Orbital ordering in manganites in the band approach

Abstract

We consider the orbital ordering in LaMnO3 and similar systems, proceeding from the band picture. We show that for the realistic magnetic structure of A-type there exists a complete nesting betweeen two eg-bands. As a result there occurs an instability towards an excitonic insulator-like state -an electron-hole pairing with the wave vector Q = (π, π), which opens a gap in the spectrum and makes the system insulating. In the resulting state there appeasr an orbital ordering -orbital density wave (ODW), the type of which coincides with those existing in LaMnO3.

Domain Formation and Orbital Ordering Transition in a Doped Jahn-Teller Insulator

“…2(d) shows how the DOS evolves from the 'clean gap' at x = 0 through the low x pseudogap to the high T c 'metallic' regime. The clean gap at x = 0, which originates from the nesting instabilities at weak coupling [21] and effective repulsions between self-trapped electrons at strong coupling, is stable in the presence of weak disorder [22]. From the combination of N (µ), ρ(T ), and σ(ω) we conclude that the electron system at T = 0 has an 'IMT' near x = 0.25.…”

Fermi surface nesting and possibility of orbital ordering in FeO

“…1 As can be seen in Fig. 2, for the case of LaMnO 3 , the inter-band susceptibility peaks around the ordering vector, q = 1 2 (G 1 + G 2 ) in (001) plane [16]. This implies an ordering in the xy plane with 2a × 2a unit cell.…”

“…1(c) we plot the GGA Fermi surface of LaMnO 3 . As shown by Efremov and Khomskii [16], such a nesting leads to an instability towards orbital ordered state at low enough temperatures. In the following we are going to argue that the same argument predicts orbital ordering for LiVO 2 and FeO.…”

“…Based on the theory of Ref. [16], we speculate this implies a possible instability towards ordering of t 2g bands on a triangular lattice of (111) plane.…”

“…Efremov and Khomskii [16] developed a theory of orbital ordering in terms of inter-band nesting between the two e g bands in manganites. According to their work, the underlying inter-band nesting can lead to instability towards a symmetry broken phase with ordering in e g orbitals.…”