和達 大樹

Interfaces in heterostructures composed of two different transition-metal oxides are now a strongly growing research topic in condensed-matter physics due to the anomalous electronic properties very different from those of the component materials. Here we present resonant soft x-ray scattering studies of SrTiO3/LaAlO3 superlattices. From the existence of the forbidden Bragg peak (003), we found evidence for differences between the bulk and the interface electronic structures. The photon-energy dependence near the O 1s edge and polarization dependence showed that there are two structures split by similar to 1 eV, which were not observed in the O 1s x-ray absorption spectrum. The origin of this splitting is still unclear, but we consider that this is not only due to the splitting of t(2g) states at the interface but also due to the difference between the O 1s binding energies for bulk and interface oxygen.

We study the importance of inter-site coherences and quantum interference effects in the x-ray absorption spectroscopy (XAS) of mixed-valent systems. We demonstrate its importance, first for a simple diatomic mixed-valent molecule to elucidate the physics involved, and finally for mixed-valent Ti oxides, using a model calculation including full spin and orbital multiplicities. Our calculations demonstrate the inefficacy of conventional approaches of describing the XAS from mixed-valent systems as incoherent combinations of XAS from the pure-valent end members, and that multiplets forbidden in the single-site approximation can be reached within a multi-site description. These conclusions play an important role in the interpretation of XAS and resonant x-ray scattering (RXS) in terms of the electronic structure of strongly correlated systems.

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.

We present an x-ray absorption study of the oxidation states of transition-metal-ions of LiMnO2 and its related materials, widely used as cathodes in Li-ion batteries. The comparison between the obtained spectrum and the configuration-interaction cluster-model calculations showed that the Mn3+ in LiMnO2 is a mixture of the high-spin and low-spin states. We found that Li deficiencies occur in the case of Cr substitution, whereas there are no Li deficiencies in the case of Ni substitution. We conclude that the substitution of charge-transfer-type Ni or Cu is effective for LiMnO2 battery materials. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3463468]

We report the observation of pressure-induced changes in the electronic structures of La1-xSrxMnO3 (LSMO) by hard x-ray photoemission spectroscopy. Application of compressive and tensile strains results in the formation of a gap at the Fermi level (E-F) and suppression of spectral weight at E-F, respectively, across magnetic phase transitions. In contrast, no detectable change is observed in the absence of phase transitions even upon application of pressure. These results indicate that the change in the electronic structure of LSMO does not originate from the lattice distortions alone, but is induced by subtle interplay among the lattice, magnetic, and orbital degrees of freedom.

We present resonant soft x-ray scattering studies of Ti 3d and O 2p states at the interfaces of SrTiO3/LaAlO3 superlattices. From reflectivity analyses, focusing on the (003) peak which is forbidden for our "ideal" superlattice structure, we concluded that the LaO vertical bar TiO2/SrO and the SrO vertical bar AlO2/LaO interfaces have distinct reconstructions, breaking the heterostructure symmetry. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3246788]

We have performed a systematic tight-binding analysis of the angle-resolved photoemission spectroscopy (ARPES) spectra of a series of transition-metal (TM) oxides A(TM)O-3 and compared the obtained electronic structure parameters with those well established values obtained from cluster-model analyses of photoemission spectra. The values of epsilon(d) - epsilon(p) from ARPES are found to be much larger than the values which would be expected for the limit of weak p-d hybridization, indicating that the "effective" TM 3d hands are originated from the strong hybridization with the O 2p bands even for light TM oxides.

We present an x-ray absorption study of the dependence of the V oxidation state on the thickness of LaVO(3) (LVO) and capping LaAlO(3) (LAO) layers in the multilayer structure of LVO sandwiched between LAO. We found that the change of the valence of V as a function of LAO layer thickness can be qualitatively explained by a transition between electronically reconstructed interfaces and a chemical reconstruction. The change as a function of LVO layer thickness is complicated by the presence of a considerable amount of V(4+) in the bulk of the thicker LVO layers.

