We have investigated the doping and temperature dependences of the pseudogap and superconducting gap in the single-layer cuprate La2-xSrxCuO4 by angle-resolved photoemission spectroscopy. The results clearly exhibit two distinct energy and temperature scales, namely, the gap around (pi, 0) of magnitude Delta* and the gap around the node characterized by the d-wave order parameter Delta0. In comparison with Bi2212 having higher Tc's, Delta0 is smaller, while Delta* and T* are similar. This result suggests that Delta* and T* are approximately material-independent properties of a single CuO2 plane, in contrast to the material-dependent Delta0, representing the pairing strength.

We have investigated the magnetic structure of an insulating lightly doped La 1Ϫx Sr x MnO 3 with xϭ0.04,0.125, and 0.17 with neutron scattering technique and bulk magnetization measurements. The magnetic structure of the xϭ0.04 sample at 5 K is a commensurate layer-type (A-type͒ antiferromagnetism with the propagation vector ͓010͔ and the moment of 3.7Ϯ0.2 B /M n, being parallel to the ͓100͔ axis in the orthorhombic Pnma symmetry. In addition, a spontaneous magnetization of 0.28Ϯ0.02 B /M n is observed at 8 K by magnetization measurement. By contrast, the xϭ0.125 sample exhibits a large ferromagnetic moment of 3.7Ϯ0.2 B /Mn and a small A-type antiferromagnetic component of 0.23Ϯ0.1 B /M n, while the xϭ0.17 sample exhibits only a ferromagnetic component of 3.6Ϯ0.2 B /Mn. These results are consistent with a spin structure derived from the conventional double exchange mechanism, but rule out the possibility of a recently suggested spiral spin structure for the insulating La 1Ϫx Sr x MnO 3 system.Distorted perovskite manganese oxides such as La 1Ϫx Sr x MnO 3 and La 1Ϫx Ca x MnO 3 transform from an antiferromagnetic insulator to a ferromagnetic metal as substitution of La ions by alkaline-earth ions. 1-4 Such behavior has been long known as metallic transport phenomena mediated by a double exchange mechanism. 5-7 The recent discovery of a gigantic magnetoresistance, 8-15 lattice-structure switching by an external field 16 as well as field-induced insulator-metal transitions 17-19 has renewed interest in these perovskite-type manganese oxide systems. Manganese ions in the parent material LaMnO 3 have a 3d 4 configuration where three electrons occupy the t 2g orbitals, while one electron occupies the e g orbital. The number of e g electrons in LaMnO 3 corresponds to half filling, and makes the parent LaMnO 3 a Motttype antiferromagnetic insulator. The substitution of La ions by divalent ions leads to doping of holes to the e g orbitals, introducing an itinerant character as well as ferromagnetism to well-doped samples. For a lightly doped insulating regime of the solid solution system La 1Ϫx Sr x MnO 3 , however, the double exchange model suggests a canted antiferromagnetism as the magnetic structure. 7 On the other hand, based on a mean field treatment of localized t 2g electrons and itinerant e g electrons with a strong Hund coupling, Inoue and Maekawa 20 recently proposed that a spiral state can be energetically more favorable than a canted antiferromagnetic state for a nearly half-doped insulating regime. This theory suggests that the spiral state continuously changes to the ferromagnetic state as a function of hole doping from half filling.In a pioneering work by Wollan and Koehler in 1955, the magnetic structures of the LaMnO 3ϩ␦ system as well as the La 1Ϫx Ca x MnO 3 system were studied by the neutron diffraction technique. 2 The spin structure of the LaMnO 3ϩ␦ system was reported to be a canted antiferromagnet where a layer type ͑a so-called A-type͒ antiferromagnetic ͑AF͒ structure coexists with a ferromagnet...

In a polar ferrimagnet GaFeO3, we have found a novel magneto-optical effect, termed x-ray nonreciprocal directional dichroism (XNDD), that the x-ray absorption at around the K edge of an Fe ion depends on whether the x-ray propagation vector is parallel or antiparallel to the outer product of the magnetization and electric-polarization vectors. The XNDD spectroscopy as demonstrated here can be a useful tool to probe the local magnetism in noncentrosymmetric systems such as magnetic interfaces and nanostructures.

