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Magnetic excitations in layered electride ${\mathrm{Y}}_{2}\mathrm{C}$ have been found by inelastic neutron scattering. We have observed weak but clear magnetic scattering around the wave number $Q=0$, but no magnetic order down to the lowest temperature measured (7 K). The imaginary part of the dynamical susceptibility deduced is well described by the Lorentz function of energy $E$ for each momentum $Q$. The width $\mathrm{\ensuremath{\Gamma}}(Q)$ of the Lorentzian is proportional to $Q({Q}^{2}+{\ensuremath{\kappa}}^{2})$ with ${\ensuremath{\kappa}}^{\ensuremath{-}1}\ensuremath{\sim}4\phantom{\rule{4pt}{0ex}}\AA{}$ at $T=7$ K. We have also found that with increasing $Q$ the magnetic form factor decays faster than that of a $4d$ electron in a single Y atom, which indicates a more extended magnetic moment in ${\mathrm{Y}}_{2}\mathrm{C}$. These results provide experimental evidence that the itinerant magnetism in ${\mathrm{Y}}_{2}\mathrm{C}$ originates from the anionic electrons that reside in the interlayers. The Curie-Weiss-like behavior of the magnetic susceptibility reported in ${\mathrm{Y}}_{2}\mathrm{C}$ is ascribed to the mode coupling effects of spin fluctuations.

Muon spin rotation and resonant soft X-ray scattering experiments on prototype multiferroics RMn2O5 (R = Y, Sm) are used to demonstrate that the local electric displacements are driven by the spin-current (SC) mechanism. Small local electric displacements were evaluated by observing spin polarization at ligand O ions, for which implanted muons served as an extremely sensitive probe. Our results for YMn2O5 provide evidence that the spin polarization of O ions forming a spin cycloid chain with Mn spins increases in proportion to the vector spin chirality (Si x Sj ) of the Mn ions. This relationship strongly indicates that the charge transfer between O and Mn ions is driven by the SC mechanism, which leads to the ferroelectricity accompanying O spin polarization.

A new type of magnetoelectric (ME) effect was discovered in ${\mathrm{Tb}}_{1\ensuremath{-}x}{\mathrm{Gd}}_{x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ in which Gd ions are substituted into ${\mathrm{TbMn}}_{2}{\mathrm{O}}_{5}$. The substitution of Gd ions increased to $x=0.5$ was found to completely suppress increases in electric polarization below $T\ensuremath{\sim}14\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, which is clearly observed in ${\mathrm{TbMn}}_{2}{\mathrm{O}}_{5}$. The electric polarization of ${\mathrm{Tb}}_{0.5}{\mathrm{Gd}}_{0.5}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ reappears below $T\ensuremath{\sim}14\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ in response to an external magnetic field applied along each crystallographic axis. This isotropic ME effect is in distinction from those exhibited by the $R{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ family and by other multiferroic compounds. Resonant x-ray scattering was used to investigate the mechanism of the ME effect, and the data clearly demonstrate that this phenomenon can be attributed to competition between two different magnetic orderings. Furthermore, the magnetic-field dependence of the resonant intensity around the O $K$ edge implies that charge transfer between O and Mn ions microscopically contributes to the ME effects. These findings indicate potential new applications for multiferroic materials.

Inelastic neutron scattering was performed for an iron-based superconductor ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{D}}_{0.1}$, where most of D (deuterium) replaces oxygen, while a tiny amount goes into the interstitial sites. By first-principles calculation, we characterize the interstitial sites for D (and for H slightly mixed) with four equivalent potential minima. Below the superconducting transition temperature ${T}_{\mathrm{c}}=26\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, excitations emerge in the range 5--15 meV, while they are absent in the reference system ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$. The strong excitations at 14.5 and 11.1 meV broaden rapidly around 15 and 20 K, respectively, where each energy becomes comparable to twice the superconducting gap. The strong excitations are ascribed to a quantum rattling, or a band motion of hydrogen, which arises only if the number of potential minima is larger than two.

