Abstract: Charge ordering and superconductivity are observed in the phase diagrams of a variety of materials such as NbSe3, layered transition-metal dichalcogenides and high-temperature copper oxide superconductors, low-dimensional organics, Ba1−xKxBiO3 and so forth. Although both conventional charge-density-wave (CDW) and superconducting transitions show an energy gap in the single-particle density of states at the Fermi level (EF), their physical properties are poles apart: insulating behaviour for the CDW and zero resistivity in superconductors. Consequently, these two ground states are believed to compete with each other. Here we provide evidence for maximized superconductivity at points in momentum (k) space that are directly connected by the CDW ordering vector. Temperature-dependent angle-resolved photoemission spectroscopy of 2H-NbSe2 across the CDW and superconducting transitions (TCDW∼33 K and Tc=7.2 K, respectively) shows CDW-induced spectral-weight depletion at the same Fermi-surface-crossing k points, which evolve into the largest superconducting gaps. These k points also show the highest electron–phonon coupling and lowest Fermi velocities. Our results demonstrate that charge order can boost superconductivity in an electron–phonon coupled system, in direct contrast to the prevailing view that it only competes with superconductivity.

Abstract: The spatial charge arrangement of a typical quasi-two-dimensional organic conductor alpha-(BEDT-TTF)2I3 is revealed by single crystal structure analysis using synchrotron radiation. The results show that the horizontal stripe type structure, which was suggested by mean field theory, is established. We also find the charge disproportion above the metal-insulator transition temperature and a significant change in transfer integrals caused by the phase transition. Our result elucidates the insulating phase of this material as a 2k_F charge density localization.

Abstract: The charge ordering (CO) and magnetic properties of La(0.25)Ca(0.75)MnO(3) compounds with average particle sizes ranging from 40 to 2000 nm have been studied. With decreasing particle size, the CO transition gradually shifts to lower temperature and becomes increasingly wide and weak; meanwhile, the ferromagnetic cluster glass state appears, and the temperature T(G) for the appearance of ferromagnetic clusters shows a nonmonotonous variation with a maximum at about 100 nm. The magnetization at low temperature shows a relatively complex change with particle size. A core-shell model was proposed to explain these anomalous behaviors, and the temperature-particle size phase diagram of La(0.25)Ca(0.75)MnO(3) was established. These results are helpful in deeply understanding the coupling between charge and spin degrees of freedom in manganites.

Abstract: In recent decades, novel magnetism of d- and f-electron compounds has been discussed very intensively both in experimental and theoretical research fields of condensed matter physics. It has been recognized that these material groups are in the same category of strongly correlated electron systems, while the low-energy physics of d- and f-electron compounds has been separately investigated in rather different manners. One of the common features of both d- and f-electron systems is certainly the existence of active orbital degree of freedom, but in f-electron materials, due to the strong spin?orbit interaction in rare-earth and actinide ions, the physics seems to be quite different from that of d-electron systems. In general, when the number of internal degrees of freedom and relevant interactions is increased, it is possible to obtain a rich phase diagram including large varieties of magnetic phases by using several kinds of theoretical techniques. However, we should not be simply satisfied with the reproduction of a rich phase diagram. It is believed that the more essential point is to seek a simple principle penetrating complicated phenomena in common with d- and f-electron materials, which opens the door to a new stage in orbital physics. In this sense, it is considered to be an important task of this paper to explain common features of magnetism in d- and f-electron systems from a microscopic viewpoint, using a key concept of orbital ordering, in addition to the review of the complex phase diagram of each material group. As a typical d-electron complex material exhibiting orbital order, we first focus on perovskite manganites, in which remarkable colossal magneto-resistance effect has been intensively studied. The manganites provide us with a good platform to understand that a simple mechanism works for the formation of complex spin, charge and orbital ordering. We also explain intriguing striped charge ordering on the orbital-ordered background in nickelates and the effect of orbital ordering to resolve spin frustration in geometrically frustrated eg electron systems. Note that orbital ordering phenomena are also found in t2g electron systems. Here we review recent advances in the understanding of orbital ordering phenomenon in Ca2RuO4. Next we discuss another spin?charge?orbital complex system such as the f-electron compound. After detailed explanation of the construction of microscopic models on the basis of a j?j coupling scheme, we introduce a d-electron-like scenario to understand novel magnetism in some actinide compounds with the HoCoGa5-type tetragonal crystal structure. Finally, we show that complicated multipole order can be understood from the spin?orbital model on the basis of the j?j coupling scheme. As a typical material with multipole order, we pick up NpO2 which has been believed to exhibit peculiar octupole order. Throughout this review, it is emphasized that the same orbital physics works both in d- and f-electron complex materials in spite of the difference between d and f orbitals.

