Fermi Surface Topology of Na_0.5CoO₂ from the Hybrid Density Functional

The Fermi surface topology of Na_0.5CoO₂ is studied using the hybrid density functional theory. We first study a single (CoO₂)^0.5- layer model with the percentage of the nonlocal Hartree--Fock exchange changing from 0% to 20%. The results show that only when the mixed nonlocal Hartree--Fock exchange is between 1% and 5%, the Fermi surface topology is similar to the experimental one. With 3% HF exchange in the hybrid density functional, considering the effects of Na ions in the Na_0.5CoO₂ system, we find that the Fermi surface is split to double holes and small gaps open near the intersections between the Brillouin zone and the Fermi surface. Our results show that both the amounts of the nonlocal Hartree--Fock exchange in the hybrid density functional and the Na ions have much influence on the Fermi surface topology.