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We present elastic and quasielastic neutron scattering measurements characterizing peculiar short-range charge-orbital and spin order in the layered perovskite material La{sub 1.5}Sr {sub 0.5}CoO {sub 4} . We find that below T{sub c}{approx}750 K holes introduced by Sr doping lose mobility and enter a statically ordered charge glass phase with loosely correlated checkerboard arrangement of empty and occupied d{sub 3z{sup 2}-r{sup 2}} orbitals (Co{sup 3+} and Co{sup 2+} ). The dynamics of the resultant mixed spin system is governed by the anisotropic nature of the crystal-field Hamiltonian and the peculiar exchange pattern produced by the orbital order. It undergoes a spin freezing transition at a much lower temperature, T{sub s}{approx}

The k-dependent electronic structure of the low temperature ferromagnetic state of La 1.2Sr1.8Mn2O7 was measured using angle-resolved photoemission spectroscopy and calculated using the local spin density approximation (LSDA). The measured near-Fermi energy states display E vs k and symmetry relationships which agree relatively well with the LSDA prediction through much of the Brillouin zone, and the locus of lowest energy excitations matches the predicted large Fermi surface quite well. However, the spectral features are too broad to be well described as Fermi-liquid-like quasiparticles, and they are strongly suppressed from the Fermi energy, i.e., there is a pseudogap in the excitation spectrum. We discuss the spectral properties in terms of strong coupling to a local effect such as a lattice distortion. [S0031-9007(98)06522-3] PACS numbers: 71.30. + h, 71.20.Lp, 72.80.Ga, 79.60.Bm

Charge ordering and orbital ordering in ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{1.5}{\mathrm{MnO}}_{4}$ have been investigated by using synchrotron x-ray diffraction. The alternating pattern of ${\mathrm{Mn}}^{3+}/{\mathrm{Mn}}^{4+}$ in real space is observed directly by making use of their anomalous dispersion. The first clear evidence of the orbital ordering of ${e}_{g}$ electrons on ${\mathrm{Mn}}^{3+}$ is also presented based on the measurements of ATS (anisotropy of the tensor of susceptibility) reflection near the Mn $K$-absorption edge. The present observations indicate that the charge ordering and orbital ordering occur simultaneously at a transition temperature higher than that of the spin ordering.

The bilayer manganite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ exhibits a transition between nearly insulating paramagnetic and metallic ferromagnetic states at ${T}_{C}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}126\mathrm{K}$. We use magnetic neutron scattering to show that the paramagnetic state contains long-lived antiferromagnetic clusters coexisting with ferromagnetic critical fluctuations. The principal implication is that at least in two dimensions, Anderson localization effects must also be taken into account in attempts to explain giant magnetoresistance in manganites.

Competition between the ferromagnetic double-exchange interaction and the antiferromagnetic charge-ordering instability gives rise to thermally as well as magnetic field (H) induced insulator-metal transitions in distorted perovskites $({\mathrm{Nd}}_{1\ensuremath{-}y}{\mathrm{Sm}}_{y}{)}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3}$ with a controlled one-electron bandwidth. A systematic study on the metal-insulator phase diagram in the $T\ensuremath{-}H$ plane with varying y as well as a high-pressure study has revealed the metastable nature of the undercooled ferromagnetic metallic state.

Elastic neutron scattering measurements characterizing the charge and magnetic order in L${\mathrm{a}}_{0.5}$S${\mathrm{r}}_{1.5}$Mn${\mathrm{O}}_{4}$ are presented. Superlattice reflections corresponding to an alternating ${\mathrm{Mn}}^{+3}/{\mathrm{Mn}}^{+4}$ pattern with wave vector ${\mathbf{q}}_{\mathrm{co}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(\frac{1}{2},\frac{1}{2},0)$ are observed below the charge ordering temperature, ${T}_{\mathrm{co}}\ensuremath{\approx}217$ K. Below the N\'eel temperature, ${T}_{N}\ensuremath{\approx}110$ K, the magnetic moments associated with these ions form an ordered structure with dimensions $2\sqrt{2}a\ifmmode\times\else\texttimes\fi{}2\sqrt{2}a\ifmmode\times\else\texttimes\fi{}2c$ relative to the underlying chemical cell.

The perovskite-type manganese oxide, ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ $(x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.5)$, shows a first-order phase transition from a ferromagnetic (FM) metal to antiferromagnetic (AFM) nonmetal at 140 K, which accompanies the transition to the charge-ordered state (generalized Wigner lattice). The nonmetal-to-metal transition temperature can be decreased down to near 0 K with a magnetic field up to 70 kOe. The temperature-field phase diagram for the charge-ordered state is presented together with a discussion on the mechanism of this field-induced phase transition.

Dielectric and thermodynamic properties have been investigated in isotopically mixed crystals ${\mathrm{K}}_{3}$${\mathrm{D}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{H}}_{\mathit{x}}$(${\mathrm{SO}}_{4}$${)}_{2}$ with isolated hydrogen bonds interconnecting the dimeric ${\mathrm{SO}}_{4}^{2\mathrm{\ensuremath{-}}}$ units. The critical temperature ${\mathit{T}}_{\mathit{c}}$ for the antiferroelectric phase transition decreases with increasing proton concentration (x) and eventually the phase transition vanishes for x\ensuremath{\ge}${\mathit{x}}_{\mathit{c}}$\ensuremath{\sim}0.66. The x-dependent behavior of ${\mathit{T}}_{\mathit{c}}$, the dielectric constant \ensuremath{\epsilon}(T), and the specific heat jump \ensuremath{\Delta}C at ${\mathit{T}}_{\mathit{c}}$ are described by simple transverse Ising Hamiltonian. This indicates a determinative role of the proton tunneling in the phase transition and related properties.