The charge disproportionation of Mn $3d$ and O $2p$ electronic states was investigated in two $({\mathrm{LaMnO}}_{3}{)}_{2}{({\mathrm{SrMnO}}_{3})}_{2}$ superlattices using the resonant x-ray scattering technique. The superlattices show significantly different conductivity even with the same composition. The large modulation of the Mn $3d$ electronic state was observed in the insulating sample, however the charge disproportionation and magnetic modulation of O $2p$ were shown in the semiconductive sample. It is suggested that the charge modulation appears in either Mn $3d$ or O $2p$, which is strongly related to strength of the $p\text{\ensuremath{-}}d$ hybridization, which is the main factor determining the electric conductivity in the superlattices.

Magnetic excitations in a heavily electron-doped antiferromagnet, ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{D}}_{0.5}$, have been investigated using powder inelastic neutron scattering. Unlike other parent compounds of the iron-based superconductors, the magnetic excitation gap in ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{D}}_{0.5}$ was not detected down to the lowest measured temperature of 4 K. This result can be understood as a result of quasi-isotropy within the $ab$ plane, which is consistent with the band calculation result that the ${d}_{xy}$ orbital plays the dominant role in the magnetism of ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{H}}_{0.5}$. In addition, the intensities of the magnetic excitations in this phase are much stronger than those in nondoped LaFeAsO. Even in the paramagnetic phase, the magnetic excitation in ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{D}}_{0.5}$ persists. These results corroborate recent studies showing that the electron doping enhances the localized nature in this system.

We investigate the thickness effects on charge and magnetic orderings in Fe perovskite oxide ${\mathrm{La}}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{FeO}}_{3}/{\mathrm{SrTiO}}_{3}$ thin films by hard x-ray and resonant soft x-ray scattering (RSXS) with changing thin-film thickness systematically. We found that the correlation lengths of the magnetic ordering along the in-plane and out-of-plane directions are comparable and proportional to the thickness, and show stronger thickness dependence than those of charge ordering. The magnetic ordered states disappear when the correlation length of magnetic ordering decreases to that of charge ordering through the intrinsic thickness effects. Surface-sensitive grazing-incident RSXS revealed that the orderings exist even in the surface region, which indicates that the observed orderings are not affected by surface effects like oxygen vacancies. Critical thickness is in 5--15 nm, which corresponds to 4--11 antiferromagnetic ordering period. This critical value seems to be common to other ferromagnetic oxide thin films.

A thin film superconductor $\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ is the first electron-doped compound of $\mathrm{BaF}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ obtained by the substitution of Ba with La within a blocking layer. By contrast, in conventional electron-doped $\mathrm{Ba}{(\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{x})}_{2}\mathrm{A}{\mathrm{s}}_{2}$, the dopant (Co) is inserted into the conduction layer. The different shapes, or geometries, of $\mathrm{FeA}{\mathrm{s}}_{4}$ are expected in the two compounds above. However, the structure of $\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ is investigated here because of being a thin film. To clarify the effect of the geometry, we make use of an up-to-date method to determine the atomic positions of the $\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ thin film using synchrotron x-ray diffraction. We established that the $\mathrm{FeA}{\mathrm{s}}_{4}$ geometry in $\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ indicates a systematic variation upon doping, which is opposite to that in $\mathrm{Ba}{(\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{x})}_{2}\mathrm{A}{\mathrm{s}}_{2}$. However, the superconducting transition temperatures nearly coincide with each other for the same amount of dopings. The present result contrasts with a suggestion that the $\mathrm{FeA}{\mathrm{s}}_{4}$ geometry strongly influences the superconductivity in iron pnictides. Hence, we propose that the carrier density is the more important parameter than the $\mathrm{FeA}{\mathrm{s}}_{4}$ geometry and that the electronic correlation plays a significant role in the superconductivity for the electron-doped $\mathrm{BaF}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$.

