On the nature of the pressure-induced insulator-to-metal transition in LaMnO$_3$

Since the discovery of colossal magnetoresistance, manganites have been intensively studied. We focus on the pressure induced insulator-to-metal (IM) transition which was found experimentally~[1] in the undoped parent compound LaMnO$_3$ with configuration $t^3_{2g}e^1_g$. This transition occurs at room temperature (above $T_N$=140 K) and at a hydrostatic pressure of 32 GPa where the Jahn-Teller distortion appears to be completely suppressed~[1]. The IM transition thus seems to be a bandwidth-driven Mott-Hubbard transition of the $e_g$ electrons. We employ the local density approximation combined with static and dynamical mean-field theories (LDA+$U$ and LDA+DMFT) and conclude that the IM transition observed at 32 GPa in paramagnetic LaMnO$_3$ at room temperature is {\it not} a Mott transition, but is caused by the overlap of the majority-spin $e_g$ bands, orbitally polarized by the Coulomb repulsion. Crucial are also the bandwidth reduction of $\sim0.6$ and $2/3$ arising from, respectively, the gap generated by the crystal-field splitting and the random, spatially uncorrelated direction of the $t_{2g}$ spin at room temperature. [1] I. Loa {\it et al}., Phys. Rev. Lett. {\bf 87}, 125501 (2001).