Ab InitioComputation of Optical Conductivity ofYBa2Cu3O7−x

Optical conductivity computed from first principles suggests that the feature near 0.37 eV is due to interband transitions from the bridging O that connects the Cu-O planes to the Cu-O chain. Screened Coulomb interaction between an electron and a hole broadens the interband peaks but does not lead to any new features that could be interpreted as excitonic. Variations in O content cause the low-energy optical feature to disappear as the bridging O states drop well below $E_{\mathrm{F}}$. We suggest that differences between single-crystal and powder data are likely due to compositional differences.