Two-dimensional (2D) electrides are a new concept material in which anionic electrons are confined in the interlayer space between positively charged layers. We have carried out angle-resolved photoemission spectroscopy measurements on ${\mathrm{Y}}_{2}\mathrm{C}$, which is a possible 2D electride, in order to verify the formation of 2D electride states in ${\mathrm{Y}}_{2}\mathrm{C}$. We clearly observe the existence of semimetallic ``electride bands'' near the Fermi level, as predicted by ab initio calculations, which conclusively demonstrates that ${\mathrm{Y}}_{2}\mathrm{C}$ is a quasi-2D electride with electride bands derived from interlayer anionic electrons.

The compounds designated as ${R}_{3}{T}_{4}{\mathrm{Sn}}_{13}$ ($R$ = rare earth and $T$ = transition metal) are hypothesized to be strongly correlated electron systems. Some of these compounds exhibit structural phase transitions with speculated charge-density-wave (CDW) formations. We carried out x-ray diffraction measurements in order to investigate the second-order phase transition of ${R}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$ ($R=\mathrm{La} \text{and} \mathrm{Ce}$) by using single-crystalline samples synthesized by the molten Sn-flux method. Both compounds exhibit the structural superlattice transformations characterized by the wave vector $\mathbit{q}=(1/2,1/2,0)$ below ${T}_{\mathrm{D}}\ensuremath{\simeq}160$ K, which originate from electronic states owing to the common Co and Sn. The space group of the low-temperature phase is evidenced as a chiral cubic $I{2}_{1}3$. The magnetic-susceptibility enhancement and the Co-ion valence shift below ${T}_{\mathrm{D}}$ are signatures of a phase transition not fully attributed to the conventional CDW formation.

In quasi-one-dimensional (quasi-1D) systems, because of the finite interchain coupling and fluctuation, the charge density wave (CDW) transition temperature ${T}_{\mathrm{CDW}}$ is much lower than the mean-field-theory predicted ${T}_{\mathrm{MF}}$ and the CDW fluctuation region exists between ${T}_{\mathrm{CDW}}$ and ${T}_{\mathrm{MF}}$. Usually, CDW fluctuation has little effect on the electronic transport properties. Moreover, the CDW state is usually insensitive to magnetic field. The work done here on a PdTeI single crystal with quasi-1D PdTe chains shows that the above statements need to be revisited. It is found that the CDW fluctuation leads to a gradual decrease of carrier concentration before the static long-range CDW order occurs at ${T}_{1}$~110 K. On the other hand, the pinning and sliding of CDW state are observed below ${T}_{2}$~6 K. When such a low ${T}_{2}$ is combined with the existence of multiple quasi-1D bands, PdTeI exhibits exotic crossover behavior from negative to huge positive magnetoresistance under magnetic field and field-induced localization. It indicates that the CDW fluctuation could have a remarkable influence on transport properties of CDW materials. Moreover, there is an interaction between the CDW state and magnetic field.

We have investigated the lattice distortion coupled to the Co $3d$-spin-state ordering in thin films of perovskite ${\mathrm{LaCoO}}_{3}$ with various epitaxial strains by measurements of the magnetization, x-ray diffraction, and optical spectra. In the system with tensile strain about 0.5%, a lattice distortion characterized by the modulation vector $q=(1/6,1/6,1/6)$ emerges at 40 K, followed by a ferromagnetic ordering at 24 K. Alternatively, in systems with tensile strain exceeding 1%, the lattice distortion characterized by $q=(1/4,1/4,1/4)$ emerges at 120 K or higher, and subsequently the ferromagnetic or ferrimagnetic ordering occurs around 90 K. The evolution of infrared phonon spectra and resonant x-ray scattering at the Co $K$ edge suggests that the population change in the Co $3d$ spin state causes the strain-induced switching of spin-state ordering as well as of magnetic ordering in this canonical spin-state crossover system.