Abstract: We present a scenario for the peculiar coexistence of charge fluctuations observed in quasi-2D $1/4$-filled organic conductors $\ensuremath{\theta}\mathrm{\text{\ensuremath{-}}}(\mathrm{BEDT}\mathrm{\text{\ensuremath{-}}}\mathrm{TTF}{)}_{2}X$ in the quantum critical regime where the charge ordering is suppressed down to zero temperature. The scenario is explored in the extended Hubbard model including electron-phonon couplings on an anisotropic triangular lattice. We find that the coexisting fluctuations emerge from two different instabilities, the ``Wigner crystallization on a lattice'' driven by the off-site Coulomb repulsion and the charge-density-wave formation due to the nesting of the Fermi surface, not from phase competition or real-space inhomogeneity. This mechanism explains the contrastive temperature dependence of two fluctuations in experiments.

Abstract: Charge-ordered rare earth manganites Nd0.5Ca0.5MnO3,La0.25Nd0.25Ca0.5MnO3, Pr0.7Ca0.3MnO3 and Pr0.6Ca0.4MnO3 are found to exhibit dielectric constant anomalies around the charge-ordering or the antiferromagnetic transition temperatures. Magnetic fields have a marked effect on the dielectric properties, indicating the presence of coupling between the magnetic and electrical order parameters. The observation of magnetoferroelectricity in these manganites is in accord with the recent theoretical predictions of Khomskii and co-workers.

Abstract: The charge state and local ordering of Mn doped into a pulsed laser deposited single-phase thin film of ZnO are investigated by using x-ray absorption spectroscopy at the O K-edge, Mn K-edge and L-edge, and x-ray emission spectroscopy at the O K-edge and Mn L-edge. This film is ferromagnetic at room temperature. EXAFS measurement shows that Mn2+ replaces the Zn site in tetrahedral symmetry, and there is no evidence for either metallic Mn or MnO in the film. Upon Mn doping, the top of O 2p valence band extends into the bandgap, indicating additional charge carriers being created.

Abstract: A macroscopic phase separation, in which ferromagnetic clusters are observed in an insulating matrix, is sometimes observed, and believed to be essential to the colossal magnetoresistive (CMR) properties of manganese oxides. The application of a magnetic field may indeed trigger large magnetoresistance effects due to the percolation between clusters allowing the movement of the charge carriers. However, this macroscopic phase separation is mainly related to extrinsic defects or impurities, which hinder the long-ranged charge-orbital order of the system. We show in the present article that rather than the macroscopic phase separation, an homogeneous short-ranged charge-orbital order accompanied by a spin glass state occurs, as an intrinsic result of the uniformity of the random potential perturbation induced by the solid solution of the cations on the A-sites of the structure of these materials. Hence the phase separation does occur, but in a more subtle and interesting nanoscopic form, here referred as "homogeneous". Remarkably, this "nanoscale phase separation" alone is able to bring forth the colossal magnetoresistance in the perovskite manganites, and is potentially relevant to a wide variety of other magnetic and/or electrical properties of manganites, as well as many other transition metal oxides, in bulk or thin film form as we exemplify throughout the article.

