The role of orbital ordering in the formation of electron structure in undoped LaMnO 3 manganites in the regime of stron
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C PROPERTIES OF SOLIDS
The Role of Orbital Ordering in the Formation of Electron Structure in Undoped LaMnO3 Manganites in the Regime of Strong Electron Correlations V. A. Gavrichkova,*, S. G. Ovchinnikova, and L. E. Yakimovb a
Kirensky Institute of Physics, Siberian Division, Russian Academy of Sciences, Krasnoyarsk, 660036 Russia b Siberian State Aerospace University, Krasnoyarsk, 660014 Russia *e-mail: [email protected] Received September 30, 2005
Abstract—The electron structure of undoped LaMnO3 and slightly doped La1 – xSrxMnO3 manganites has been calculated within the framework of a generalized tight binding method with explicit allowance for strong intraatomic electron correlations. According to the results of these calculations, the ground state in orbitally disordered undoped LaMnO3 ferromagnets would be metallic despite the Mott–Hubbard correlation gap in the spectrum of quasiparticles. Owing to the orbital ordering, the insulating state is stabilized in both antiferromagnetic and paramagnetic phases. In-gap states of a polaron nature with a spectral weight proportional to the dopant concentration have been found near the top of the valence band in La1 – xSrxMnO3 . As the doping level increases, a metal state appears in the ferromagnetic phase, which has a metallic character for one spin subband and an insulating character for the other subband (representing the so-called half-metallic state). PACS numbers: 71.10.–w DOI: 10.1134/S1063776106060112
1. INTRODUCTION A starting point in most discussions concerning the mechanisms of magnetoresistance, metal–insulator transition, and ferromagnet–paramagnet (FM–PM) transition in manganites is the model of double exchange [1]. According to Anderson and Hasegawa [2] and de Gennes [3], the physics of double exchange consists in the hopping amplitude t depending on the spin configuration in two nearest neighbor sites. The double exchange model provides an intuitively clear explanation both for the interrelation of spin and charge degrees of freedom and for the mobility of carriers. The main problem consists in the fact that this model cannot quantitatively describe the magnitude of the conductivity change upon the metal–insulator transition [4]. Indeed, in a PM state (T > TC), the angle between two adjacent spins is 90° and, hence, the amplitude of the hopping integral teff decreases by a factor of cos(90°/2) = 0.7 as compared to the value in the FM state, which implies the same decrease in the conductivity. However, as is well known, a decrease in the conductivity upon the FM PM transition in experiment reaches 2–3 orders of magnitude. The discrepancy reaching orders of magnitude indicates that some other factors are responsible for a change in the conductivity observed upon the FM PM transition. Another conclusion is that the quasiparticle band width also decreases by a factor of 0.7 relative to the value in the FM state. This implies a small increase in the density of
states at the Fermi level (EF) in the PM phase. This conclusion also contradicts experimental f
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