We report on the electronic ground state of a layered perovskite vanadium oxide ${\mathrm{Sr}}_{2}{\mathrm{VO}}_{4}$ studied by the combined use of synchrotron radiation x-ray diffraction (SR-XRD) and muon spin rotation/relaxation $(\ensuremath{\mu}\mathrm{SR})$ techniques, where $\ensuremath{\mu}\mathrm{SR}$ measurements were extended down to 30 mK. We found an intermediate orthorhombic phase between ${T}_{\mathrm{c}2}\ensuremath{\sim}130$ K and ${T}_{\mathrm{c}1}\ensuremath{\sim}100$ K, whereas a tetragonal phase appears for $Tg{T}_{\mathrm{c}2}$ and $Tl{T}_{\mathrm{c}1}$. The absence of long-range magnetic order was confirmed by $\ensuremath{\mu}\mathrm{SR}$ at the reentrant tetragonal phase below ${T}_{\mathrm{c}1}$, where the relative enhancement in the $c$-axis length versus that of the $a$-axis length was observed. However, no clear indication of the lowering of the tetragonal lattice symmetry with superlattice modulation, which is expected in the orbital order state with superstructure of ${d}_{yz}$ and ${d}_{zx}$ orbitals, was observed by SR-XRD below ${T}_{\mathrm{c}1}$. Instead, it was inferred from $\ensuremath{\mu}\mathrm{SR}$ that a magnetic state developed below ${T}_{\mathrm{c}0}\ensuremath{\sim}10$ K, which was characterized by the highly inhomogeneous and fluctuating local magnetic fields down to 30 mK. We argue that the anomalous magnetic ground state below ${T}_{\mathrm{c}0}$ originates from the coexistence of ferromagnetic and antiferromagnetic correlations.

Gd${}_{1-x}$Tb${}_{x}$MnO${}_{3}$ oxides are characterized by two main competing states that exhibit both ferroelectric and multiferroic ordering. Both states involve cycloidal magnetic order that drives ferroelectricity. In their new article, a collaboration of researchers from Japan discovered that exposure of Gd${}_{0.5}$Tb${}_{0.5}$MnO${}_{3}$ to x-ray irradiation induces a reversible transition into a new hidden state, which is also characterized by cycloidal order, but with a different modulation wave vector. The ability to manipulate the state with x-rays suggests a novel route for photocontrol of multiferroic materials.

A synchrotron x-ray diffraction study of metal-insulator transitions in W-doped ${\text{VO}}_{2} ({\text{V}}_{1\ensuremath{-}x}{\text{W}}_{x}{\text{O}}_{2})$ thin films has been carried out. The insulating phase for $x\ensuremath{\le}0.07$ exhibits cell-doubling with the V dimerization similar to bulk ${\text{VO}}_{2}$, while the insulating phase for $x\ensuremath{\ge}0.11$ does not. This result suggests that the electronic structure of the $x\ensuremath{\ge}0.11$ insulators should be different from that of the $x\ensuremath{\le}0.07$ ones and bulk-insulating phase of ${\text{VO}}_{2}$. The temperature and $x$ dependence of superlattice reflection as observed casts doubt about the direct relationship between the dimerization of V ions and metal-insulator transition. The temperature dependence of the electrical resistivity rather implies the Mott-Anderson localization nature of the insulating phases. X-ray-induced persistent phase transitions are observed at low temperatures in each insulating phase in the vicinity of the boundary to the metallic phase regardless of the difference in the electronic structure. Gradual peak shift suggests that the x-ray irradiation produces nanometer metallic regions.

