Jahn-Teller Effect and Superexchange in Half-Doped Manganites

We investigate the stability of the charge exchange (CE) phase within a microscopic model which describes a single plane as in La0.5Sr1.5MnO4. The model includes Coulomb interactions (on-site and intersite), the Jahn-Teller term and the superexchange interactions due to eg and t2g electrons. By investigating the phase diagram at T = 0 in mean-field approximation we conclude that the superexchange interactions can stabilize the CE phase, but only if they are stronger than estimated from spectroscopy. Journal reference: O. Sikora and A. M. Oleś, Phys. Stat. Sol. (b) 236, 380 (2003). The doped manganites belong to a very interesting class of transition metal oxides, with orbital degrees of freedom and several magnetic phases stable in various doping regimes. The so-called charge exchange (CE) phase, composed of one-dimensional (1D) ferromagnetic (FM) zigzag chains with an antiferromagnetic (AF) coupling between them, has attracted a lot of attension recently, and the origin of its stability is still under debate [1]. The charge and orbital ordering sets in La0.5Ca0.5MnO3 at TCO ≃ 225 K, and is followed by a magnetic transition at TN ≃ 155 K [2]. The CE-phase was also observed in one-plane La0.5Sr1.5MnO4 compound, with a similar sequence of phase transitions (a structural transition at TCO = 255 K, a magnetic transition at TN ≃ 110 K [3]), and a rather pronounced orbital order [4]. Recent experiments show that the charge order is particularly pronounced in this case, and the Jahn-Teller (JT) distortions around the occupied Mn centers are induced [5]. The reasons for appearing of the CE phase in La0.5Sr1.5MnO4 are not yet fully understood. We investigate the stability of the single-plane CE-type phase with respect to three other phases: the C phase with staggered linear FM chains, the G-type AF (G) phase, and the FM plane of the A-type AF (A) phase. These phases are characterized by different orbital structure, as shown in Fig. 1. The tight binding model without any interactions applied to the CE phase gives a band insulator due to a particular conflict of orbital phases which frustrates the kinetic energy [6]. This favors the observed zigzag CE phase with respect to the C phase. The CE phase is destabilized by large on-site Coulomb interactions U , and we have shown that either Coulomb intersite or the JT interaction can stabilize the CE phase [7]. This effect is enhanced when the superexchange energy is included. However, when we take into account A-AF phase or G-AF phase, the energy of CE phase is higher for U not bigger then about 7t and realistic values of other parameters. We have verified that the CE phase can be stabilized in the present model only by increasing the superexchange above that which follows from the spectroscopic parameters. We consider below a model which describes the electronic properties of the single-plane compounds (La0.5Sr1.5MnO4), and includes spin, orbital and charge degrees of freedom, as well as the cooperative JT distortions. We consider the Hamiltonian composed of four terms: H = Ht +Hint +HJT +HSE, (1) 1) E-mail: [email protected] phys. stat. sol. – 2 –