Eur. Phys. J. B (2012) 85: 55
https://doi.org/10.1140/epjb/e2011-20613-0

Regular Article

Competition between crystal field splitting and Hund’s rule coupling in two-orbital magnetic metal-insulator transitions

¹ Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031, P.R. China
² Department of Physics, Chinese University of Hong Kong, Shatin, New Territory, Hong Kong, P.R. China

^a e-mail: zou@theory.issp.ac.cn

Received: 25 July 2011
Received in final form: 12 October 2011
Published online: 8 February 2012

Abstract

Competition between crystal field splitting and Hund’s rule coupling in magnetic metal-insulator transitions of half-filled two-orbital Hubbard model is investigated by multi-orbital slave-boson mean field theory. We show that with the increase of Coulomb interaction, the system firstly transits from a paramagnetic (PM) metal to a Néel antiferromagnetic (AFM) Mott insulator, or to a nonmagnetic orbital insulator, depending on the competition of crystal field splitting and the Hund’s rule coupling. The AFM Mott insulating, PM metallic and orbital insulating phases are not, partially and fully orbital polarized, respectively. For a small J_H and a finite crystal field, the orbital insulator is robust. These results demonstrate that large crystal field splitting favors the formation of the orbital insulating phase, while large Hund’s rule coupling tends to destroy it, driving the low-spin to high-spin transition.