X-ray diffraction measurements of $({\mathrm{NH}}_{3}){\mathrm{K}}_{3}{\mathrm{C}}_{60}$ have revealed a structural phase transition at ${T}_{s}=150 \mathrm{K},$ which is attributed to the orientational order-disorder transition of the $\mathrm{K}\ensuremath{-}{\mathrm{NH}}_{3}$ pair at the octahedral site of the ${\mathrm{C}}_{60}$ lattice. The low-temperature phase has a face-centered-orthorhombic structure derived by doubling the unit lattice vectors of the high-temperature phase along three axes. The superlattice intensity increases continuously below ${T}_{s},$ which means that the transition is second order. At $100 \mathrm{K}lTl{T}_{s},$ a negative thermal expansion is observed along the a and b axes. This is closely related to the contraction of K-N interatomic distance. We also discuss the relation between the superconductivity and the local symmetry of ${\mathrm{C}}_{60}$ in the crystal.

We studied the spin-state responses to light impurity doping in low-spin perovskite LaCoO$_{3}$ (Co^3+: d^6) through magnetization and X-ray fluorescence measurements of single-crystal LaCo$_{0.99}$$M_{0.01}$O$_{3}$ ($M$ = Cr, Mn, Fe, Ni). In the magnetization curves measured at 1.8 K, a change in the spin-state was not observed for Cr, Mn, or Fe doping but was observed for Ni doping. Strong magnetic anisotropy along the [100] easy axis was also found in the Ni-doped sample. The fluorescence measurements revealed that the valences were roughly estimated to be Cr^3+, Mn^4+, Fe^(3+delta)+, and Ni^3+. From the observed chemical trends, we propose that the chemical potential is a key factor in inducing the change of the low-spin state. By expanding a model of the ferromagnetic spin-state heptamer generated by hole doping, we discuss the emergence of highly anisotropic ferromagnetic spin-state clusters induced by low-spin Ni^3+ with Jahn-Teller activity. We also discuss applicability of the present results to mantle materials and impurity-doped pyrites with Fe (d^6).

We have found persistent and bidirectional x-ray photoinduced phase transition (PIPT) between the charge-orbital ordered and the ferromagnetic phase in a few percent Fe-doped layered manganite La${}_{0.5}$Sr${}_{1.5}$Mn${}_{1\ensuremath{-}x}$Fe${}_{x}$O${}_{4}$. Although partial x-ray PIPT has been observed in undoped manganite as well, the photoinduced effect has been confirmed to be much enhanced by impurity doping. The partial substitution of Mn ions with Fe ions was found to suppress the long-range charge-orbital order and to form a phase-separated ground state between the charge-orbital ordered and the ferromagnetic phases. Such bi-stability arising from competing phases plays an essential role in persistent and reversible PIPT phenomena.

The magnetic susceptibility (\ensuremath{\chi}) and magnetization of oxygen (${\mathrm{O}}_{2}$) monolayers and multilayers physisorped on exfoliated graphite have been investigated in order to elucidate the magnetic and melting transitions in the two-dimensional (2D) system. An anisotropy of \ensuremath{\chi} in the dense monolayer phase has been observed below ${\mathit{T}}_{\mathit{N}}$=11.9\ifmmode\pm\else\textpm\fi{}0.1 K. The magnetization process shows the precursor of the spin-flop transition, which is estimated to occur in the magnetic-field region 6H7 T at T=5.0 K. These experimental results confirm the existence of magnetic long-range order below ${\mathit{T}}_{\mathit{N}}$. The exchange field between nearest-neighbor ${\mathrm{O}}_{2}$ molecules in a monolayer is estimated as 70% of that of bulk ${\mathrm{O}}_{2}$. The anisotropy in the fluid-II phase has also been observed, which means that ${\mathrm{O}}_{2}$ molecules in this phase have orientational order. In the bilayer region, the second ${\mathrm{O}}_{2}$ layer is not only ordered magnetically, but the magnetic ordering of the first layer is also destroyed in a magnetic field when the second layer coverage exceeds 65% of the full coverage of the second layer. These results are discussed in relation to a random-exchange field effect between the first and second layers, which are incommensurate to each other. The feature of melting transitions in the multilayer region is described in terms of surface melting. \textcopyright{} 1996 The American Physical Society.

