H. Wadati

Where are the extra d electrons in transition-metal-substituted iron pnictides?

Transition-metal substitution in Fe pnictides leading to superconductivity is usually interpreted in terms of carrier doping to the system. We report on a density functional calculation of the local substitute electron density and demonstrate that substitutions like Co and Ni for Fe do not carrier dope but rather are isovalent to Fe. We find that the extra d electrons for Co and Ni are almost totally located within the muffin-tin sphere of the substituted site. We suggest that Co and Ni act more like random scatterers scrambling momentum space and washing out parts of the Fermi surface.

Engineering a Spin-Orbital Magnetic Insulator by Tailoring Superlattices.

In 5d Ir oxides with an interplay of spin-orbit coupling and electron correlations, we have tailored a spin-orbital magnetic insulator out of a semimetal SrIrO(3) by tuning the structure through superlattices [(SrIrO(3))(m), SrTiO(3)] (m=1,2,3,4, and ∞). We observed the systematic decrease of the magnetic ordering temperature and the resistivity as a function of m. The transition from the semimetal to the insulator is found to be closely linked to the appearance of magnetism at m≃3. Long range magnetic ordering was realized even in the m=1 single layer superlattice, implying that the design and realization of novel electronic phases is feasible at the level of a single atomic layer in complex Ir oxides.

Charge ordering inLa1.8−xEu0.2SrxCuO4studied by resonant soft x-ray diffraction

Resonant soft X-ray scattering with photon energies near the O K and the Cu L3 edges was used to study charge ordering in the system La_{1.8-x}Eu_{0.2}Sr_xCuO_4 as a function of temperature for x = 0.125 and 0.15. From the superstructure diffraction intensities a charge ordering with a doping dependent wave vector is derived which is in this system well below the transition temperature of the low-temperature tetragonal phase but well above the onset of spin ordering. This indicates that charge ordering is the primary driving force for the formation of stripe-like phases in two-dimensional doped cuprates. Analysis of the lineshape of the scattered intensity as a function of photon energy yields evidence for a high hole concentration in the stripes.

Origin of the large polarization in multiferroic YMnO 3 thin films revealed by soft- and hard-x-ray diffraction

We investigated the magnetic structure of an orthorhombic YMnO(3) thin film by resonant soft x-ray and hard x-ray diffraction. We observed a temperature-dependent incommensurate magnetic reflection below 45 K and a commensurate lattice-distortion reflection below 35 K. These results demonstrate that the ground state is composed of coexisting E-type and cycloidal states. Their different ordering temperatures clarify the origin of the large polarization to be caused by the E-type antiferromagnetic states in the orthorhombic YMnO(3) thin film.

Inherent charge transfer layer formation at La0.6Sr0.4FeO3∕La0.6Sr0.4MnO3 heterointerface

We have investigated the Mn3d spectral function in La0.6Sr0.4FeO3(LSFO)∕La0.6Sr0.4MnO3(LSMO) heterointerfaces as well as of La1−xSrxMnO3 films using Mn2p−3d resonant photoemission spectroscopy. The strong enhancement of the Mn3d spectra at the Mn2p−3d threshold enables us to extract the Mn3d spectra of LSMO layers in the vicinity of the interface with the LSFO overlayers. We have found that the spectral intensity of eg↑ states near the Fermi level is gradually reduced with increasing LSFO overlayer thickness and is finally saturated at 5–7ML. The close similarity in reduction of the intensity of the eg↑ states between the LSFO∕LSMO interface and hole-doped LSMO films indicates the occurrence of charge transfer at the interface between the LSMO and LSFO layers.

Insulator-to-Superconductor Transition upon Electron Doping in a BiS2-Based Superconductor Sr1−xLaxFBiS2

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 3 K. The value of Tc gradually increases with decreasing x from 0.6 to 0.45 and immediately decreases down to zero at the critical concentration of x ∼ 0.4, resulting in an insulator–superconductor transition highly sensitive to the carrier density. Thermodynamic measurements furthermore have revealed the possible enhancement o...

Robust Ti4+ states in SrTiO3 layers of La0.6Sr0.4MnO3∕SrTiO3∕La0.6Sr0.4MnO3 junctions

We have investigated the interfacial electronic structure of La0.6Sr0.4MnO3 (LSMO)∕SrTiO3 (STO)/LSMO heterojunctions utilizing the elemental selectivity of photoemission spectroscopy. The Ti 2p core-level spectra clearly show Ti4+ states and do not exhibit any indication of Ti3+ states in TiO2 layers irrespective of a different kind of adjacent atomic layer with different chemical carrier concentration. This result indicates that the Ti ions in the TiO2 atomic layers preserve their tetravalent states even in the vicinity of the valence-mismatched interface between LSMO and STO, reflecting the chemical stability of the Ti4+ states.

Manifestation of correlation effects in the photoemission spectra of Ca1-xSrxRuO3

We have measured soft x-ray photoemission and O 1s x-ray absorption spectra of Ca1-xSrxRuO3 thin films prepared in situ. The coherent and incoherent parts have been identified in the bulk component of the photoemission spectra, and spectral weight transfer from the coherent to the incoherent part has been observed with decreasing x, namely, with increasing orthorhombic distortion. We propose that, while the Ru 4d one-electron bandwidth does not change with x, the distortion, and hence the splitting of the t(2g) band, effectively increases electron correlation strength. Although strong mass enhancement is found in the electronic specific heat data, the coherent part remains wide, suggesting enhanced band narrowing only in the vicinity of E-F.

Band structure and Fermi surface of La 0.6 Sr 0.4 MnO 3 thin films studied by in situ angle-resolved photoemission spectroscopy

We have performed an in situ angle-resolved photoemission spectroscopy (ARPES) on single-crystal surfaces of

Electronic structure of the hole-doped delafossite oxides CuCr 1-xMgxO2

{\mathrm{La}}_{0.6}{\mathrm{Sr}}_{0.4}{\mathrm{MnO}}_{3}