Doping dependence of spin and orbital correlations in layered manganites

Abstract

We investigate the interplay between spin and orbital correlations in monolayer and bilayer manganites using an effective spin-orbital t-J model which treats explicitly the eg orbital degrees of freedom coupled to classical t2g spins. Using finite clusters with periodic boundary conditions, the orbital many-body problem is solved by exact diagonalization, either by optimizing spin configuration at zero temperature, or by using classical Monte-Carlo for the spin subsystem at finite temperature. In undoped two-dimensional clusters, a complementary behavior of orbital and spin correlations is found -the ferromagnetic spin order coexists with alternating orbital order, while the antiferromagnetic spin order, triggered by t2g spin superexchange, coexists with ferro-orbital order. With finite crystal field term, we introduce a realistic model for La1−xSr1+xMnO4, describing a gradual change from predominantly out-of-plane 3z 2 − r 2 to in-plane x 2 − y 2 orbital occupation under increasing doping. The present electronic model is sufficient to explain the stability of the CE phase in monolayer manganites at doping x = 0.5, and also yields the C-type antiferromagnetic phase found in Nd1−xSr1+xMnO4 at high doping. Also in bilayer manganites magnetic phases and the accompanying orbital order change with increasing doping. Here the model predicts C-AF and G-AF phases at high doping x > 0.75, as found experimentally in La2−2xSr1+2xMn2O7.

Orbital Correlations in Monolayer Manganites - From Spin t-J Model to Orbital t-J Model

“…clusters with periodic boundary conditions, which are large enough to capture the essential interactions deciding about the nature of the ground state [29], as we also concluded recently [32].…”

“…In contrast to the undoped case, the magnetic interactions at half doping are in general anisotropic and the orbital state can be then tuned to support FM spin order along a certain direction by double exchange mechanism, while the superexchange is predominantly AF in the other direction. In this way the C-AF (studied before in the ladder geometry [37]) and CE-AF phase may arise and strongly compete with each other due to rather similar kinetic and interaction energies [32], as long as κ = 0. The situation changes however when the JT interactions are considered, as finite κ = 0.2t used for the data of Fig.…”

“…In the absence of JT interactions (at κ = 0), the CE phase arises as a compromise between competing AF and FM interactions, roughly speaking in a range of 0 < J < 0.1t and V < 1.5t [32]. In contrast to the undoped case, the magnetic interactions at half doping are in general anisotropic and the orbital state can be then tuned to support FM spin order along a certain direction by double exchange mechanism, while the superexchange is predominantly AF in the other direction.…”

Theory for charge and orbital density-wave states in manganite La0.5Sr1.5MnO4Yao1, Chen2, Gao3 et al. 2013Phys. Rev. B4010View full textAdd to dashboardCite

“…We arrange the matrix index from 1 to 4 as (αβ) = (11), (22), (12), and (21). The interaction matrixÛ c is of the formÛ c =Û 1 ⊕Û 2 , whereÛ 1 = V ( q)σ 0 + (V ( q) + U 0 )σ 1 , andÛ 2 = −U 0 σ 0 with σ 0 an identity matrix and σ 1 the first Pauli matrix.…”

“…For undoped manganite LaSrMnO 4 , the high temperature orbital ordering could be achieved from the strong coupling approach 11 . And for the half-doped La 0.5 Sr 1.5 MnO 4 , the low temperature phase transition has been studied previously 12 . But various angle-resolved photoemission spectroscopy (ARPES) experiments on different layered manganites suggest an essential connection between the Fermi surface (FS) nesting and the charge/orbital ordering in this family of materials.…”