The motion of guest Gd ions in oversized boron cages in GdB${}_{6}$ was investigated from phonon spectra measurements obtained by inelastic x-ray scattering. The measured phonon modes soften by about 10$%$ from 300 K down to ${T}_{\mathrm{N}}=16$ K, in particular, the longitudinal phonon for the propagation vector ${\mathbit{q}}_{1}=(1/2,0,0)$ that characterizes the distorted structure below ${T}_{\mathrm{N}}$. Besides, the dispersion relation curves show kinklike anomalies at ${\mathbit{q}}_{\mathrm{k}}=(0.38,0.38,0)$. The observed results imply that the motion of the guest Gd ion interplays with the $f$ electrons magnetoelastically and with carriers via Fermi surface nesting. The anomalous properties previously reported for this material far above ${T}_{\mathrm{N}}$ originate from the strong electron-phonon coupling, which causes the motion of guest ions as precursors to the first-order phase transition.

We have observed a persistent x-ray induced insulator-metal transition at low temperature for an epitaxial film of electron-doped VO${}_{2}$, a material as characterized by strong electron-electron and electron-lattice interactions. The volume fraction of the photo-generated metallic patches, ranging from 0 to 100% of the whole film, can be scaled well with the total dose of x-ray irradiation, irrespective of the x-ray intensity. This indicates the monomolecular process of the insulator-metal phase conversion that corresponds to the instantaneous creation of the metallic patch extending over as many as 10${}^{5}$ V sites per one x-ray absorbed photon. The typical percolation behavior is observed in the conductivity change with the finely photo-controlled volume ratio of the metallic phase.

Multiferroic properties have been investigated for single crystals of the triangular-lattice antiferromagnet NiBr${}_{2}$ with long-wavelength ($\ensuremath{\sim}7$ nm) cycloidal spin structure whose spin-spiral plane is parallel to (001). X-ray diffraction revealed a magnetoelastic lattice modulation with half the periodicity of the magnetic modulation below the cycloidal ordering temperature (${T}_{\mathrm{IC}}\ensuremath{\sim}23$ K), indicating the elliptically distorted nature of the transverse helix. Field-reversible spontaneous polarization ($P$) appears in the [1$\overline{1}$0] direction perpendicular to the spin rotation axis ($\ensuremath{\parallel}$[001]) below ${T}_{\mathrm{IC}}$. $P$ shows nontrivial dependence on the magnitude and direction of the poling magnetic field ($H$), suggesting the possible $H$ selection of the propagation vector of the helix from the sixfold-degenerate directions, even for the in-plane cycloidal spin structure, through magnetic domain control in the higher-temperature spin-collinear antiferromagnetic phase.

A new type of colossal magnetoresistance, which is extremely sensitive to the magnetic-field direction, was discovered in a magnetoplumbite-type hexagonal cobalt oxide SrCo${}_{12}$O${}_{19}$ synthesized under a high pressure of 3 GPa. SrCo${}_{12}$O${}_{19}$ has a unique combination of physical properties (i.e., uniaxial magnetic anisotropy originating from Ising spins on Co${}^{3+}$O${}_{5}$ bipyramids and a metal-insulator transition taking place on a three-dimensional network of Co${}^{3+/4+}$O${}_{6}$ octahedra). The resistivity decreases by a factor of 1000 when a magnetic field of 10 T is applied along the hexagonal axis (magnetic easy axis) in the insulating phase, whereas it is rather robust against the field perpendicular to this axis. These results demonstrate a metal-insulator crossover dominated by the orientation order of built-in Co${}^{3+}$ Ising spins embedded in the conductive Co${}^{3+/4+}$-O matrix lattice.

