Fermi-surface pockets in $YBa_2Cu_3O_{6.5}$ : A comparison of ab initio techniques

We study the Fermi surface of metallic, non-magnetic \textit{ortho}-II YBa$_2$Cu$_3$O$_{6.5}$ using three different density-functional-based band-structure techniques (GGA, GGA+U, PSIC). The calculated Fermi surface exhibits no pockets in GGA+U and PSIC, a minor one in GGA. Upon shifting the Fermi level in the vicinity of the calculated value, we instead observe several pocket structures. We calculate their cross-sectional areas and cyclotron masses. Overall, our calculations show no solid evidence of the existence of electron-like --nor, in fact, of any-- Fermi surface pockets in this phase. This suggests that the origin of the pockets should be sought for in other, different phases.