We have investigated the optical conductivity spectra of La2-2xSr1+2xMn2O7 (0.3

The perovskite ${\mathrm{LaCoO}}_{3}$ evolves from a nonmagnetic Mott insulator to a spin cluster ferromagnet (FM) with the substitution of ${\mathrm{Sr}}^{2+}$ for ${\mathrm{La}}^{3+}$ in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$. The clusters increase in size and number with $x$ and the charge percolation through the clusters leads to a metallic state. Using elastic neutron scattering on ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ single crystals, we show that an incommensurate spin superstructure coexists with the FM spin clusters. The incommensurability increases continuously with $x$, with the intensity rising in the insulating phase and dropping in the metallic phase as it directly competes with the commensurate FM, itinerant clusters. The spin incommensurability arises from local order of ${\mathrm{Co}}^{3+}\mathrm{\text{\ensuremath{-}}}{\mathrm{Co}}^{4+}$ clusters but no long-range static or dynamic spin stripes develop. The coexistence and competition of the two magnetic phases explain the residual resistivity at low temperatures in samples with metalliclike transport.

Inelastic cold-neutron scattering on ${\mathrm{LaCoO}}_{3}$ provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic correlations that are dynamic follow the activation to the excited state, identified as the intermediate $S=1$ spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$, the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.

The charge density distribution has been determined for a transition metal cyanide, RbMn[Fe(CN) 6 ], by means of the maximum entropy-Rietveld method combined with the highly angularly resolved synchrotron radiation x-ray powder diffraction at SPring-8 BL02B2. We directly observed a charge transfer from the Mn site to the Fe site in the low-temperature phase. On the basis of a local density approximation calculation, we discuss the origin for the anisotropic bonding electron distribution around the Mn 3 + ion in the low-temperature phase.

We have observed magnetic diffuse signals of the bilayer manganite La1.1Sr1.9Mn2O7 by neutron scattering. Even at the low temperature, 20 K, there is a diffuse signal, which is much stronger than that observed in the high temperatures, 144 and 288 K. This signal cannot be characterized by a bilayer structure as observed at 144 K nor by a monolayer structure as at 288 K, but by a high degeneracy of a canted spin structure. Thus, an observation of this diffuse signal is the direct evidence for the establishment of the canted spin structure in La1.1Sr1.9Mn2O7.

Optical spectroscopy was used to determine nearest-neighbor spin correlations in pyrochlore molybdates ${R}_{2}{\mathrm{M}\mathrm{o}}_{2}{\mathrm{O}}_{7}$ ($R=\mathrm{Y}$, Sm, and Nd), which exhibit ferromagnetic metal to spin-glass insulator transition as the $R$ ion size decreases. Using an analysis based on the orbitally degenerate Hubbard model, we could estimate important physical parameters, such as the effective on-site Coulomb energy ${U}_{\mathrm{e}\mathrm{f}\mathrm{f}}$ and the Hund rule exchange coupling ${J}_{\mathrm{H}}$. We demonstrated experimentally and theoretically that the effective superexchange interaction between the Mo ions depends on ${J}_{\mathrm{H}}$/${U}_{\mathrm{e}\mathrm{f}\mathrm{f}}$, which determines the phase boundary of the magnetic ground states.

The relationship between charge and spin degrees of freedom in a geometrically frustrated system, AlV 2 - x Cr x O 4 spinel, is investigated. Upon Cr doping, the charge-ordered phase of AlV 2 O 4 is rapidly suppressed and a charge-disordered phase grows up instead. It is found that the magnetic ground state is a spin-glass state dominated by geometrical frustration for both phases, but larger spin entropy remains down to low temperatures in the charge-ordered phase, possibly owing to its two-dimensional character.

We systematically investigated optical properties of ${\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ single crystals ( $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.40$, 0.50, 0.55, and 0.65). They are similar in their spin-orbital (SO) disordered states at room temperature. At low temperature, the crystals enter into various SO ordered states, i.e., $F$-, $\mathrm{CE}$-, $A$-, and $C$-type orderings, and their mid-infrared absorptions become quite different. The remarkable variation can be explained by polaron dynamics which depend on the ordering patterns. This SO pattern dependent polaron model can also explain the pseudo CE-type ordering case, demonstrating that this scheme can explain the carrier dynamics in complex SO configurations.

Temperature variation of resistivity and specific heat have been measured for prototypical half-metallic ferromagnets, R_0.6Sr_0.4MnO_3, with controlling the one-electron bandwidth W. We have found variation of the temperature scalings in the resistivity from T^2 (R = La, and Nd) to T^3 (R = Sm), and have interpreted the $T^3-law in terms of the anomalous single-magnon scattering (AMS) process in the half-metallic system., To appear in Phys. Rev. Lett., 3 pages + 4 EPS figures

The temperature-pressure phase diagram was determined for a crystal of squaric acid which consists of molecular sheets of [${\mathrm{O}}_{4}$${\mathrm{O}}_{4}$${]}^{2\mathrm{\ensuremath{-}}}$ skeletons and intervening protons. Under hydrostatic pressures, the paraelectric phase composed of the polar (${\mathit{C}}_{1\mathit{h}}$-distorted) molecules is realized even near 0 K due to a joint quantum motion of the protons and deforming molecular skeletons. Further application of pressure causes a disappearance of the \ensuremath{\pi}-bond alternation of the constituent molecule at all temperatures caused by enhanced motion of the protons.