Single crystals of electron-doped ${\text{SrMnO}}_{3}$ with a cubic perovskite structure have been systematically investigated as the most canonical (orbital-degenerate) double-exchange system, whose ground states have been still theoretically controversial. With only $1--2\text{ }\mathrm{%}$ electron doping by Ce substitution for Sr, a $G$-type antiferromagnetic metal with a tiny spin canting in a cubic lattice shows up as the ground state, where the Jahn-Teller polarons with heavy mass are likely to form. Further electron doping above 4%, however, replaces this isotropic metal with an insulator with tetragonal lattice distortion, accompanied by a quasi-one-dimensional $3{z}^{2}\ensuremath{-}{r}^{2}$ orbital ordering with the $C$-type antiferromagnetism. The self-organization of such dilute polarons may reflect the critical role of the cooperative Jahn-Teller effect that is most effective in the originally cubic system.

Crystal-field excitation spectra of $\text{Pr}\text{ }4f$ electron state in the filled skutterudite ${\text{PrOs}}_{4}{\text{P}}_{12}$ measured by inelastic neutron scattering experiment evolve distinctly with temperature variation. The spectral widths of excitations from the ground-state singlet to the two triplets and their temperature dependences are well reproduced by the theory based on the exchange coupling between $4f$ and conduction electrons. The shift of level energies by around 1 meV below 60 K indicates modification of crystal-field potential with variation of thermal population of $4f$ electron crystal-field states. The spectral evolution is consistent with the specific heat data that deviates from the calculated one for the well-localized $4f$ electron. The result supports the effect of $p\text{\ensuremath{-}}f$ hybridization between $\text{Pr}\text{ }4f$ and $\text{P}\text{ }p$ states in ${\text{PrOs}}_{4}{\text{P}}_{12}$.

Structural relations between two ground states of the axial next nearest neighbor Ising compound NaV{sub 2}O{sub 5}, C{sub 1/4} and C{sub 0} phases below and above the transition pressure P{sub C}=1 GPa, were investigated by x-ray diffraction and scattering techniques. The structure of the C{sub 0} phase is well explained by the A (A{sup '}) pattern, which is one of four layers (AAA{sup '}A{sup '}) of the C{sub 1/4} phase, however, the amount of the atomic shifts under the conditions 1.6 GPa and 6 K is 27% that under ambient pressure. On the other hand, resonant x-ray scattering showed that the charges are disproportionated under high pressure. Based on these facts, it was concluded that charge disproportionation corresponds to the Ising variable in NaV{sub 2}O{sub 5}, where the atomic shifts are regarded as linearly coupled to the Ising spins. These results lead to the hypothesis that the competitive interactions between the Ising spins may result from the Ising spin-phonon coupling.

The effects of hydrostatic pressure on a structure and the corresponding resonant x-ray scattering (RXS) spectrum have been investigated for the hole-orbital-ordered compound ${({\mathrm{C}}_{2}{\mathrm{H}}_{5}\mathrm{N}{\mathrm{H}}_{3})}_{2}\mathrm{Cu}{\mathrm{Cl}}_{4}$. We have found a structural phase transition at ${P}_{\mathrm{c}}\ensuremath{\sim}4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, as suggested by the Raman scattering measurement, by observing a peak splitting resulting from an orthorhombic-to-monoclinic symmetry breaking. The reduction of the Jahn-Teller distortion (JTD) toward ${P}_{\mathrm{c}}$ is also ascertained by structural analyses. The gradual change in the color of the crystal is also observed in connection with the structural change near ${P}_{\mathrm{c}}$. The red-colored transparency of the crystal indicates that the system is still insulative above ${P}_{\mathrm{c}}$. The observed RXS and fluorescence spectrum at ambient pressure were precisely analyzed. We experimentally confirmed that the RXS intensity comes from the polarization of the density of states of ${p}_{\mathrm{x}}$ and ${p}_{\mathrm{y}}$ symmetries, which is mainly dominated not by the on-site $3d\text{\ensuremath{-}}4p$ Coulomb interaction but by the JTD of the $\mathrm{Cu}{\mathrm{Cl}}_{6}$ octahedron. charged Cl ions neighboring the Cu site. It is also expected that the RXS intensity is proportional to JTD; however, our RXS study under a high pressure shows no striking change as JTD is suppressed by the application of pressure. On the other hand, the RXS intensity becomes zero above ${P}_{\mathrm{c}}$. The results indicate that the local environment of the ${\mathrm{Cu}}^{2+}$ ion markedly changes. Two possible structures of ${\mathit{EA}}_{2}\mathrm{Cu}{\mathrm{Cl}}_{4}$ above ${P}_{\mathrm{c}}$ are proposed.

