ARPES on the non-magnetic low-spin transition metal oxides

In this talk I will present the results of angle-resolved photo-electron spectroscopy (ARPES) on correlated transition metal oxides (TMOs) and compare them to density functional theory (DFT) calculations. We have selected non-magnetic low-spin TMO’s to minimize the effect of magnetic correlations and anti-ferromagnetic ordering on the band dispersions. On a moderately correlated side I will discuss about Nb_0.75O_0.75 and on a strongly correlated side - about LaSrCoO₄.

Nb_0.75O_0.75 is a metallic system, which crystallizes in a simple cubic structure. The moderate correlations are expected in the Nb – 4d bands. A good agreement between our ARPES spectra and calculated band structure is found in general, especially for the part of the valence band near the Fermi level. This result supports the importance of metal-metal bonding in this compound and consideration of Nb_0.75O_0.75  structure as a three-dimensional (3D) network of corner-sharing Nb₆O₁₂ metal clusters. Additional energy shifts are required for O– 2p manifold and part of Nb – 4d complex in theoretical band structure in order to fully match theory with experiment. Additional bands, which are observed in ARPES data, are identified as surface bands located in the pockets of the bulk projected one-dimensional band structure.

LaSrCoO₄ is a much stronger correlated system than Nb_0.75O_0.75 and, therefore, we expect less agreement between ARPES and DFT theory. LaSrCoO₄ is an insulator, iso-structural to the famous superconducting 214-cuprates. Our ARPES study addresses the 3D-momentum space characterization of LaSrCoO₄ (LSCO) as it is expected to be a multiorbital interacting electron system. The observed k_z-dispersions in layered cobaltate suggest the existence of non-negligible effects in its electronic structure depending on the photon energy. The first ARPES excitation below the Fermi level possesses large spectral weight and a broad band width. Despite such correlations effects, we observe the in-plane dispersions within this band and can identify its orbital character with the help of LDA+U calculation.