We have studied the valence redistribution of V in LaAlO3/LaVO3/LaAlO3 trilayers, which are composed of only polar layers grown on SrTiO3 (001) substrates, by core-level photoemission spectroscopy. We have found that the V valence is intermediate between V3+ and V4+ for thin LaAlO3 cap layers, decreases with increasing cap-layer thickness, and finally recovers the bulk value of V3+ at similar to 10 unit-cell thickness. In order to interpret these results, we propose that the atomic reconstruction of the polar LaAlO3 surface competes with the purely electronic V valence change so that the polar catastrophe is avoided at the cost of minimum energy.

We have performed an in situ photoemission study of Nd1-xSrxMnO3 (NSMO) thin films grown on SrTiO3 (001) substrates by laser molecular-beam epitaxy. The lattice constants of the thin films were relaxed from those of the substrates, and the transport properties were almost the same as those of bulk NSMO. From core-level photoemission studies, we found that the behavior of the chemical-potential shift was not much different from that of the bulk NSMO samples. In the valence-band spectra, finite intensity at the Fermi level was observed even in the insulating phase. The band dispersions of Nd0.6Sr0.4MnO3 obtained by angle-resolved photoemission spectroscopy were almost the same as those of La0.6Sr0.4MnO3 thin films. These results showed that NSMO is closely related to La1-xSrxMnO3 in terms of the band structure, except for a decrease in the coherent spectral weight.

Resonant soft x-ray scattering with photon energies near the O K and the Cu L-3 edges was used to study the charge ordering in the system La1.8-xEu0.2SrxCuO4 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 structural changes and magnetic interactions are not in general the driving force for the formation of stripelike 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.

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the O 2p valence-band structure of Nb-doped SrTiO3, in which a dilute concentration of electrons are doped into the d(0) band insulator. We have found that ARPES spectra at the valence-band maxima at the M [k=(pi/a,pi/a,0)] and R [k=(pi/a,pi/a, pi/a)] points start from similar to 3.3 eV below the Fermi level (E-F), consistent with the indirect band gap of 3.3 eV and the EF position at the bottom of the conduction band. The peak position of the ARPES spectra were, however, shifted toward higher binding energies by similar to 500 meV from the 3.3 eV threshold. Because the bands at the M and R points have pure O 2p character, we attribute this similar to 500 meV shift to strong coupling of the oxygen p hole with optical phonons, in analogy with the peak shifts observed for d-electron photoemission spectra in various transition-metal oxides.

We have performed an in situ photoemission study of Pr(1-x)Ca(x)MnO(3) (PCMO) thin films grown on LaAlO(3) (001) substrates and observed the effect of epitaxial strain on the electronic structure. We found that the chemical potential shifted monotonically with doping, unlike bulk PCMO, implying the disappearance of incommensurate charge fluctuations of bulk PCMO. In the valence-band spectra, we found a doping-induced energy shift toward the Fermi level (E(F)) but there was no spectral weight transfer, which was observed in bulk PCMO. The gap at E(F) was clearly seen in the experimental band dispersions determined by angle-resolved photoemission spectroscopy and could not be explained by the metallic band structure of the C-type antiferromagnetic state, probably due to localization of electrons along the ferromagnetic chain direction or due to another type of spin-orbital ordering.

We have studied the electronic structure of multilayers composed of a band insulator LaAlO(3) (LAO) and a Mott insulator LaVO(3) (LVO) by means of hard x-ray photoemission spectroscopy, which has a probing depth as large as similar to 60 A. The Mott-Hubbard gap of LVO remained open at the interface, indicating that the interface is insulating unlike the LaTiO(3)/SrTiO(3) multilayers. We found that the valence of V in LVO was partially converted from V(3+) to V(4+) only at the interface on the top side of the LVO layer and that the amount of V(4+) increased with LVO layer thickness. We suggest that the electronic reconstruction to eliminate the polarity catastrophe inherent in the polar heterostructure is the origin of the highly asymmetric valence change at the LAO/LVO interfaces.