We investigated orbital ordering states in the vicinity of the Mott transition in $R\mathrm{Ti}{\mathrm{O}}_{3}$ and $\mathrm{La}\mathrm{Ti}{\mathrm{O}}_{3.02}$ ($R=\mathrm{Gd}$,Sm,Nd,La) using resonant x-ray scattering (RXS) at the $1s\ensuremath{\rightarrow}3d$ transition energy of the $K$ absorption edge of Ti. We measured the energy and azimuthal angle dependences of the resonant signal at the reflections of $Q=(1\phantom{\rule{0.3em}{0ex}}0\phantom{\rule{0.3em}{0ex}}0)$, (0 1 1), and (0 0 1). The RXS intensity gradually decreased with decreasing the $\mathrm{Gd}\mathrm{Fe}{\mathrm{O}}_{3}$-type distortion. Although there is a magnetic phase boundary between $\mathrm{Gd}\mathrm{Ti}{\mathrm{O}}_{3}$ (ferromagnet) and $\mathrm{Sm}\mathrm{Ti}{\mathrm{O}}_{3}$ (antiferromagnet), the ratio among the magnitudes at those reflections was almost the same. The azimuthal angle dependence also showed an identical periodicity at each scattering vector. With further decreasing the $\mathrm{Gd}\mathrm{Fe}{\mathrm{O}}_{3}$-type distortion, the magnitude of the RXS in $\mathrm{La}\mathrm{Ti}{\mathrm{O}}_{3}$ became small and almost isotropic. This means that the orbital state of $\mathrm{La}\mathrm{Ti}{\mathrm{O}}_{3}$ is different from those of $R\mathrm{Ti}{\mathrm{O}}_{3}$ ($R=\mathrm{Y}$,Gd,Sm). Furthermore, when we approached the Mott transition by hole doping, the signal of the RXS disappeared in $\mathrm{La}\mathrm{Ti}{\mathrm{O}}_{3.02}$, which is just located on the metal-insulator boundary. This indicates a disappearance of the orbital ordering at the Mott transition.

The orbitally ordered state of ${\mathrm{YTiO}}_{3}$ has been investigated utilizing resonant x-ray scattering (RXS) near the Ti K-edge. The RXS intensities have been observed at the $1\stackrel{\ensuremath{\rightarrow}}{s}4p$ dipole transition energy (the main edge). The atomic scattering tensor of the Ti ion has quantitatively been determined by measurements of the azimuthal angle and polarization dependence of the signal at several scattering vectors. On the basis of the tensor, we discuss not only the scattering mechanism but also the wave function of the orbitally ordered state, finding it to be a linear combination of two ${t}_{2g}$ orbitals. In addition, RXS at the $1\stackrel{\ensuremath{\rightarrow}}{s}3d$ transition energy (the pre-edge) was also observed. This signal was found to have the same azimuthal angle and polarization dependence as that at the main edge.

We studied optical absorption of single-walled carbon nanotubes by varying the dielectric environment. For the two different components of the broad UV-absorption feature conventionally referred to as the $\ensuremath{\pi}$ plasmon, we find that the component at 5.0--5.3 eV exhibits remarkable spectral changes, which we attribute to a dipolar radial surface plasmon. However, the component at $\ensuremath{\sim}4.5\text{ }\text{eV}$ remains unchanged, raising a fundamental question as to its conventional attribution. We discuss its relation with the absorption feature at $\ensuremath{\sim}4.5\text{ }\text{eV}$ in graphite arising from an interband transition.