hartree-Fock calculations on Mn 3d-O 2p lattice models show that intra-layer second and third neighbor superexchange interactions J(2) and J(3) as well as inter-layer superexchange interactions J(c), J(c1), and J(c2) are enhanced due to Mn-O-O-Mn pathways, which are activated by the smallness of charge-transfer energy. The present analysis indicates that the ferromagnetic J(c1) and antiferromagnetic J(c2) are responsible to the antiferromagnetic inter-layer coupling and that the intra-layer exchange interactions with the ferromagnetic J(2) and antiferromagnetic J(3) have no frustration effect. (C) 2013 Elsevier Ltd. All rights reserved.

We report the detailed electronic structure of a hole-doped delafossite oxide CuCr1-xMgxO2 (0 &lt;= x &lt;= 0.03) studied by photoemission spectroscopy (PES), soft x-ray absorption spectroscopy (XAS), and band-structure calculations within the local-density approximation + U (LDA + U) scheme. Cr/Cu 3p-3d resonant PES reveals that the near-Fermi-level leading structure has primarily the Cr 3d character with a minor contribution from the Cu 3d through Cu 3d-O 2p-Cr 3d hybridization, having good agreement with the band-structure calculations. This indicates that a doped hole will have primarily the Cr 3d character. Cr 2p PES and L-edge XAS spectra exhibit typical Cr3+ features for all x, while the Cu L-edge XAS spectra exhibited a systematic change with x. This indicates now that the Cu valence is monovalent at x = 0 and the doped hole should have Cu 3d character. Nevertheless, we surprisingly observed two types of charge-transfer satellites that should be attributed to Cu+ (3d(10)) and Cu2+ (3d(9)) like initial states in Cu 2p-3d resonant PES spectrum of at x = 0, while Cu 2p PES spectra with no doubt shows the Cu+ character even for the lightly doped samples. We propose that these contradictory results can be understood by introducing not only the Cu 4s state, but also finite Cu 3d, 4s-Cr 3d charge transfer via O 2p states in the ground-state electronic configuration.

Temperature dependent Co K-edge extended X-ray absorption fine structure is used to investigate local disorder in LiCoO2 nanoparticles. We find that the nanostructuring has direct influence on the bondlength characteristics. The results reveal a substantial decrease in the force constant of Co-O bonds (the Co-O bonds becoming more flexible), while that for the Co-Co bonds showing hardly any change (or increases slightly) in LiCoO2 nanoparticles with respect to the bulk. Therefore, both random disorder and Co-O bondlength flexibility should be the factors to limit the battery characteristics of the LiCoO2 nanoparticles.

We report normal and superconducting properties of the Rashba-type noncentrosymmetric compound CaIrSi3 using single crystalline samples with nearly 100% superconducting volume fraction. The electronic density of states revealed by the hard x-ray photoemission spectroscopy can be well explained by the relativistic first-principles band calculation. This indicates that strong spin-orbit interaction indeed affects the electronic states of this compound. The obtained H-T phase diagram exhibits only approximately 10% anisotropy, indicating that the superconducting properties are almost three dimensional. Nevertheless, strongly anisotropic vortex pinning is observed.

The effect of x rays on an orbital and charge ordered epitaxial film of a Pr0.5Ca0.5MnO3 is presented. As the film is exposed to x rays, the antiferromagnetic response increases and concomitantly the conductivity of the film improve. These results are discussed in terms of a persistent x-ray induced doping, leading to a modification of the magnetic structure. This effect allows writing electronic and magnetic information in the film and represents a novel way of manipulating magnetism.

The role of Co substitution in the low-energy electronic structure of Ca(Fe0.944Co0.056)(2)As-2 is investigated by resonant photoemission spectroscopy and density-functional theory. The Co 3d state center of mass is observed at 250 meV higher binding energy than that of Fe, indicating that Co possesses one extra valence electron and that Fe and Co are in the same oxidation state. Yet, significant Co character is detected for the Bloch wave functions at the chemical potential, revealing that the Co 3d electrons are part of the Fermi sea determining the Fermi surface. This establishes the complex role of Co substitution in CaFe2As2 and the inadequacy of a rigid-band shift description.

We have studied the local structure of LiCoO2 nanoparticles by Co K-edge x-ray absorption spectroscopy as a function of particle size. Extended x-ray absorption fine structure data reveal substantial changes in the near neighbor distances and the associated mean square relative displacements with decreasing particle size. X-ray absorption near edge structure spectra show clear local geometrical changes with decreasing particle size, similar to those that appear in the charging (delithiation) process. The results suggest that the LiCoO2 nanoparticles are characterized by a large atomic disorder confined to the Co-O octahedra, similar to the distortions generated during the delithiation, and this disorder should be the primary limiting factor for a reversible diffusion of Li ions when nanoparticles of LiCoO2 are used as cathode material in rechargeable Li ion batteries.