Perovskite-type BiNiO(3) is an insulating antiferromagnet in which a charge disproportionation occurs at the Bi site. La substitution for Bi suppresses the charge disproportionation and makes the system metallic, and for 0.05 <= x <= 0.1 a broad metal-insulator transition (MIT) occurs as a function of temperature. We have measured the temperature dependence of the photoemission and x-ray absorption (XAS) spectra of Bi(1-x)La(x)NiO(3) to investigate how the electronic structure changes across the MIT. From the Ni 2p XAS spectra of x=0.05, we found almost no change in the valence of Ni across the MIT. In the valence-band photoemission spectra, the Fermi cutoff disappeared for x=0.05 at a low temperature, whereas for x=0.1 and 0.2, it remained at all temperatures but the intensity at the Fermi level decreased gradually with decreasing temperature. Our experimental results suggest that the MIT is caused by the localization of holes in the O 2p band and that the "insulating" phase below the MIT is indeed a mixture of insulating and metallic regions.

We have studied the chemical potential shift and changes in the electronic density of states near the Fermi level (E (F)) as functions of temperature and carrier concentration in Pr1-x Ca (x) MnO3 (PCMO) and Nd1-x Sr (x) MnO3 (NSMO) by measurements of photoemission spectra. Suppression of the chemical potential shift as a function of carrier concentration has been observed in PCMO and NSMO near and in the composition range where the CE-type antiferromagnetic charge-ordered (CO) phase appears at low temperatures. This result indicates that there is charge self-organization on a microscopic scale such as stripe formation in this composition range. In the ferromagnetic metallic phase of NSMO, we found a large temperature-dependent chemical potential shift at low temperatures and attributed this to double-exchange mechanism. Suppression of the temperature-dependent chemical potential shift near Curie temperature was observed, possibly associated with the formation of correlated polarons. In the valence band near the E (F) of PCMO, spectral weight was transferred with hole doping, leading to a finite intensity at E (F) even in the paramagnetic insulating phase above the CO phase for x greater than or similar to 0.3, and as the temperature was lowered, a clear gap opening was observed in the CO phase.

We report the observation of changes in the electronic structure of La(1-x)Sr(x)MnO(3) (LSMO) across the filling-control metal-insulator (MI) transition by means of in situ angle-resolved photoemission spectroscopy (ARPES) of epitaxial thin films. The Fermi surface gradually disappears near the MI transition by transferring spectral weight from the coherent band near the Fermi level (E(F)) to the lower Hubbard band, whereas a pseudogap behavior also exists in the ARPES spectra in the close vicinity of E(F) for metallic LSMO. These results indicate that the spectral weight transfer derived from strong electron-electron interaction dominates the gap formation in LSMO associated with the filling-control MI transition.

We have measured photoemission spectra of two kinds of TiO2-capped VO2 thin films, namely, that with a rutile-type TiO2 capping layer (r-TiO2/VO2) and that with an amorphous TiO2 capping layer (a-TiO2/VO2). Below the metal-insulator transition temperature of the VO2 thin films, similar to 300 K, TiO2 and VO2 form an interface between a band insulator with the d(0) electronic configuration and a Mott (or Mott-Peierls) insulator with the d(1) electronic configuration. Metallic states were, however, not observed at the interfaces, in contrast to the interfaces between the d(0) band insulator SrTiO3 and the d(1) Mott insulator LaTiO3. We discuss possible origins of the difference between TiO2/VO2 and SrTiO3/LaTiO3, and suggest the importance of the polarity discontinuity in the latter interface. The stronger incoherent part was observed in r-TiO2/VO2 than in a-TiO2/VO2, suggesting Ti-V atomic diffusion due to the higher deposition temperature for r-TiO2/VO2.

