The Effects of Correlation on the Fermi Surface of
Superconducting Sodium Cobaltate

Michelle Johannes

Computational Materials Science Branch, Naval Research Laboratory, 4555 Overlook Ave, Washington D.C. 20375, USA

NaxCoO2yH2O is a recently discovered superconductor with an unresolved pairing symmetry. Understanding the Fermi surface is an essential element of understanding the superconductivity, but theoretical (LDA or LSDA) and experimental (ARPES) results disagree. Though two distinct types of surfaces are predicted by calculation: one large hole surface of a1g symmetry and six identical small hole surfaces of eg symmetry, only the large surface has been detected by ARPES measurements. These measurements also indicate a rather large degree of renormalization, leading to claims of strong correlation that may also be responsible for the suppression of the smaller hole pockets. Although LSDA+U calculations seemingly resolve the problem by removing the small pockets, a more thorough treatment of correlation, using DMFT shows that dynamical correlations actually stabilize them. One possible answer is that the electronic structure is very sensitive to relaxation of the oxygen atoms and that subtle surface effects come into play. Whether or not these small hole pockets exist remains an open question of some importance, the answer to which may provide insight into the degree of correlation in the system and inform our theoretical methods of treating it.