Hall effect in CoO2 Layer with Hexagonal Structure

The Hall effect in layered cobalt oxides with hexagonal structure is examined. We have pointed out that the large thermopower in the cobalt oxides is caused by the degeneracy of $t_{2g}$ orbitals in Co ions. Here, we show that the orbital degeneracy brings about a Kagomé lattice electronic structure hidden in the CoO$_{2}$ triangular crystal lattice. This is because the electron hopping occurs between Co ions via neighboring oxygens by exchanging the orbitals in the triangular lattice. The importance of $t_{2g}$ orbital degeneracy on the transport properties of the cobalt oxides under the magnetic field at high temperatures is discussed in light of the theory.