We characterized the electronic structure of LaVOx thin films by x-ray photoemission spectroscopy. We could distinguish the photoemission spectra among the x=3 phase (LaV3+O3), the x=4 phase (LaV5+O4), and the mixed phase regime. In the mixed phase, the surface was found to be mostly LaVO4. We could observe the evolution of the electronic structure in the growth phase diagram by systematically measuring core-level photoemission spectra. The V4+ component present in the x=3 phase may be due to charge transfer to avoid the polar catastrophe of the polar LaVO3 thin-film surface and/or to the oxidation of the surface. (C) 2007 American Institute of Physics.

Epitaxial Ba0.5Sr0.5TiO3 thin films were prepared on Nb-doped SrTiO3 (100) substrates by the pulsed laser deposition technique and were studied by measuring the Ti 2p -> 3d resonant photoemission spectra in the valence-band region as a function of film thickness, both at room temperature and low temperature. The results demonstrated an abrupt variation in the spectral structures between 2.8 nm (similar to 7 ML) and 2.0 nm (similar to 5 ML) Ba0.5Sr0.5TiO3 films, suggesting that there exists a critical thickness for phase change in the range of 2.0-2.8 nm. This may be ascribed mainly to the intrinsic size effects. (C) 2007 American Institute of Physics.

We have performed an in situ photoemission study of Pr1-xCaxMnO3 thin films grown on LaAlO3 (001) by laser molecular beam epitaxy. By utilizing Mn 2p -> 3d resonant photoemission, we obtained the Mn 3d partial density of states. There was no density of states at the Fermi level, consistent with the insulating behavior of this material. Chemical potential pinning, which was observed in bulk samples, was not observed in the present study, consistent with the suppression of charge ordering under the compressive strain from the substrate. (c) 2006 Elsevier B.V. All rights reserved.

We have performed Ti 2p core-level photoemission and X-ray absorption spectroscopic (XAS) studies on the insulating barrier SrTiO3 (STO) layer sandwiched by La0.6Sr0.4MnO3 (LSMO) layers. The elemental selectivity of the techniques enabled us to extract the electronic structure of the constituent layers in the vicinity of the heterointerfaces. The Ti 2p core-level and XAS spectra clearly showed Ti4+ states and did not exhibit any indication of Ti3+ states. 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. (c) 2006 Elsevier B.V. All rights reserved.

We have performed an in situ angle-resolved photoemission spectroscopy (ARPES) on single-crystal surfaces of La0.6Sr0.4MnO3 (LSMO) thin films to investigate the electronic structure near the Fermi level (E-F). ARPES spectra near E-F reveal the existence of a dispersive Mn-3d derived conduction band indicative of an electron pocket centered at the Gamma point. Comparing the experimental band structure with the local density approximation + U band-structure calculation, the estimated size of the Fermi surface is consistent with the prediction of the calculation, while the measured bandwidth is significantly narrower than the calculation. The discrepancy between the two suggests that the renormalization effect is significant due to the strong electron correlation in LSMO. (c) 2006 Elsevier B.V. All rights reserved.

Transition-metal (TM) oxides have been attracting great interest because of their remarkable physical properties such as metal-insulator transition, colossal magnetoresistance, and high-temperature superconductivity. These properties are considered to originate from strong electron correlation. Nowadays a lot of progress has been made in the photoemission studies of TM oxides with threedimensional perovskite structures owing to the availability of high-quality single crystals of bulk and epitaxial thin films. Even angle-resolved photoemission has become possible through the progress in synthesizing high-quality samples. Here, we give an overview of high-resolution photoemission studies on perovskite-type Ti, V, Mn, Fe, Ni and Ru oxides, and show how high-resolution photoemission has contributed to the understanding of the electronic structures of these materials. © 2007 Springer.

We used x-ray photoemission spectroscopy to investigate the electronic structure of one to five unit cell thick layers of the Mott insulator LaVO3 embedded in LaAlO3. By limiting the upper layer of LaAlO3 to three unit cells, the underlying LaVO3 could be probed. The V 2p core-level spectra had both V3+ and V4+ components, and above two unit cell thick LaVO3, the structures exhibited spectra similar to bulk samples. The atomically flat surfaces enabled the study of the emission angle dependence, which indicates that the V4+ is localized to the topmost layer. These results demonstrate the potential for probing interface electronic structure in oxide ultrathin films by surface spectroscopy. (c) 2006 American Institute of Physics.

