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We have systematically studied the strong correlation effects in A-site ordered perovskites CaCu3Ti4-xRuxO12 (x = 0, 1, 3.5, and 4) by using photoemission and inverse photoemission spectroscopies. In x = 0, 1, 3.5, the peak positions of the strongly correlated Cu 3d states around -3.8 eV and Ti 3d states around 3.6 eV little change. On the other hand, in x = 4, the Cu 3d states split into two peaks around -2.5 and -4 eV. These indicate that Ti plays an important role to retain the strong correlation effects. In addition, the multiplet structures of Cu 3d final states from -8 to -14 eV become weak as Ru increases, indicating the reduction of the localized characters of Cu 3d states. At the Fermi level, we observe the absence of spectral weight in x = 0, 1 and the development of Ru 4d in-gap states in x = 3.5, 4, which give rise to the metal-insulator transition between x = 1 and x = 3.5., Comment: 12 pages, 3 figures

We have investigated the electronic structure of A-site ordered CaCu$_3$Ti$_4$O$_{12}$ as a function of temperature by using angle-integrated and -resolved photoemission spectroscopies. Intrinsic changes of the electronic structure have been successfully observed in the valence band region by the careful consideration of charging effects. The obtained photoemission results have revealed that the intensity of the nearly non-dispersive Cu 3$d$-O 2$p$ hybridized bands at the binding energy of $\sim$2 eV increases with decreasing temperature from 300 to 120 K. This suggests that the density of the localized states, caused by the strong correlation effects, enhances as temperature decreases., Comment: 13 pages, 3 figures

We have performed the photoemission and inverse photoemission experiments to elucidate the origin of Mott insulating states in A-site ordered perovskite CaCu$_3$Ti$_4$O$_{12}$ (CCTO). Experimental results have revealed that Cu 3$d$-O 2$p$ hybridized bands, which are located around the Fermi level in the prediction of the local-density approximation (LDA) band calculations, are actually separated into the upper Hubbard band at $\sim$ 1.5 eV and the lower Hubbard band at $\sim$ $-$1.7 eV with a band gap of $\sim$ 1.5-1.8 eV. We also observed that Cu 3$d$ peak at $\sim$ $-$3.8 eV and Ti 3$d$ peak at $\sim$ 3.8 eV are further away from each other than as indicated in the LDA calculations. In addition, it is found that the multiplet strucutre around $-$9 eV includes a considerable number of O 2$p$ states. These observations indicate that the Cu 3$d$ and Ti 3$d$ electrons hybridized with the O 2$p$ states are strongly correlated, which originates in the Mott-insulating states of CCTO., Comment: 11 pages, 3 figures

Bulk-sensitive hard x-ray photoemission spectroscopy (HAXPES) reveals for as-grown epitaxial films of half-metallic ferromagnetic CrO2(100) a pronounced screening feature in the Cr 2p3/2 core level and an asymmetry in the O 1s core level. This gives evidence of a finite, metal-type Fermi edge, which is surprisingly not observed in HAXPES. A spectral weight shift in HAXPES away from the Fermi energy is attributed to single-ion recoil effects due to high energy photoelectrons. In conjunction with inverse PES the intrinsic correlated Mott-Hubbard-type electronic structure is unravelled, yielding an averaged Coulomb correlation energy Uav ~ 3.2 eV., Comment: Manuscript 12 pages, 4 figures; Supplemental material 7 pages, 5 figures

We report angle-resolved photoemission spectroscopy (ARPES) results of A-site ordered perovskite CaCu$_3$Ti$_4$O$_{12}$. We have observed the clear band dispersions, which are shifted to the higher energy by 1.7 eV and show the band narrowing around 2 eV in comparison with the local density approximation calculations. In addition, the high energy multiplet structures of Cu 3$d^8$ final-states have been found around 8 - 13 eV. These results reveal that CaCu$_3$Ti$_4$O$_{12}$ is a Mott-type insulator caused by the strong correlation effects of the Cu 3$d$ electrons well hybridized with O 2$p$ states. Unexpectedly, there exist a very small spectral weight at the Fermi level in the insulator phase, indicating the existence of isolated metallic states., Comment: 5 pages, 5 figures

