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

Abstract

The ground state of a double-exchange model for orbitally degenerate eg electrons with Jahn-Teller lattice coupling and weak disorder is found to be spatially inhomogeneous near half filling. Using a real space Monte-Carlo method we show that doping the half-filled orbitally ordered insulator leads to the appearance of hole-rich disordered regions in an orbitally ordered environment. The doping driven orbital order to disorder transition is accompanied by the emergence of metallic behavior. We present results on transport and optical properties along with spatial patterns for lattice distortions and charge densities, providing a basis for an overall understanding of the low doping phase diagram of La1−xCaxMnO3.PACS numbers: 75.47.Lx, 81.16.Rf Hole-doped perovskite manganites, for example La 1−x Ca x MnO 3 (LCMO), are well known for their colossal magnetoresistance (CMR) effect [1,2]. The 'optimally doped' CMR compounds with x ∼ 0.3 have been the focus of numerous theoretical studies and are qualitatively understood in terms of the interplay of the double exchange mechanism, electron-phonon interactions, and disorder in a single electronic band [3,4,5,6]. Less attention has been given to the low-doping regime, where the two-band character of manganites is crucial and in addition to charge, spin, and lattice variables, the orbital degrees of freedom and their ordering become important. The undoped compounds are orbitally ordered (OO), A-type antiferromagnetic insulators [7] with large Jahn-Teller (JT) distortions of the MnO 6 octahedra [8], which lift the degeneracy of the two Mn-e g levels. Electronic and cooperative lattice effects lead to a staggered ordering of the lattice distortions and the orbital occupancies. Upon doping, the antiferromagnetic insulator evolves into a ferromagnetic insulator, with weakened orbital order, and eventually undergoes a transition to an orbitally disordered ferromagnetic metal (OD-FM-M) [9]. Despite the achieved progress towards an understanding of magnetic and orbital ordering in the undoped compounds [10], efforts for analyzing the doping driven transition from the orbitally ordered insulating to the orbitally disordered metallic phase have remained limited. A simple view of this transition rests on an entirely classical picture in terms of random fields introduced by the doped holes [11].The critical hole doping x OD for the loss of orbital order is close to the doping x IMT for the insulator-metal transition (IMT) in LCMO, where x IMT ∼ 0.22 [12]. In lower bandwidth materials like Pr 1−x Ca x MnO 3 (PCMO) the insulating phase persists to even larger hole concentrations [13]. NMR and neutron scattering experiments suggest, that the doping regime x x IMT is spatially inhomogeneous in LCMO with coexisting 'hole poor' orbitally ordered and 'hole rich' orbitally disordered regions [14], and the observation of confined 'spin waves' confirms the existence of magnetic clusters on the nanoscale [15]. It has remained unclear how the JT insulator evolves from the homogeneous OO sta...

Maximally localized Wannier functions in LaMnO3within PBE +U, hybrid functionals and partially self-consistent GW: an efficient route to constructab initiotight-binding parameters for egperovskites

“…The tabulated TB parameters should provide an interpretative direction for more sophisticated many-body model Hamiltonian investigations of similar systems [14,76,77,78].…”

Disorder-induced orbital ordering in doped manganites

“…[23][24][25] In the present study, we employ the traveling cluster approximation, 25 which indeed has been very successful in analyzing similar models in the recent past. 10,11,26 …”

“…[4][5][6][7] Therefore, analyzing the effects of disorder in manganites has become an active area of research. [8][9][10][11] Disorder is generally viewed as an agent for suppressing the ordering tendencies of the microscopic degrees of freedom. Experiments on the half-doped ͑x = 0.5͒ manganites show that quenched disorder indeed spoils the long-range ordering of the charge, orbital, and spin variables leading, in some cases, to a short-range ordering of these microscopic degrees of freedom.…”

Electronic and magnetic reconstructions in manganite superlattices

“…The computational cost thereby decreases to O(N N 3 c ) and allows to treat system sizes up to ∼ 40 × 40, with an 8 × 8 moving cluster. The method and the associated transport calculations for N = 24×24 were extensively benchmarked and successfully applied in several earlier studies 41,42,47 . At each system sweep we additionally solve for the Coulomb potentials φ i in H lrc self consistently until the electron density at each site is converged.…”