We have performed an angle-resolved photoemission spectroscopy study of La0.6Sr0.4FeO3 using in situ prepared thin films and determined its band structure. The experimental band dispersions could be well explained by an empirical band structure assuming the G-type antiferromagnetic state. However, the Fe 3d bands were found to be shifted downward relative to the Fermi level (E-F) by similar to 1 eV compared with the calculation and to form a gap of similar to 1 eV at E-F. We attribute this observation to a strong localization effect of doped holes due to polaron formation.

We have measured photoemission spectra of SrTiO3/LaTiO3 superlattices with a topmost SrTiO3 layer of variable thickness. A finite coherent spectral weight with a clear Fermi cutoff was observed at chemically abrupt SrTiO3/LaTiO3 interfaces, indicating that an "electronic reconstruction" occurs at the interface between the Mott insulator LaTiO3 and the band insulator SrTiO3. For SrTiO3/LaTiO3 interfaces annealed at high temperatures (similar to 1000 degrees C), which leads to Sr/La atomic interdiffusion and hence to the formation of La1-xSrxTiO3-like material, the intensity of the incoherent part was found to be dramatically reduced whereas the coherent part with a sharp Fermi cutoff was enhanced due to the spread of charge. These important experimental features are well reproduced by layer dynamical-mean-field-theory calculation.

We have studied the chemical potential shift and changes in the electronic density of states near the Fermi level (E-F) as a function of carrier concentration in Pr1-xCaxMnO3 (PCMO, 0.2 <= x <= 0.65) through the measurements of photoemission spectra. The results showed that the chemical potential shift was suppressed for x greater than or similar to 0.3, where the charge-exchange (CE) type antiferromagnetic charge-ordered state appears at low temperatures. We consider this observation to be related to charge self-organization such as stripe formation on a microscopic scale in this composition range. Together with the previous observation of monotonous chemical potential shift in La1-xSrxMnO3, we conclude that the tendency toward the charge self-organization increases with decreasing bandwidth. In the valence band, spectral weight of the Mn 3d e(g) electrons in PCMO was transferred from similar to 1 eV below E-F to the region near E-F with hole doping, leading to a finite intensity at E-F even in the paramagnetic insulating phase for x greater than or similar to 0.3, probably related with the tendency toward charge self-organization. The finite intensity at E-F in spite of the insulating transport behavior is consistent with fluctuations involving ferromagnetic metallic states.

We have measured the temperature dependence of the photoemission and x-ray absorption spectra of La1-xSrxFeO3 (LSFO) epitaxial thin films with x = 0.67, where charge disproportionation (3Fe(3.67+) -> 2Fe(3+) + Fe5+) resulting in long-range spin and charge ordering is known to occur below T-CD = 190K. With decreasing temperature we observed gradual changes of the spectra with spectral weight transfer over a wide energy range of similar to 5 eV. Above T-CD the intensity at the Fermi level was relatively high compared to that below T-CD but still much lower than that in conventional metals. We also found a similar temperature dependence for x = 0.4, and to a lesser extent for x = 0.2. These observations suggest that a local charge disproportionation occurs not only in the x = 0.67 sample below T-CD but also over a wider temperature and composition range in LSFO. This implies that the tendency toward charge disproportionation may be the origin of the unusually wide insulating region of the LSFO phase diagram.

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.(c) 2006 American Institute of Physics.