Material specific electronic band structure of the electron-doped high-Tc cuprate Nd(1.85)Ce(0.15)CuO(4) (NCCO) is calculated within the pseudo gap regime, using the recently developed generalized LDA+DMFT+Sigma_k scheme. LDA/DFT (density functional theory within local density approximation) provides model parameters (hopping integral values, local Coulomb interaction strength) for the one-band Hubbard model, which is solved by DMFT (dynamical mean-field theory). To take into account pseudogap fluctuations LDA+DMFT is supplied with "external" k-dependent self-energy Sigma_k, which describes interaction of correlated conducting electrons with non-local Heisenberg-like antiferromagnetic (AFM) spin fluctuations responsible for pseudo gap formation. Within this LDA+DMFT+Sigma_k approach we demonstrate the formation of pronounced "hot-spots" on the Fermi surface (FS) map in NCCO, opposite to our recent calculations for Bi(2)Sr(2)CaCu(2)O(8-d) (Bi2212), which have produced rather extended region of FS "destruction". There are several physical reasons for this fact: (i) the "hot-spots" in NCCO are located closer to Brillouin zone center; (ii) correlation length of AFM fluctuations \xi is larger for NCCO; (iii) pseudogap potential \Delta is stronger, than in Bi2212. Comparison of our theoretical data with recent bulk sensitive high-energy angle-resolved photoemission (ARPES) data for NCCO provides good semiquantitative agreement. Based on that comparison alternative explanation of the van-Hove singularity at -0.3 eV is proposed. Optical conductivity both for Bi2212 and NCCO is also calculated within LDA+DMFT+Sigma_k and compared with experimental results, demonstrating satisfactory agreement., Comment: 8 pages, 10 figures, 1 table

Angle resolved photoemission spectroscopy (ARPES) has been performed on quasi-one dimensional Li0.9Mo6O17 using photon energy 500 eV. Measured band dispersions are in agreement with those from both low photon energy measurements and band structure calculations. The momentum integrated ARPES spectrum is well fit by the finite temperature Luttinger liquid (LL)spectral function, with an anomalous exponent 0.6 that is the same within experimental uncertainty as the value found with photon energy 30 eV. These identical findings at both low and high photon energies are entirely consistent with reasoning based on the crystal structure, that the quasi-one dimensional chains lie two layers below the cleavage plane so that the observed spectroscopic LL behavior of Li0.9Mo6O17 is a bulk property., Comment: Accepted for publication in Physical Review B

We have performed the bulk-sensitive high-resolution soft x-ray photoemission study of a Kondo semiconductor CeRhAs and related compounds CeNiSn and CePdSn. The comparison of the spectra of polycrystalline CePdSn on the fractured and scraped surfaces shows that the fracturing of the samples is much better than the scraping in order to obtain intrinsic photoemission spectra. The Ce 4d core-level spectra show clear differences in the electronic states among the materials., Comment: 4 pages including 3 figures, to appear in Journal of Electron Spectroscopy and Related Phenomena

We have studied the electronic structure and charge ordering (Verwey) transition of magnetite (Fe3O4) by soft x-ray photoemission. Due to the enhanced probing depth and the use of different surface preparations we are able to distinguish surface and volume effects in the spectra. The pseudogap behavior of the intrinsic spectra and its temperature dependence give evidence for the existence of strongly bound small polarons consistent with both dc and optical conductivity. Together with other recent structural and theoretical results our findings support a picture in which the Verwey transition contains elements of a cooperative Jahn-Teller effect, stabilized by local Coulomb interaction.

We report the Fermi surfaces of the superconductor Sr2RuO4 and the non-superconductor Sr1.8Ca0.2RuO4 probed by bulk-sensitive high-energy angle-resolved photoemission. It is found that there is one square-shaped hole-like, one square-shaped electron-like and one circle-shaped electron-like Fermi surface in both compounds. These results provide direct evidence for nesting instability giving rise to magnetic fluctuations. Our study clarifies that the electron correlation effects are changed with composition depending on the individual band., Comment: 5 pages, 3 figures including 2 color figures