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The Cu-O pair-distribution function in a Bi 2 Sr 2 CaCu 2 O 8ϩy superconductor has been measured by polarized Cu K-edge extended x-ray-absorption fine structure at TϽT c . The results show an anomalous long Cu-O ͑planar͒ distance, 1.96 Å, assigned to distorted CuO 2 stripes of width W intercalated with undistorted stripes of width L. From the measurement of Lϭ15Ϯ0.5 Å we have calculated the energies E n of the bottom of the one-dimensional subbands of the superlattice and found that the Fermi level E F is tuned to a ''shape resonance'' E F ϪE n Ͻប D , where D is the Debye frequency, giving the T c amplification.

We have studied metallic SrVO3 and CaVO3 by inverse photoemission and high-resolution photoemission. In going from Sr to Ca, considerable spectral weight is transferred from the coherent band to the upper and lower Hubbard bands. Meanwhile, the overall intensity rather than the width of the coherent band decreases, implying that the bandwidth remains finite as the system approaches the Mott transition. The result implies that the e6'ect of long-range Coulomb interaction as well as short-range interaction becomes increasingly important towards the transition.It is well known that electron correlation enhances the conduction electron mass m * in metals. The mass should diverge toward a metal-insulator transition if the transition is of second order and the carrier number remains finite up to the transition point. ' Recent studies of a filling-control system La& Sr"Ti03 suggest that such a mass divergence indeed occurs in the electronic specific heats and the magnetic susceptibilities. ' In bandwidthcontrol systems, where the ratio between the interaction strength U and the bandwidth 8' is varied for a fixed band filling, a mass divergence has also been predicted by Brinkman and Rice. Recent studies of the infinitedimension Hubbard model at half filling indicate that the coherent quasiparticle (QP) band, which crosses the Fermi level (EF), is narrowed with U/W while the spectral intensity at E~r emains unaltered.In previous work, ' we made photoemission studies of various compounds with a d ' configuration ranging from a Mott-Hubbard insulator to a normal metal (YTi03, LaTi03, SrVO3, VO2, and Re03) in order to investigate how the single-particle spectral function p(co) evolves with U/W across the transition. The result has shown that, with increasing U/W; spectral weight is transferred from the coherent part (QP excitations) around E~to the incoherent part (reminiscent of the lower Hubbard band) -1.5 eV below EF, but that the overall intensity rather than the bandwidth of the coherent part appears to decrease in contrast to what has been predicted by the Hubbard-model calculations. From analysis of the photoemission spectra using a phenomenological self-energy correction, it has been suggested that the mass enhancement associated with the spectral weight transfer is largely compensated for by the band widening which is represented by an increasing degree of the k dependence of the self-energy. However, the limited energy resolution in the previous work ' has precluded detailed and unambiguous information about the low-energy electronic structure of these compounds such as the presence or absence of the narrow QP band at Ez, as predicted theoretically.In order to clarify these points, we have performed detailed photoemission and inverse-photoemission spectroscopy [bremsstrahlung isochromat spectroscopy (BIS)] studies of two metallic compounds CaVO& and SrVO3.With much improved energy resolution, we have established that the intensity at EF indeed decreases with U/W, and that there is no detectable narrow peak in the...

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