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RESEARCH PRODUCT

Optical studies of gap, hopping energies, and the Anderson-Hubbard parameter in the zigzag-chain compoundSrCuO2

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We have investigated the electronic structure of the zig-zag ladder (chain) compound ${\mathrm{SrCuO}}_{2}$ combining polarized optical absorption, reflection, photoreflectance, and pseudo-dielectric-function measurements with the model calculations. These measurements yield an energy gap of 1.42 eV (1.77 eV) at 300 K along (perpendicular to) the Cu-O chains. We have found that the lowest-energy gap, the correlation gap, is temperature independent. The electronic structure of this oxide is calculated using both the local-spin-density approximation with gradient correction method and the tight-binding theory for the correlated electrons. The calculated density of electronic states for noncorrelated and correlated electrons shows quasi-one-dimensional character. The correlation gap values of 1.42 eV (indirect transition) and 1.88 eV (direct transition) have been calculated with the electron hopping parameters $t=0.30\mathrm{eV}$ (along a chain), ${t}_{\mathrm{yz}}=0.12\mathrm{eV}$ (between chains), and the Anderson-Hubbard repulsion on copper sites $U=2.0\mathrm{eV}.$ We concluded that ${\mathrm{SrCuO}}_{2}$ belongs to the correlated-gap insulators.