Transport and magnetic properties along with a high-resolution valence band photoemission study of disordered double perovskite Sr2TiIrO6have been investigated. The insulator to insulator transition along with a magnetic transition concurrently occur at 240 K. The comparison of the valence band photoemission with band structure calculations suggests that the spin orbit coupling as well as the electron correlation are necessary to capture the line shape and width of the Ir 5d band. Room temperature valence band photoemission spectra show a negligibly small intensity at the Fermi energy, EF. A Fermi cut-off is observed at low temperatures employing high resolution. The spectral density of states at room temperature exhibits energy dependence signifying the role of electron-electron interaction. This energy dependence changes to below the magnetic transition evidencing the role of the electron-magnon coupling in a magnetically ordered state. The evolution of a pseudogap () explains the sudden increase in resistivity below 50 K in this disordered system. The temperature-dependent spectral density of states at EF exhibiting behaviour verifies the Altshuler-Aronov theory for correlated disordered systems.

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We investigate the electronic structure of ternary palladates APd3O4 (A = Sr, Ca) using valence band photoemission spectroscopy and band structure calculations. Energy positions of various features and overall width of the experimental valence band spectra are well captured by band structure calculations using hybrid functional. Band structure calculations within local density approximations lead to metallic ground state while the calculations using hybrid functional provide band gap of 0.25 eV and 0.22 eV for CaPd3O4 and SrPd3O4 respectively, suggesting moderate to strong electron correlation strength in these narrow band gap semiconducting palladates. High resolution spectra reveal negligibly small intensity at Fermi level, E F, for parent compounds, while hole doped SrPd3O4 (by 15% Li substitution at Sr site) exhibits a Fermi cut-off suggesting metallic character in contrast to semiconducting transport. These observations reveal the importance of localization of electrons in case where the Fermi edge falls in the mobility edge.

Here we have investigated the role of electron phonon coupling on the Raman spectrum of narrow bandgap semiconductors APd3O4 (A = Ca, Sr) and hole-doped system Sr0.85Li0.15Pd3O4. Four Raman active phonons are observed at room temperature for all three compounds as predicted by factor group analysis. The lowest energy phonon (∼190/202 cm−1) associated with Pd vibrations is observed to exhibit an asymmetric Fano-like lineshape in all the three compounds, indicating the presence of an interaction between the phonon and the electronic continuum. The origin of the electronic continuum states and electron–phonon coupling are discussed based on our laser power- and temperature-dependent Raman results. We have observed an enhanced strength of electron–phonon coupling in Sr0.85Li0.15Pd3O4 at low temperatures which can be attributed to the metallicity in this doped compound.

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We investigate the role of electron correlation in the electronic structure of honeycomb lattice Li2RuO3 using photoemission spectroscopy and band structure calculations. Monoclinic Li2RuO3 having Ru network as honeycomb lattice undergoes magneto-structural transition at T c ∼ 540 K from high temperature phase C2/m to low temperature dimerized phase P21/m. Room temperature valence band photoemission spectra reveal an insulating ground state with no intensity at Fermi level (E F). Ru 4d band extracted from high and low photon energy valence band photoemission spectra reveal that the surface and bulk electronic structures are very similar in this system. Band structure calculations using generalized gradient approximation leads to metallic ground state while screened hybrid (YS-PBE0) functional reveals opening up of a gap in almost degenerate d zx /d yz orbitals, whereas d xy orbital is already gapped. Ru 3d core level spectra with prominent unscreened feature provides direct evidence of strong electron correlation among Ru 4d electrons which is also manifested by |E − E F|2 dependence of spectral density of states in the vicinity of E F in the high-resolution spectra, establishing Li2RuO3 as Mott insulator.

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