Anodization of a-Fe2O3 (hematite) electrodes in alkaline electrolyte under constant potential conditions the electrode surface in a way that an additional current wave occurs in the cyclic voltammogram. The energy position of this current wave is closely below the potential of the anodization treatment. Continued cycling or exchanging of the electrolyte causes depletion of this new feature. The O 1s and Fe 2p core-level X-ray photoelectron spectra (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectra of such conditioned hematite exhibit a chemical shift towards higher binding energies, in line with the general perception that anodization generates oxide species with dielectric properties. The valence band XPS and particularly the iron resonant valence band photoemission spectra, however, are shifted towards the opposite direction, that is, towards the Fermi energy, suggesting that hole doping on hematite has taken place during anodization. Quantitative analysis of the Fe 2p resonant valence band photoemission spectra shows that the spectra obtained at the Fe 2p absorption threshold are shifted by virtually the same energy as the anodization potential towards the Fermi energy. The tentative interpretation of this observation is that anodization forms a surface film on the hematite that is specific to the anodization potential.

Single crystals of SrFe1-xCoxO3 solid solution with high valence ions (Fe4+/Co4+) were synthesized by combining floating-zone and high-pressure oxygen-annealing techniques. As the Co content (x) is increased, the ground state changes from the helimagnetic state (x &lt;= 0.05) through cluster glass to the ferromagnetic state (x &gt;= 0.2), with a high Curie temperature ranging between 245 and 337 K. We found that, within the helimagnetic state (x &lt;= 0.05), several different magnetic phases, which possibly reflect versatile three-dimensional helical orders with multiple wave vectors, emerge depending on temperature and magnetic field strength. In addition, we observed Co-concentration-dependent systematic evolution of the complex magnetic phase diagram. Resonant soft x-ray diffraction measurements revealed a gradual decrease in the wave vector of the helix toward the ferromagnetic phase.

We performed soft X-ray resonant scattering at the MnL (2,3)- and OK edges of YMn2O5. While the resonant intensity at the Mn L (2,3) edges reflects the magnetic order parameter, the resonant scattering at the O K edge is found to be directly related to the macroscopic ferroelectric polarization. The latter observation reveals the important role of the spin-dependent Mn-O hybridization for the multiferroicity of YMn2O5. We present details about how to obtain correct energy dependent lineshapes and discuss the origin of the resonant intensity at the O K edge.

X-ray absorption spectroscopy is one of the most widely used experimental techniques to study the electronic and spatial structure of materials. Fluorescence yield mode is bulk-sensitive, but has several serious problems coming from saturation effects. In this study, we show the usefulness of partial fluorescence yields in addressing these problems. We discuss the different behaviors of La2NiMnO6 and LiMnO2 at the Mn 2p absorption edges. The total fluorescence yield produces misleading spectra for LiMnO2 due to the absence of high-Z (Z: atomic number) elements. We conclude that the measurement of the inverse partial fluorescence yield is essential in studies of LiMnO2, which is a hotly debated Li-ion battery material. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4711801]

LaFe3/4Ni1/4O3 was subjected to oxygen near edge x-ray absorption fine structure (NEXAFS) spectroscopy for 300 K < T < 773 K. The spectra show in the pre-edge a small hole doped peak originating from Ni substitution. The relative spectral weight of this transition to the weight of the hybridized O(2p) - Fe(3d) transitions scales with T and has a maximum at around 600 K. The characteristic energies of the thermal activated spectral intensity and conductivity suggest that the concentration of charge transferred electrons from O(2p) to Ni(3d) increases and that the pre-edges account in part for the polaron activated transport.

In order to understand the influence of strain and film thickness on the electronic structure of thin films of strongly correlated oxides, we have applied hard x-ray photoemission (HXPS) at 6 keV, soft x-ray photoemission (XPS) at 1.5 keV, and transmission electron microscopy to epitaxial LaNiO3 films deposited on two substrates: LaAlO3 (compressive strain) and (LaAlO3)(0.3)(Sr2AlTaO6)(0.7) (tensile strain). Using inelastic attenuation lengths in LaNiO3 determined from the HXPS data, we have decomposed valence-band spectra into layer-specific contributions. This decomposition is validated by comparing with the results of first-principles calculations using a hybrid functional. The resultant thin-film LaNiO3 densities of states exhibit significant differences in spectral weights for the thinnest LaNiO3 films. A gap opening consistent with a metal-to-insulator transition is observed for the thinnest 2.7 nm LaNiO3 film on an (LaAlO3)(0.3)(Sr2AlTaO6)(0.7) substrate, with a similar gap opening also being observed in complementary soft x-ray photoemission at 1.5 keV for a thinner 1.4 nm film on an LaAlO3 substrate. A metal-to-insulator transition in very thin nm-scale films of LaNiO3 is thus suggested as a general phenomenon.

