Charge density waves and the Coulomb correlation effects in Na2Ti2P2O (P=Sb, As)

Abstract

To explore the origin of the phase transitions in Na2Ti2P2O (P = Sb, As), we have investigated their band structures and phonon dispersions based on the ab initio density functional theory. We have found that the phonon softening instabilities occur for both compounds at q(X) and q(M), which lead to charge density wave (CDW) instabilities through the electron-phonon coupling. When the Coulomb correlation effect of Ti d electrons is taken into account, the CDW transition to a 2 x 1 x 1 supercell driven by the normal mode at q(X) produces the most stable state for both compounds. In the CDW ground states, Na2Ti2Sb2O and Na2Ti2As2O have the partial and full gap openings in the band structures, respectively, which are in good agreement with the observed transport and angle-resolved photoemission spectroscopy results. Our paper reveals that the Coulomb correlation effects of Ti d electrons are essential to properly describe the CDW transitions in Na2Ti2P2O.