Hofstadter Butterfly and Broken-Symmetry Quantum Hall States in α-Type Organic Dirac Fermion Systems

Abstract

The electronic state of α-type organic Dirac fermion systems such as α-(ET)_2I_3 or α-(BETS)_2I_3 has been studied under magnetic fields using the four-band tight-binding model with Peierls phase factors. The validity of the Dirac fermion picture in these materials was confirmed by the generated Hofstadter butterfly and its Chern numbers. The four-component envelope function of the N = 0 Landau level with valley degeneracy was studied. It was found that the two degenerate valley states have different weights on A and A' molecules connected by inversion. This feature is also recognized for the N = 0 spin-split Landau levels under the Zeeman effect and the spin-orbit interaction. The spontaneous valley symmetry breaking in the N = 0 Landau levels due to the exchange interaction results in the ν= 1 and -1 quantum Hall states accompanied by the spatial charge and spin modulations in a unit cell.

16 pages, 4 figures

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