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Hotta, Takashi; Moraghebi, M.*; Feiguin, A.*; Moreo, A.*; Yunoki, Seiji*; Dagotto, E.*
Physical Review Letters, 90(24), p.247203_1 - 247203_4, 2003/06
Times Cited Count：83 Percentile：91.21(Physics, Multidisciplinary)Novel ground-state spin structures in undoped and doped manganites are here investigated based on the orbital-degenerate double-exchange model, by using mean-field and numerical techniques. In undoped manganites, a new antiferromagnetic (AFM) state, called the E-type phase, is found adjacent in parameter space to the A-type AFM phase. Its structure is in agreement with recent experimental results. This insulating E-AFM state is competing with a ferromagnetic metallic phase as well, suggesting that large magneto-resistant effects could exist even in undoped manganese oxides. For doped layered manganites, the phase diagram includes another new AFM phase of the 
-type. Experimental signatures of the new phases are discussed.
Hotta, Takashi
Physical Review B, 67(10), p.104428_1 - 104428_8, 2003/03
Times Cited Count：21 Percentile：68.18(Materials Science, Multidisciplinary)The existence of a novel metal-insulator transition in the ferromagnetic state of models for undoped manganites is here discussed using numerical techniques applied to the 
-orbital degenerate Hubbard model tightly coupled with Jahn-Teller distortions. The ground-state phase diagram is presented in the plane defined by the electron-phonon coupling 
and Coulomb interaction 
. In contrast to the standard one-band Hubbard model for cuprates, the metallic phase is found to exist for finite values of both and in the present -orbital Hubbard model even at half-filling, due to the Fermi-surface topology which is incompatible with the staggered orbital ordering concomitant to the insulating phase. Based on the present results, a possible scenario for Colossal Magneto-Resistive effect is discussed in undoped manganites.
