Electron correlations—the origin of the CE phase in bilayer La_2−2xSr_1+2xMn₂O₇manganites

Abstract

We introduce the effective Hamiltonian which describes eg electrons in doped bilayer La2−2xSr1+2xMn2O7 manganites and study magnetic and charge order using correlated wavefunctions. The Hamiltonian includes: the kinetic energy, the crystal field splitting between x2−y2 and 3z2−r2 orbitals, on-site interactions—Coulomb U and Hund’s exchange JH between eg electrons, antiferromagnetic superexchange interaction J′>0 between t2g core S = 3/2 spins, and finally the coupling between eg electrons and Jahn–Teller local modes. The model reproduces reasonably well the evolution of magnetic order with increasing hole doping x in the experimentally accessible range from x = 0.3 up to x = 0.9. Electron correlations are found to be of crucial importance for the stability of the recently experimentally identified CE phase in a half-doped (x = 0.5) bilayer (with zig-zag ferromagnetic chains in each plane coupled antiferromagnetically with neighbouring chains in the same and in the other layer).

A possibility of high spin hole states in doped CoO2layered systems

“…These simplifying assumptions allow one to make only qualitative predictions (concerning the JT effect), but any quantitative analysis would require more precise information about the coupling constants. In manganites typical values for g JT and K JT are: g JT = 3.8 eVÅ −1 and K JT = 13 eVÅ −2 , respectively (see [19] and references therein). For cobalt oxides the values of g JT and K JT are not known.…”

“…This ansatz was successfully applied to several systems, inter alia to cuprates [42], nickelates [43], manganites [19] and chemical bonds in molecular systems [44]. Here the HF computations were run starting from each one of many different initial conditions (to get unbiased results we considered up to 10000 nonhomogeneous random charge and spin arrangements for each doping level).…”

“…The symbol δ in δn iµσ indicates that only that part of n iµσ operator is included which annihilates one electron in an occupied single particle state belonging to the HF ground state |Φ 0 , and creates an electron in one of the virtual empty states. The above local operators O m correspond to the subselection of presumably most important electronpair excitations within the ab initio configuration-interaction method (for details see [19] and [43]). …”

Jahn–Teller distortions and the magnetic order in the perovskite manganites

“…The crystal field splitting between z and x orbitals (2) was assumed to be zero (this approximation seems reasonable in quasi-cubic 3D manganites [13] though some authors [12] assume instead small finite values). Below we describe the numerical simulatons performed on finite 3D clusters with the above model (1) The variants of the described model reproduced the observed sequence of magnetic phases for increasing hole doping in 2D monolayer and bilayer manganites [32,33].…”

“…In the next step each HF wave functions was independently modified to improve the energy and to include the electron correlations using the so-called local ansatz (for details see refs. [30][31][32][33]). We used the ansatz for the correlated ground state,…”