Abstract: Elemental chromium orders antiferromagnetically near room temperature, but the ordering temperature can be driven to zero by applying large pressures. We combine diamond anvil cell and synchrotron x-ray diffraction techniques to measure directly the spin and charge order in the pure metal at the approach to its quantum critical point. Both spin and charge order are suppressed exponentially with pressure, well beyond the region where disorder cuts off such a simple evolution, and they maintain a harmonic scaling relationship over decades in scattering intensity. By comparing the development of the order parameter with that of the magnetic wave vector, it is possible to ascribe the destruction of antiferromagnetism to the growth in electron kinetic energy relative to the underlying magnetic exchange interaction.

Abstract: Charge ordering accompanied by lattice distortion in quasi-two dimensional organic conductors \theta-(ET)2X (ET=BEDT-TTF) is studied by using an extended Hubbard model with Peierls-type electron-lattice couplings within the Hartree-Fock approximation. It is found that the horizontal-stripe charge-ordered state, which is experimentally observed in \theta-(ET)2RbZn(SCN)4, is stabilized by the self-consistently determined lattice distortion. Furthermore, in the presence of the anisotropy in nearest-neighbor Coulomb interactions Vij, the horizontal charge order becomes more stable than any other charge patterns such as diagonal, vertical and 3-fold-type states. At finite temperatures, we compare the free energies of various charge-ordered states and find a first-order transition from a metallic state with 3-fold charge order to the insulating state with the horizontal charge order. The role of lattice degrees of freedom in the realization of the horizontal charge order and the relevance to experiments on \theta-(ET)2X are discussed.

Abstract: Neutron scattering experiments were performed on single crystals of layered cobalt-oxides La 2- x Ca x CoO 4 (LCCO) to characterize the charge and spin orders in a wide hole-doping range of 0.3≦ x ...

Abstract: Charge-ordered La0.5Ca0.5MnO3 film has been deposited on (100)-LaAlO3 substrate using rf magnetron sputtering method. X-ray diffraction analysis shows that the bulk has orthorhombic structure and the film has the better epitaxial character. The charge-ordering transition temperature is about 280 K by fitting the resistance-temperature curve using the variable-range hopping model. The decrease in the resistance of the film irradiated by the laser is caused by the delocalization effect of the correlated electron system. The photoinduced relaxation exhibits different processes when the laser is on and off, which can be attributed to thermal fluctuation.

Abstract: Polycrystalline samples of Bi0.5−yLaySr0.5MnO3 (0.0≤y≤0.4) (BLSMO) have been synthesized to investigate the Bi3+ lone-pair effect on the long-range charge-ordering (CO) state. Since the ionic size of La3+ is similar to that of Bi3+ and the Mn valence state does not change with La doping, we obtained the Bi lone-pair effect on the CO state without disturbance by other effects. The resistivity ρ(T) and thermoelectric power S(T) of BLSMO have been measured. A hysteretic behaviour was observed in both ρ(T) and S(T) for y = 0.1 and 0.2. From the onset of the hysteretic behaviour, we defined a charge-ordering temperature (TCO) and compared it to that of Bi1−xSrxMnO3 (BSMO). Finally, we found that the Bi3+ lone pairs play an important role in the anomalously high TCO in BSMO.

Abstract: The colossal electroresistance in manganites accompanies the insulator-to-metal phase transition induced by the electric field. A phenomenological phase transition model is proposed to study this electric field induced collapse of the charge-ordered phase. The hysteresis of the phase transition is well explained using the effective medium approximation. The volume fraction of the metallic region at the metal-to-insulator transition point is estimated as 30%.

