Makoto Naka

Electronic Ferroelectricity in a Dimer Mott Insulator

Motivated by recent experiments in κ-(BEDT-TTF) 2 Cu 2 (CN) 3 , we studied dielectric and magnetic properties of a dimer-Mott insulator. We derived the effective Hamiltonian where the number of electrons per dimer is taken to be one. Charge and magnetic structures at a finite temperature were obtained by using the mean-field and Monte-Carlo methods. Magnetic and ferroelectric phases are exclusive with each other. Dielectric fluctuation is remarkable near the phase boundary between the dimer-Mott and ferroelectric phases. Implications of the present results for κ-(BEDT-TTF) 2 Cu 2 (CN) 3 were discussed.

Magnetodielectric phenomena in a charge- and spin-frustrated system of layered iron oxide

Dielectric and magnetic phenomena in spin- and charge-frustrated system

Collective excitation of an electric dipole on a molecular dimer in an organic dimer-Mott insulator

R{\text{Fe}}_{2}{\text{O}}_{4}

Phase diagram and collective excitations in an excitonic insulator from an orbital physics viewpoint

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Doubly degenerate orbital system in honeycomb lattice: Implication of orbital state in layered iron oxide

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Magnetoelectric effect in organic molecular solids

is a rare-earth metal ion) are studied. An electronic model for charge, spin, and orbital degrees in a pair of triangular-lattice planes is derived. We analyze this model by utilizing the mean-field approximation and the Monte Carlo simulation in a finite size cluster. A threefold-type charge-ordered structure with charge imbalance between the planes is stabilized in finite temperatures. This polar charge order is reinforced by spin ordering of Fe ions. This magnetodielectric phenomenon is caused by spin frustration and charge-spin coupling in the exchange interaction. We show cross-correlation effect in magnetic- and electric-field responses. Oxygen-deficiency effect as an impurity effect in a frustrated charge-spin coupled system is also examined.

Collective Charge Excitation in a Dimer Mott Insulating System

The terahertz response in 10-100 cm(-1) was investigated in an organic dimer-Mott (DM) insulator κ-(ET)(2)Cu(2)(CN)(3) that exhibits a relaxorlike dielectric anomaly. An ~30 cm(-1) band in the optical conductivity was attributable to collective excitation of the fluctuating intradimer electric dipoles that are formed by an electron correlation. We succeeded in observing photoinduced enhancement of this ~30 cm(-1) band, reflecting the growth of the electric dipole cluster in the DM phase. Such optical responses in κ-(ET)(2)Cu(2)(CN)(3) reflect an instability near the boundary between the DM-ferroelectric charge ordered phases.

Emergence of charge degrees of freedom under high pressure in the organic dimer-Mott insulator β′-(BEDT - TTF) 2 ICl 2

An excitonic-insulating system is studied from a viewpoint 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 pseudospin 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. Transitions to the excitonic phases are classified to an Ising-type transition resulted from a spontaneous breaking of the

Long-Period Charge Correlations in Charge-Frustrated Molecular θ-(BEDT-TTF)2X

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Theory of Valence Transition in BiNiO_{3}.

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 using the generalized spin-wave approximation. Characteristics in the Goldstone modes in the excitonic-insulating phases are studied through the calculations of the dynamical correlation functions for the spins and pseudospin 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. Relationships of the present results to the perovskite cobalt oxides are discussed.