We have performed an in situ angle-resolved photoemission spectroscopy (ARPES) on single-crystal surfaces of La0.6Sr0.4MnO3 (LSMO) thin films grown on SrTiO3 (001) substrates by laser molecular beam epitaxy, and investigated the electronic structure near the Fermi level (E-F). The experimental results were compared with the band-structure calculation based on local density approximation + U. The band structure of LSMO thin films consists of several highly dispersive O 2p derived bands in the binding energy range of 2.0-6.0 eV and Mn 3d derived bands near E-F. ARPES spectra around the Gamma point show a dispersive band near E-F indicative of an electron pocket centered at the Gamma point, although the suppression of spectral weight occurs in the vicinity of E-F. Compared with the band-structure calculation, the observed conduction band is assigned to the Mn 3de(g) majority-spin band responsible for the half-metallic nature of LSMO. We have found that the estimated size of the Fermi surface is consistent with the prediction of the band-structure calculation, while the band width becomes significantly narrower than the calculation. Also, the intensity near E-F is strongly reduced. The origin of these discrepancies between the experiment and the calculation is discussed based on the strong electron correlation and the strong electron-phonon coupling.

We have performed an in situ angle-resolved photoemission spectroscopy (ARPES) on single-crystal surfaces of La0.6Sr0.4MnO3 (LSMO) thin films grown on SrTiO3 (001) substrates by laser molecular beam epitaxy, and investigated the electronic structure near the Fermi level (E-F). The experimental results were compared with the band-structure calculation based on local density approximation + U. The band structure of LSMO thin films consists of several highly dispersive O 2p derived bands in the binding energy range of 2.0-6.0 eV and Mn 3d derived bands near E-F. ARPES spectra around the Gamma point show a dispersive band near E-F indicative of an electron pocket centered at the Gamma point, although the suppression of spectral weight occurs in the vicinity of E-F. Compared with the band-structure calculation, the observed conduction band is assigned to the Mn 3de(g) majority-spin band responsible for the half-metallic nature of LSMO. We have found that the estimated size of the Fermi surface is consistent with the prediction of the band-structure calculation, while the band width becomes significantly narrower than the calculation. Also, the intensity near E-F is strongly reduced. The origin of these discrepancies between the experiment and the calculation is discussed based on the strong electron correlation and the strong electron-phonon coupling.

We have investigated the Mn 3d partial density of states in SrTiO3(STO)/La0.6Sr0.4MnO3(LSMO) heterointerfaces as well as of LSMO films using Mn 2p-3d resonant photoemission spectroscopy. The strong enhancement of the Mn 3d spectra at the Mn 2p-3d threshold enables us to extract the Mn 3d spectra of LSMO layers in the vicinity of the interface with the STO overlayers. We have found that the spectral intensity of e(g up arrow) states near the Fermi level is drastically reduced when the LSMO film is capped with STO overlayers. The close similarity in reduction of the intensity of the e(g up arrow) states between the STO/LSMO interface and hole-doped LSMO films suggests that the hole doping into the LSMO layer close to the STO layer originates from the chemical carrier-concentration modulation at the valence-mismatched interface composed of the stacking sequence -TiO2-SrO-MnO2-La0.6Sr0.4O-, which is inevitable for multilayers based on perovskite oxides. (C) 2006 American Institute of Physics.

Perovskite-type BiNiO3 is an insulating antiferromagnet in which a charge disproportionation occurs at the Bi site. La substitution for Bi suppresses the charge disproportionation and makes the system metallic. We have measured the photoemission and x-ray-absorption (XAS) spectra of Bi1-xLaxNiO3 to investigate how the electronic structure changes with La doping. From Ni 2p XAS, we observed an increase of the valence of Ni from 2+ toward 3+. Combined with the core-level photoemission study, it was found that the average valence of Bi remains similar to 4+ and that the Ni valence behaves as similar to(2+x)+, that is, La substitution results in a hole doping at the Ni sites. In the valence-band photoemission spectra, we observed a Fermi cutoff for x>0, consistent with the metallic behavior of the La-doped compounds. The Ni 2p XAS, Ni 2p core-level photoemission, and valence-band photoemission spectra were analyzed by configuration-interaction cluster-model calculation, and the spectral line shapes were found to be consistent with the gradual Ni2+-> Ni3+ valence change.

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.

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.

