Phase transition between the Mott state and a ferromagnetic state in an extended Nagaoka system

Orateur : Seiji MIYASHITA (University of Tokyo)

We will study properties of an extended Nagaoka system, where we find phase transitions among various magnetic states as a function of the chemical potential. As mechanisms of itinerant ferromagnetism, various systems, e.g., Nagaoka-ferromagnetism[1], flat-band ferromagnetism[2], a hybridized model[3], etc. have been studied extensively. In Nagaoka- ferromagnetic systems, the ferromagnetic state appears when an electron is removed from the system. In order to realize the process of removal of electron adiabatically as a function of parameter, we introduce a model in which a part of the system plays a role of particle bath. There we can control the density of electron of the rest of the system (subsystem) in a small lattice as a function of the chemical potential the part of particle bath [4]. We extend the model and we study properties of the model in extended lattices, and study properties in the thermodynamic limit [5]. The system exhibits a ground state phase transition between the Mott singlet state and a ferromagnetic state. Moreover it exhibits various transitions among various values of the total spins. We investigate the distribution of electron spins, the magnetization process in the magnetic field, the spin correlation functions, and also the entangle entropy. We found the transitions take place as a function of electron density and we may regard the present procedure as an emulation of doping process. [1] Y. Nagaoka, Phys. Rev. 147, 392 (1966), H. Tasaki, Prog. Theor. Phys. 99, 489 (1998). [2] A. Mielke and H. Tasaki, Commun. Math. Phys. 158, 341 (1993). [3] Y. Watanabe and S. Miyashita, J. Phys. Soc. Jpn. 66, 2123(1997) ; 68, 3087 (1999). [4] S. Miyashita, Prog. Theor. Phys., 120, 4, 785 792 (2008) , S. Miyashita, M. Ogata, and H. De Raedt, Phys. Rev. B 80, 174422 (1-6) (2009). [5] H. Onishi and S. Miyashita, in preparation.

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