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the undoped and electron-doped iron pnictides BaFe 2−x Co x As 2 (Ba122) (x=0, 0.14) and studied the Fermi surfaces (FSs) and band dispersions near the Fermi level. The FS sheets we observed are consistent with the shrinkage of the hole-like pockets around the Brillouin Zone (BZ) center and the expansion of the electron pockets around the BZ corner in the electrondoped compound as compared to the undoped parent compound. Band dispersions and FSs around the BZ center strongly depend on the photon energy, indicating the three-dimensional (3D) electronic structure. This observation suggests that the antiferromagnetism and superconductivity in the pnictides may have to be considered including the orbital-dependent 3D electronic structure, where FS nesting is not necessarily strong.The discovery of superconductivity (SC) in layered iron pnictides 1 with the critical temperature T c reaching ∼56 K 2 has opened a new route for the high-T c research in addition to that of the cuprates, bringing new challenges to the materials science community on both experimental and theoretical sides.This new class of iron-based systems share some common properties with the cuprates such as the layered crystal structures 1 and antiferromagnetic (AFM) ordering in the parent compounds. 3, 4 However, many differences exist between the two families especially in their electronic structures. These differences started to appear from the early stage when local-density-approximation (LDA) band-structure calculations predicted that all the Fe 3d-derived bands exist near the Fermi level (E F ), resulting in complicated hole-and electron-like Fermi surface (FS) sheets, 5-7 whereas only a single band with one FS

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Ca 1−x Sr x VO 3 is a Mott-Hubbard-type correlated electron system whose bandwidth can be varied by the V-O-V bond angle, but the actual effect of bandwidth control on the electronic structure has been controversial in previous photoemission experiments. In this work, band dispersions and Fermi surfaces of SrVO 3 and CaVO 3 are studied by angle-resolved photoemission spectroscopy.Near the Fermi level (E F ), three bands forming cylindrical Fermi surfaces derived from the three V 3d t 2g orbitals have been observed. The observed band widths for both compounds are almost half of those predicted by local-density-approximation band-structure calculation, confirming mass renormalization caused by electron correlation. It has been clearly demonstrated that the width of the d band in CaVO 3 is narrower than that in SrVO 3 , qualitatively consistent with the result of band-structure calculation. Roles of the orthorhombic lattice distortion and electron correlation in the observed band narrowing are discussed. The complex nature of correlated electrons in transition-metal oxides (TMO's) causes various interesting phenomena including high-T c superconductivity and colossal magnetoresistance 1 . Light transition-metal oxides such as perovskite-type Ti and V oxides are ideal systems to study the fundamental physics of electron correlation because they are prototypical Mott-Hubbard-type systems, in which the O 2p band is located below the transition-metal 3d band, in the framework of the Zaanen-Sawatzky-Allen classification scheme. 2 Those systems have a relatively small number of electrons in the degenerate t 2g bands, and their electronic properties have been modeled by the Hubbard model without explicit consideration of the oxygen p orbitals. In the Mott-Hubbard regime, the MIT occurs when the ratio of the on-site Coulomb repulsion (U) to the one-electron band width (W ) exceeds a critical value (U/W ) c . In those light TMO's, the electronic properties are those of normal Fermi liquids on the metallic side, while they are Mott insulators (with orbital ordering) on the insulating side of the MIT. Electron correlation effects and metal-insulator transitions (MITs) in light TMO's have been described based on theoretical predictions of dynamical mean-field theory (DMFT) for the Hubbard model 3 .To address the nature of electron correlation in light TMO's experimentally, photoemission spectroscopy has provided rich information. In the photoemission spectra of the perovskite-type Ti and V oxides, 4-7 the coherent part around the Fermi level (E F ) corresponding to band-like electronic excitations and incoherent part 1-2 eV away from E F corresponding to atomic-like excitations or the remnant of the lower Hubbard band (LHB) have been observed. DMFT has predicted that the effective mass of conduction electron is enhanced concomitant with decreasing spectral weight in the coherent part 3 and that spectral weight transfer occurs from the coherent part to incoherent part as the system approaches an MIT from the metallic side. For ex...

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