Ordering and fluctuation of orbital and lattice distortion in perovskite manganese oxides

Abstract

Roles of orbital and lattice degrees of freedom in strongly correlated systems are investigated to understand electronic properties of perovskite Mn oxides such as La1−xSrxMnO3. An extended double-exchange model containing Coulomb interaction, doubly degenerate orbitals and Jahn-Teller coupling is derived under full polarization of spins with two-dimensional anisotropy. Quantum fluctuation effects of Coulomb interaction and orbital degrees of freedom are investigated by using the quantum Monte Carlo method. In undoped states, it is crucial to consider both the Coulomb interaction and the Jahn-Teller coupling in reproducing characteristic hierarchy of energy scales among charge, orbital-lattice and spin degrees of freedom in experiments. Our numerical results quantitatively reproduce the charge gap amplitude as well as the stabilization energy and the amplitude of the cooperative Jahn-Teller distortion in undoped compounds. Upon doping of carriers, in the absence of the Jahn-Teller distortion, critical enhancement of both charge compressibility and orbital correlation length is found with decreasing doping concentration. These are discussed as origins of strong incoherence in charge dynamics. With the Jahn-Teller coupling in the doped region, collapse of the Jahn-Teller distortion and instability to phase separation are obtained and favorably compared with experiments. These provide a possible way to understand the complicated properties of lightly doped manganites.

Phase diagram of a generalized Hubbard model applied to orbital order in manganites

“…͑The real value for it in manganites is about 4ϳ10. 4 ͒ In the undoped case, ϭ/2 means a staggered (͉x 2 Ϫy 2 ͘ϩ͉3z 2 Ϫr 2 ͘)/(͉x 2 Ϫy 2 ͘Ϫ͉3z 2 Ϫr 2 ͘) orbital ordering, which is consistent with the result by Motome and Imada from quantum Monte Carlo calculation, 13 but slightly different from the experimentally observed ͉3x 2 Ϫr 2 ͘/͉3y 2 Ϫr 2 ͘. 19 This may be due to some other physical mechanism, e.g, a Jahn-Teller effect which is present in real materials but not included here.…”

“…19 This may be due to some other physical mechanism, e.g, a Jahn-Teller effect which is present in real materials but not included here. 13 On the other hand, it is found that the ͉3x 2…”

“…12,2 This means that a perfect spin-polarized two-dimensional ͑2D͒ plane is always retained over large region of doping. Therefore the spin degree of freedom may be discarded and the following effective 2D Hamiltonian with only orbital degree of freedom is proposed: 13,14 …”

Neutron scattering study of charge-ordering in R1/3Sr2/3FeO3 (R=La, Pr, Nd, Sm, and Y)

“…The details are presented in the followings. Hamiltonians in both cases incorporating the electron-acoustic phonon coupling will be expressed as [66,67,68], 19) where…”

Microwave properties of Nd0.5Sr0.5MnO3: a key role of the (x2−y2)-orbital effects

“…Even in the fully spin-polarized FM state and the hightemperature PM state the (x 2 -y 2 )-type orbital degree of freedom may play an important role causing quasi-2D orbital fluctuations and establishing the two-dimensionality of the 6 spin dynamics [6,9]. Some important features of the quasi-2D orbital fluctuations in the FM state and their effect on the optical conductivity (including the low-energy range) have been studied theoretically [13][14][15]. Using a numerical approach Ishihara et al [13] concluded that the quasi-2D nature of the orbital fluctuations in a 3D cubic system leads to a so-called orbital liquid state.…”