Nature of the Mott Transition in Ca2RuO4

We study the origin of the temperature-induced Mott transition in Ca2RuO4. As a method we use the local-density approximation + dynamical mean-field theory. We show the following. (i) The Mott transition is driven by the change in structure from long to short c-axis layered perovskite (L-Pbca -> S-Pbca); it occurs together with orbital order, which follows, rather than produces, the structural transition. (ii) In the metallic L-Pbca phase the orbital polarization is similar to 0. (iii) In the insulating S-Pbca phase the lower energy orbital, similar to xy, is full. (iv) The spin-flip and pair-hopping Coulomb terms reduce the effective masses in the metallic phase. Our results indicate that a similar scenario applies to Ca2-xSrxRuO4 (x <= 0: 2). In the metallic x <= 0: 5 structures electrons are progressively transferred to the xz/yz bands with increasing x; however, we find no orbital-selective Mott transition down to similar to 300 K.