Phase diagram and collective excitation in excitonic insulator: from the orbital physics viewpoint

Abstract

Excitonic insulating system is studied from the viewpoints of the orbital physics in strongly correlated electron systems. An effective model Hamiltonian for low-energy electronic states is derived from the two-orbital Hubbard model with a finite energy difference corresponding to the crystalline field splitting. The effective model is represented by the spin operators and the pseudo-spin operators for the spin-state degrees of freedom. The ground state phase diagram is analyzed by the mean-field approximation. In addition to the low-spin state and high-spin state phases, two kinds of the excitonic insulating phases emerge as a consequence of the competition between the crystalline field effect and the Hund coupling. The excitonic transition is classified to be an Ising-like transition reflecting a spontaneous breaking of the $Z_2$ symmetry. Magnetic structures in the two excitonic insulating phases are different from each other; an antiferromagnetic order and a spin nematic order. Collective excitations in each phase are examined by using the generalized spin-wave method. The Goldstone modes in the excitonic insulating phases appear in the dynamical correlation functions for the spins and pseudo-spin operators. Both the transverse and longitudinal spin excitation modes are active in the two excitonic insulating phases in contrast to the low-spin state and high-spin state phases. Connections of the present results to the perovskite cobalt oxides are discussed.

15 pages, 9 figures

References in corpus (12)

• The quadrupolar phases of the S=1 bilinear-biquadratic Heisenberg model on the triangular lattice
• Non-thermal separation of electronic and structural orders in a persisting charge density wave
• High-spin to low-spin and orbital polarization transitions in multiorbital Mott systems
• Excitonic condensation in systems of strongly correlated electrons
• Charge-density wave induced by combined electron-electron and electron-phonon interactions in 1$T$-TiSe${}_2$: A variational Monte Carlo study
• Excitonic spin density wave state in iron pnictides
• Phase separation in doped systems with spin-state transitions
• Ferroelectric properties of charge-ordered alpha-(BEDT-TTF)2I3
• Finite-Temperature Phase Diagram of Quasi-One-Dimensional Molecular Conductors: Quantum Monte Carlo Study
• Charge-Ordered State versus Dimer-Mott Insulator at Finite Temperatures
• Polar Charge Fluctuation and Superconductivity in Organic Conductor
• Charge Dynamics in a Correlated Fermion System on a Geometrically Frustrated Lattice

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• Magnetostriction studies up to megagauss fields using fiber Bragg grating technique
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• Magneto-Electric Effect in a Spin-State Transition System
• Photoinduced excitonic magnetism in a multiorbital Hubbard system