Effects of Electron Correlation, Orbital Degeneracy and Jahn-Teller Coupling in Perovskite Manganites

Abstract

Roles of Coulomb interaction, orbital degeneracy and Jahn-Teller coupling in double-exchange models are examined for Mn perovskite oxides. We study the undoped Mott insulator as well as metal-insulator transitions by hole doping, and especially strong incoherence of ferromagnetic metal. We derive models where all the spins are fully polarized in two-dimensional planes as in the experimental indications, and investigate their ground-state properties by quantum Monte Carlo method. At half filling where the number of eg electron is one per site on average, the Coulomb interaction opens a Mott gap and induces a staggered orbital ordering. The opening of the Mott gap is, however, substantially slower than the mean-field results if the Jahn-Teller coupling is absent. The synergy between the strong correlation and the Jahn-Teller coupling largely enhances the Mott gap amplitude and reproduces realistic amplitudes and stabilization energy of the Jahn-Teller distortion. Upon doping, the orbital ordering stabilized by the Coulomb interaction is destroyed immediately. Toward the metal-insulator transition, the short-ranged orbital correlation is critically enhanced in metals, which should be related to strong incoherence of charge dynamics observed in experiments. Our model, moreover, exhibits a uniform ordering of d x 2 −y 2 orbital in a wide region of doping in agreement with experimental indications.KEYWORDS: perovskite manganites, double exchange model, Coulomb interaction, orbital degeneracy, JahnTeller coupling, Mott gap, orbital ordering, Jahn-Teller distortion, metal-insulator transition, ferromagnetic metal, incoherent charge dymanics, quantum Monte Carlo method 'Simple' double-exchange (DE) models, that is, models with a single non-interacting conduction band ferromagnetically coupled with localized spins, have been intensively studied 1, 2, 3, 4, 5) to understand physical properties of Mn perovskite oxides, especially collosal magnetoresistance. 6,7,8,9,10) These models have successfully explained several experimental aspects at least qualitatively; for instance, ferromagnetic metals in a hole-doped region with a transition to a paramagnetic state by raising temperature, and a large negative magnetoresistance near the transition. All these properties are due to the so-called double-exchange mechanism; doped holes tend to gain kinetic energy by aligning localized spins in parallel. 1, 2, 3)However, many open problems still remain. In particular, we focus here the following: (i) All the materials are the Mott insulator with a charge gap of order of eV at half filling (one e g electron per Mn site on average), where two-dimensional (2D) anisotropy realizes in spin and orbital orderings, and a cooperative JahnTeller (JT) distortion. 7,11,12,13,15,14) (ii) Upon doping of holes, a metal-insulator (MI) transition occurs from the anisotropic Mott insulator to an isotropic ferromagnetic metal through disordered ferromagnetic insulator. 6, 9, 16) (iii) In the ferromagnetic metallic state, charge dynamics shows strong inco...

Incoherent Charge Dynamics in Perovskite Manganese Oxides

“…These parameters are known to reproduce well the experimental results at half filling around U/t 0 ∼5. 18,19) We here calculate σ(ω) for U/t 0 =6 and the hole-doping concentration δ is 0.2, 25) which are close values to the ones suggested from the experimental and previous theoretical works. In the above parameters,t 0 is found to be ∼0.2 when the mean-field solution is at (q 2i , q 3i ) ∼ (0, 0.031).…”

“…It is known from the quantum Monte Carlo calculation that the JT energy gain from the staggered and long-ranged distortion is the largest at half filling and becomes smaller with increasing hole-doping. 18,19) The JT energy gain and the ZPE become competing at about 10%-doping, above which such long-ranged distortion melts. This is why it is necessary to realize short-ranged correlations of distortions in the calculations.…”

Phonon-Assisted Magnetic Mott-Insulating State in the Charge Density Wave Phase of Single-Layer 1T−NbSe2

“…We believe that the cooperative enhancement of the Mott gap due to electronic and lattice degrees of freedom is revelant far beyond the family of 1T dichalcogenides, but it is a genuine feature of correlated insulators in the presence of strong Jahn-teller distortion. A similar mechanism was suggested to be relevant in manganites and manganese oxides [30] or in the fullerides like K 4 C 60 [31]. 1TNbSe 2 is an ideal playground to study Structure ∆E(U = 0, Exp.)…”

Pressure-Induced Metal-Insulator Transition inLaMnO3Is Not of Mott-Hubbard Type

“…Previous calculations for LaMnO 3 with electronic correlations, but simplified hopping integrals, include Ref. [15]. We neglected the (orbital) off-diagonal elements of the on-site Green function, forcing the DMFT density to have the same symmetry as the LDA crystal field (Fig.…”