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We use a surface-selective angle-resolved photoemission spectroscopy and unveil the electronic nature on the topmost layer of Sr_{2}RuO_{4} crystal, consisting of slightly rotated RuO_{6} octahedrons. The γ band derived from the 4d_{xy} orbital is found to be about three times narrower than that for the bulk. This strongly contrasts with a subtle variation seen in the α and β bands derived from the one-dimensional 4d_{xz/yz}. This anomaly is reproduced by the dynamical mean-field theory calculations, introducing not only the on-site Hubbard interaction but also the significant Hund's coupling. We detect a coherence-to-incoherence crossover theoretically predicted for Hund's metals, which has been recognized only recently. The crossover temperature in the surface is about half that of the bulk, indicating that the naturally generated monolayer of reconstructed Sr_{2}RuO_{4} is extremely correlated and well isolated from the underlying crystal.

We show that finite temperature variational cluster approximation (VCA) calculations on an extended Falicov-Kimball model can reproduce angle-resolved photoemission spectroscopy (ARPES) results on ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ across a semiconductor-to-semiconductor structural phase transition at 325 K. We demonstrate that the characteristic temperature dependence of the flat-top valence band observed by ARPES is reproduced by the VCA calculation on the realistic model for an excitonic insulator only when the strong excitonic fluctuation is taken into account. The present calculations indicate that ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ falls in the Bose-Einstein condensation regime of the excitonic insulator state.

The electronic structure of the formally ${\mathrm{Cu}}^{3+}$ metallic ${\mathrm{LaCuO}}_{3}$ has been studied by photoemission and x-ray-absorption spectroscopy. By analyzing the valence-band and Cu $2p$ core-level photoemission spectra using a ${\mathrm{CuO}}_{6}$ cluster model, the charge-transfer energy is estimated to be $\ensuremath{-}1\mathrm{eV},$ indicating that the ground state is dominated by the ${d}^{9}L$ configuration with which the ${d}^{8}$ configuration is strongly hybridized, where $L$ denotes a ligand hole. However, agreement between the experimental results and the cluster-model calculations is not satisfactory for the detailed line shape of the main peaks. Especially, the Cu $2p$ x-ray-absorption spectrum cannot be well explained by the single-site cluster-model calculation, suggesting the importance of intercluster interaction. On the other hand, the existence of the charge-transfer satellite in the Pauli-paramagnetic state is explained by Hartree-Fock band-structure and self-energy calculations using parameters deduced from the analysis of the photoemission spectra.

We have studied the low-energy electronic structure of a Kondo insulator YbB{sub 12} by high-resolution photoemission spectroscopy. A {open_quote}{open_quote}Kondo peak{close_quote}{close_quote} is observed {approximately}25 meV below the Fermi level, which agrees well with the Kondo temperature, whereas the gap at the Fermi level is found much smaller, indicating that the magnetic properties at higher temperatures (75 K) are indeed determined by the Kondo effect in spite of the gap formation at lower temperatures. A renormalized band picture is presented to describe the coexistence of the Kondo peak and the transport gap as well as the highly asymmetric line shape of the Kondo peak. {copyright} {ital 1996 The American Physical Society.}

We have performed a temperature dependent angle-resolved photoemission spectroscopy (ARPES) study of the tri-layer high-$T_c$ cuprate superconductor (HTSC) Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$ (Bi2223), and have shown that the \textquotedblleft effective\textquotedblright superconducting (SC) gap $\Delta_{\rm{sc}}$ defined at the end point of the Fermi arc and the $T_c$ (= 110 K) approximately satisfies the weak-coupling BCS-relationship 2$\Delta_{\rm{sc}}$ = 4.3$k_{\rm{B}}T_c$. Combining this result with previous ARPES results on single- and double-layer cuprates, we show that the relationship between 2$\Delta_{\rm{sc}}$ = 4.3$k_{\rm{B}}T_c$ holds for various HTSCs. Furthermore, at $T$ $\sim$ $T_c$, the quasi-patricle width at the end point of the Fermi arc is found to coincide with $\Delta_{\rm{sc}}$, consistent with the context of Planckian dissipation.

We report the first observation of the multilayer band splitting in the optimally doped trilayer cuprate ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{10+\ensuremath{\delta}}$ (Bi2223) by angle-resolved photoemission spectroscopy. The observed energy bands and Fermi surfaces are originated from the outer and inner ${\mathrm{CuO}}_{2}$ planes (OP and IP). The OP band is overdoped with a large $d$-wave gap around the node of ${\ensuremath{\Delta}}_{0}\ensuremath{\sim}43\text{ }\text{ }\mathrm{meV}$ while the IP is underdoped with an even large gap of ${\ensuremath{\Delta}}_{0}\ensuremath{\sim}60\text{ }\text{ }\mathrm{meV}$. These energy gaps are much larger than those for the same doping level of the double-layer cuprates, which leads to the large ${T}_{c}$ in Bi2223. We propose possible origins of the large superconducting gaps for the OP and IP: (1) minimal influence of out-of-plane disorder and a proximity effect and (2) interlayer tunneling of Cooper pairs between the OP and IP.

We report on a photoemission study of ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ that has a quasi-one-dimensional structure and an insulating ground state. Ni $2p$ core-level spectra show that the Ni $3d$ subshell is partially occupied and the Ni $3d$ states are heavily hybridized with the Se $4p$ states. In angle-resolved photoemission spectra, the valence-band top is found to be extremely flat, indicating that the ground state can be viewed as an excitonic insulator state between the Ni $3d$--Se $4p$ hole and the Ta $5d$ electron. We argue that the high atomic polarizability of Se plays an important role to stabilize the excitonic state.

Tandis que les structures proeminentes communes aux energies de liaison elevees (>2 eV) sont attribuees aux etats 2p de l'oxygene dans les plans Bi-O, les emissions proches du niveau de Fermi sont differentes les unes des autres. Ce dernier point montre que ces etats sont dus a l'oxygene 2p hybride par les niveaux 3d du metal de transition

Using hard x-ray (HX; hnu=5.95 keV) synchrotron photoemission spectroscopy (PES), we study the intrinsic electronic structure of La(1-x)Sr(x)MnO(3) (LSMO) thin films. Comparison of Mn 2p core-levels with soft x-ray (SX; hnu approximately 1000 eV) PES shows a clear additional well-screened feature only in HX PES. Takeoff-angle dependent data indicate its bulk (> or =20 A) character. The doping and temperature dependence track the ferromagnetism and metallicity of the LSMO series. Cluster model calculations including charge transfer from doping-induced states show good agreement, confirming this picture of bulk properties reflected in Mn 2p core-levels using HX PES.

High-energy $(h\ensuremath{\nu}=5.95\mathrm{keV})$ synchrotron photoemission spectroscopy (PES) is used to study bulk electronic structure of ${\mathrm{Na}}_{0.35}{\mathrm{CoO}}_{2}\ensuremath{\cdot}1.3{\mathrm{H}}_{2}\mathrm{O},$ the layered superconductor. In contrast to three-dimensional doped Co oxides, Co $2p$ core level spectra show well-separated ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions. Cluster calculations suggest low-spin ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ character, and a moderate on-site Coulomb correlation energy ${U}_{\mathrm{dd}}\ensuremath{\sim}3\char21{}5.5\mathrm{eV}.$ Photon dependent valence band PES identifies Co $3d$ and O $2p$ derived states, in near agreement with band-structure calculations.