Optical conductivity of ortho-II YBa2Cu3O6.5

The ortho-II phase of YBa2Cu3O6.5 is characterized by a periodic alternation of empty Cu and filled Cu-O b-axis chains doubling the unit cell in the a direction. The extra oxygen in the full chains gives rise to an attractive potential for the holes in the planes. The planar bands split in two with a gap opening at the new Brillouin zone boundary k(x)=+/-pi/2, which we estimate from local-density approximation calculations. Using a planar model which treats the d-wave superconductivity in a mean field approximation, we show that interband transitions produce a strongly anisotropic feature in the optical conductivity controlled by a region in k-space close to (pi/2,pi/2). The edge position of this feature gives information on the temperature dependence of quasiparticle spectrum in this region. Bilayer splitting would show up as a double edge shape.