Abstract: The magnetocaloric effect in single crystalline Nd05Sr05MnO3 (NSMO 05) is investigated by computing the field dependent entropy change (ΔS) and adiabatic temperature change (ΔTad) At the charge ordering temperature (TCO), the value of ΔSmax is found to be much higher than ΔSmax reported in polycrystalline samples This “giant” entropy change is attributed to interplay (stronger in single crystals) among spin, charge, lattice, and orbital degrees of freedom resulting in a field induced transition at TCO In contrast, the change in entropy associated with Curie temperature (TC) is very low The direct measurements of the field induced temperature change (ΔT) are in agreement with the computed value of ΔS The presence of short-range correlations with charge/orbital order (COO) above and below TC may be responsible for the suppression of the negative MCE at TC A critical exponent analysis of the paramagnetic (PM) to ferromagnetic (FM) transition using magnetization data yields mean-field-like values, wh

Abstract: Phase transitions in 1/4-filled quasi-one-dimensional molecular conductors are studied theoretically on the basis of extended Hubbard chains including electron-lattice interactions coupled by interchain Coulomb repulsion. We apply the numerical quantum transfer-matrix method to an effective one-dimensional model, treating the interchain term within mean-field approximation. Finite-temperature properties are investigated for the charge ordering, the "dimer Mott" transition (bond dimerization), and the spin-Peierls transition (bond tetramerization). A coexistent state of charge order and bond dimerization exhibiting dielectricity is predicted in a certain parameter range, even when intrinsic dimerization is absent.

Abstract: Phase transitions in 1/4-filled quasi-one-dimensional molecular conductors are studied theoretically on the basis of extended Hubbard chains including electron–lattice interactions coupled by inter...

Abstract: Pr0.57Ca0.43MnO3 nanoparticles with an average particle size of similar to 20 nm have been synthesized using hydrothermal method in combination with post-annealing, and characterized using X-ray diffraction, X-ray photoelectron spectrometer, high-resolution transmission electron microscopy and superconducting quantum interference device magnetometery. The results show that the hydrothermal synthesis of Pr1-xCaxMnO3 compound below 240 degrees C is difficult. The Pr0.57Ca0.43MnO3 nanoparticles obtained by annealing the hydrothermal products at 900 degrees C for 2 h present an orthorhombic perovskite structure with the same lattice as bulk Pr0.6Sr0.4MnO3. Magnetic characterization reveals that the low-temperature antiferromagnetic and charge ordering transitions identified in bulk Pr0.57Ca0.43MnO3 are completely suppressed in the nanoparticles, while a ferromagnetic transition occurs at -110 K. The spin-freezing behavior at low temperature for the Pr0.57Ca0.43MnO3 nanoparticles is demonstrated. (c) 2006 Elsevier B.V. All rights reserved.

Abstract: Systematic study on magnetic and electrical transport properties has been performed on the Cr-doped Pr05Ca05Mn1−xCrxO3 (x=0, 003, 005, 007, 010) system The results show that Cr-doping leads to melting of the charge ordering and induces phase separation For the sample x=010, reentrant spin glass behavior is observed and the slowing down model is applicable The reentrant spin glass behavior and the phase coexistence of charge ordering (CO) and the ferromagnetic metal phase (FMM) are responsible for the changing magnetoresistance effect of the system The phase separation or coexistence of the CO and FM phase induced by Cr-doping plays an important role in the low temperature properties for the system

Abstract: The magnetic properties of GdBaMn2O5.0, which exhibits charge ordering, are studied from 2 to 400 K using single crystals. In a small magnetic field applied along the easy axis, the magnetization M shows a temperature-induced reversal which is sometimes found in ferrimagnets. In a large magnetic field, on the other hand, a sharp change in the slope of M(T) coming from an unusual turnabout of the magnetization of the Mn sublattices is observed. Those observations are essentially explained by a molecular field theory which highlights the role of delicate magnetic interactions between Gd3+ ions and the antiferromagnetically coupled Mn2+/Mn3+ sublattices.

Abstract: The low-temperature structures of M(pyz)V4O10 (M = Co, Zn) in the spin-gapped state have been investigated by single-crystal X-ray diffraction and electronic structure calculations and provide evidence for a new type of spin-antiferroelectric transition for M = Zn. This new type of phase transition involves the onset of charge ordering within spin ladders that can be mediated by the metal-organic chains via a modification of the inter-ladder interactions.