How Cation-Covalency controls Electron Localization in t2g Perovskites

In the series of perovskites, SrVO3-CaVO3-LaTiO3-YTiO3, the transition-metal d electron becomes increasingly localized and undergoes a Mott transition. By defining a low-energy Hubbard Hamiltonian in the basis of NMTO Wannier functions for the t2g LDA band and solving it in the single-site dynamical mean-field approximation, it was recently shown [1] that simultaneously with the Mott transition, the orbital fluctuations are strongly suppressed. Here, we explain the underlying chemical mechanisms [2]: Proceeding through the series, covalency between the empty large-cation-d states and the occupied oxygen-p states causes the oxygen octahedra to rotate and tilt. Moreover, since those oxygen-p orbitals which bond to the cations are the same as those which bond to the transition-metal t2g orbitals, the t2g Wannier functions exhibit residual covalency. This splits the t2g levels and reduces the width of the lowest subband. Cation covalency thus causes the electron to localize in LaTiO3 and YTiO3. [1] E. Pavarini, S. Biermann, A. Poteryaev, A.I. Lichtenstein, A. Georges, O.K. Andersen, Phys. Rev. Lett. 92 (2004) 176403. [2] E. Pavarini, A. Yamasaki, J. Nuss, O.K. Andersen, New Journal of Physics 7 (2005) 188