We describe the design, construction, and performance of a 4-circle in-vacuum diffractometer for resonant elastic soft x-ray scattering. The diffractometer, installed on the resonant elastic and inelastic x-ray scattering beamline at the Canadian Light Source, includes 9 in-vacuum motions driven by in-vacuum stepper motors and operates in ultra-high vacuum at base pressure of 2 x 10(-10) Torr. Cooling to a base temperature of 18 K is provided with a closed-cycle cryostat. The diffractometer includes a choice of 3 photon detectors: a photodiode, a channeltron, and a 2D sensitive channelplate detector. Along with variable slit and filter options, these detectors are suitable for studying a wide range of phenomena having both weak and strong diffraction signals. Example measurements of diffraction and reflectivity in Nd-doped (La,Sr)(2)CuO(4) and thin film (Ga,Mn) As are shown. (C) 2011 American Institute of Physics. [doi:10.1063/1.3607438]

We have studied the electronic structure of multiferroic BiCoO3 using x-ray photoemission spectroscopy (XPS), x-ray absorption spectroscopy (XAS), and subsequent model calculations. The XAS results show that the Co3+ ion takes the high-spin d(6) configuration which usually prefers G-type antiferromagnetic state. The XPS results and model Hartree-Fock calculations show that, in case of BiCoO3, small charge-transfer energy plays an essential role to enhance the Co-O-O-Co superexchange pathways. It is found that the combination of ferro-type orbital ordering of Co 3d t(2g) and the Co-O-O-Co superexchange interaction gives rise to C-type antiferromagnetic state in BiCoO3. The present analysis suggests that, in addition to the Bi-O bonds responsible for the noncentrosymmetric deformation, the O-O bonds are important to stabilize the multiferroic phase of BiCoO3.

We report an x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) study on K(2)V(8-x) Ti(x)O(16) (x = 0.0, 0.3, and 4.0) in which the metal-insulator transition (MIT) and the tetragonal-to-monoclinic structural transition show different responses against the Ti doping. The spectral change of V 2p XAS across the MIT is similar to that observed in VO(2), indicating that the V-V pairing is responsible for the MIT. In the insulating phase, the V 2p XPS spectra are split into the V(3+) and V(4+) features showing the localized character of the V 3d electrons. In addition, the V 2p XAS spectra of the monoclinic insulating phase suggest that the V-V pairing is weakened, and, instead, the V(3+)-V(4+) coupling is strengthened by orbital ordering. In contrast to VO(2), the existence of the V(3+) component in K(2)V(8-x) Ti(x)O(16) gives additional orbital instability in the insulating phase, which competes with the V-V pairing.

Eu doped rare-earth orthovanadates are known to be good red phosphor materials. In particular, LaVO4:Eu is a promising candidate due to the low Eu-site point symmetry, and thus high dipole transition probability within Judd-Ofelt theory. However, the low solubility limit (&lt;3 mol %) of Eu in LaVO4 prevents its efficient use as a phosphor. We present optical evidence of enhanced Eu solubility as high as 10 mol % in LaVO4:Eu thin films grown by pulsed laser deposition and postannealing. The photoluminescent intensity exceeded that of YVO4:Eu thin films when excited below the host bandgap, indicating stronger direct emission of Eu in LaVO4. (C) 2011 American Institute of Physics. [doi:10.1063/1.3554749]

We demonstrate a method of x-ray absorption spectroscopy (XAS) that is bulk sensitive, like traditional fluorescence yield measurements, but is not affected by self-absorption or saturation effects. This measure of XAS is achieved by scanning the incident photon energy through an absorption edge and using an energy-sensitive photon detector to measure the partial fluorescence yield (PFY). The x-ray emission from any element or core-hole excitation that is not resonant with the absorption edge under investigation is selected from the PFY. It is found that the inverse of this PFY spectrum, which we term inverse partial fluorescence yield (IPFY), is linearly proportional to the x-ray absorption cross-section without any corrections due to saturation or self-absorption effects. We demonstrate this technique on the Cu L(2,3) and Nd M(4,5) absorption edges of the high-T(c) cuprate La(1.475)Nd(0.4)Sr(0.125)CuO(4) by measuring the O K(alpha) PFY and comparing the total electron yield, total fluorescence yield, and IPFY spectra.

We demonstrate a method of x-ray absorption spectroscopy (XAS) that is bulk sensitive, like traditional fluorescence yield measurements, but is not affected by self-absorption or saturation effects. This measure of XAS is achieved by scanning the incident photon energy through an absorption edge and using an energy-sensitive photon detector to measure the partial fluorescence yield (PFY). The x-ray emission from any element or core-hole excitation that is not resonant with the absorption edge under investigation is selected from the PFY. It is found that the inverse of this PFY spectrum, which we term inverse partial fluorescence yield (IPFY), is linearly proportional to the x-ray absorption cross-section without any corrections due to saturation or self-absorption effects. We demonstrate this technique on the Cu L(2,3) and Nd M(4,5) absorption edges of the high-T(c) cuprate La(1.475)Nd(0.4)Sr(0.125)CuO(4) by measuring the O K(alpha) PFY and comparing the total electron yield, total fluorescence yield, and IPFY spectra.