Antiferromagnetism and phase separation in electronic models for doped transition-metal oxides

We investigate the ground state properties of electronic models for doped transition-metal oxides. An effective t J like Hamiltonian is derived from the case of strong Hund coupling between the conduction electrons and localized spins by means of the projection technique. An attractive interaction for conduction electrons and an antiferromagnetic coupling of the localized spin are obtained. A large ratio of the attraction to effective electron hopping, which is modulated by the spin background, will lead to the phase separation. The antiferromagnetic phase and the phase separation appear in the case of either high or low density of electrons. The possible relevance of the phase separation to the charge stripe phase in doped transition-metal oxides is discussed. PACS numbers: 71.10.Hf, 75.30.Mb, 75.60.-d Typeset using REVTEX 1 The problem of doped Mott insulators has attracted much attention because of its relevance to high temperature superconductivity and colossal magnetoresistance effect. Recent experiments of doped lanthanum cuperate [1], nickelate [2] and manganite [3] families of materials exhibit a new type of charge ordering and spin ordering in an extensive region. For example, the charge and spin stripe phases were observed in La2−xSrxNiO4 samples [4]. Along the charge stripe, there is strong antiferromagnetic correlation. It is also shown experimentally that the charge ordering collapses in the presence of an external magnetic field, which can destroy antiferromagnetic ordering [5]. Many efforts have been devoted to understand the origin of the phenomena and its intrinsic relevance to various anomalous transport properties. In this paper, starting from an electronic model for doped transition-metal oxides, we derive an effective t-J like Hamiltonian for the case of strong Hund coupling. An attractive interaction between conduction electrons, which is associated with the antiferromagnetic correlation, is obtained. Both the attraction and electron hopping are modulated by the configuration of two localized spins on the nearest neighbor sites. A larger ratio between them will lead to the phase separation, which is expected in terms of the ideas of frustrated phase separation for the charge stripe phase [6]. An antiferromagnetic background leads to attraction between electrons. We find that the phase separation with electron-rich and -poor regimes has a lower energy than an antiferromagnetic phase with a uniform density of charge. A phase diagram for the model is presented. The possible relevance to the phase separation and the charge stripe phases in doped lanthanum manganites and nickelates are also discussed. An electronic model to describe doped transition metal oxides is a Kondo-like lattice Hamiltonian with the strong Hund coupling