Band-structure calculations of Fermi-surface pockets in ortho-II YBa2Cu3O6.5

We explore whether the quantum oscillation signals recently observed in ortho-II YBa2Cu3O6.5 (OII-Y123) may be explained by conventional density-functional band-structure theory. Our calculations show that the Fermi surface of OII-Y123 is extremely sensitive to small shifts in the relative positions of the bands. With rigid band shifts of around +/- 30 meV small tubular pockets of Fermi surface develop around the Y point in the Brillouin zone. The cross-sectional areas and band masses of the quantum oscillatory orbits on these pockets are close to those observed. The differences between the band structure of OII-Y123 and YBa2Cu4O8 (Y124) are discussed with reference to the very recent observation of quantum oscillations in Y124.