Electronic phase separation and charge ordering in (Sr,La)2MnO4: Indication of triplet bipolarons

Abstract

Coexistence of two phases, having different in-plane dimensions, is found in the planar Sr2−xLaxMnO4 for 0 < x ≤ 0.38 at room temperature. Furthermore, charge ordering with modulation wave vector (π2, π2) is discovered for x=0.5 below ∼ 250 K. Together with transport and magnetic measurements, these results indicate that the phase separation is due to the competition between the single and the double occupancies of the Mn 3d orbitals by the doped electrons. The doubly-occupying electrons form on-site triplets which are mobile at high temperatures and form a bipolaron lattice below ∼ 250 K.