We have investigated the electronic structure of a La0.6Sr0.4FeO3 (LSFO)/La0.6Sr0.4MnO3 (LSMO) heterointerface using Mn 2p-3d resonant photoemission spectroscopy and X-ray absorption spectroscopy. We have found that the spectral intensity of e(g up arrow) states near the Fermi level is gradually reduced with increasing LSFO overlayer thickness. The spectral behavior is quite similar to that of hole-doped LSMO films, indicating hole-doping in LSMO layers. We have also found that Fe 2p X-ray absorption spectrum of a LSMO/LSFO/LSMO trilayer shows the spectral evidence for electron doping in the corresponding LSFO layer. These results indicate the occurrence of charge transfer at interfaces between LSMO and LSFO layers. (c) 2005 Elsevier B.V. All rights reserved.

We have performed in situ photoemission and O 1s X-ray absorption studies of La1-xSrxFeO3 (0 <= x <= 0.67) epitaxial thin films grown by laser molecular beam epitaxy. We found that hole doping induces spectral weight transfer from below the Fermi level to above it across the gap in a highly non-rigid-band-like manner. We also observed the temperature dependence on the spectra of the x = 0.67 sample, which shows a charge disproportionation at 190 K. With decreasing temperature, we observed gradual changes of the spectra with spectral weight transfer primarily within the e, band. (c) 2005 Elsevier B.V. All rights reserved.

We have performed an in situ angle-resolved photoemission study of atomically flat surfaces of La1-xSrMnO3 (LSMO) single-crystal thin films grown on SrTiO3 (0 0 1) substrates by laser molecular beam epitaxy, and investigated the changes in the band structure induced by hole-doping. The band structures of LSMO x = 0.4 thin films consist of several highly dispersive O 2p-derived bands at the binding energies of 2.5-7 eV, almost dispersionless Mn 3d t(2g) bands at similar to 2 eV, and a dispersive Mn 3d e(g)-derived majority band at similar to 0.5 eV We find that the energy positions of these bands monotonically shift toward higher binding energy with decreasing hole concentration in a rigid-band manner, whereas the lower binding energy part of the e(g)-derived emission within 1.5 eV of the Fermi level (E-F) gradually disappears. These results suggest that the simple rigid-band model does not describe the electronic structure near EF of LSMO. (c) 2005 Elsevier B.V. All rights reserved.

We have studied the electronic structure of epitaxially grown thin films of La1-xSrxFeO3 by in situ photoemission spectroscopy (PES) and x-ray-absorption spectroscopy (XAS) measurements. The Fe 2p and valence-band PES spectra and the O 1s XAS spectra of LaFeO3 have been successfully reproduced by configuration-interaction cluster-model calculation and, except for the satellite structure, by band-structure calculation. From the shift of the binding energies of core levels, the chemical potential was found to be shifted downward as x was increased. Among the three peaks in the valence-band spectra of La1-xSrxFeO3, the peak nearest to the Fermi level (E-F), due to the "e(g) band," was found to move toward E-F and became weaker as x was increased, whereas the intensity of the peak just above E-F in the O 1s XAS spectra increased with x. The gap at E-F was seen for all values of x. These results indicate that changes in the spectral line shape around E-F are dominated by spectral weight transfer from below to above E-F across the gap and are therefore highly nonrigid-bandlike changes.

We have investigated the Mn 3d spectral function in La0.6Sr0.4FeO3(LSFO)/La0.6Sr0.4MnO3(LSMO) heterointerfaces as well as of La1-x SrxMnO3 films using Mn 2p-3d resonant photoemission spectroscopy. The strong enhancement of the Mn 3d spectra at the Mn 2p-3d threshold enables us to extract the Mn 3d spectra of LSMO layers in the vicinity of the interface with the LSFO overlayers. We have found that the spectral intensity of e(gup arrow) states near the Fermi level is gradually reduced with increasing LSFO overlayer thickness and is finally saturated at 5-7 ML. The close similarity in reduction of the intensity of the e(gup arrow) 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. (C) 2004 American Institute of Physics.