Yoshiaki Ōno

First- and second-order phase transitions in the Holstein-Hubbard model

We investigate metal-insulator transitions in the half-filled Holstein-Hubbard model as a function of the on-site electron-electron interaction U and the electron-phonon coupling g. We use several different numerical methods to calculate the phase diagram, the results of which are in excellent agreement. When the electron-electron interaction U is dominant, the transition is to a Mott insulator; when the electron-phonon interaction dominates, the transition is to a localized bipolaronic state. In the former case, the transition is always found to be second order. This is in contrast to the transition to the bipolaronic state, which is clearly first order for larger values of U. We also present results for the quasiparticle weight and the double occupancy as functions of U and g.

Orbital Order, Structural Transition, and Superconductivity in Iron Pnictides

We investigate the 16-band d-p model for iron pnictide superconductors in the presence of the electron-phonon coupling g with the orthorhombic mode which is crucial for reproducing the recently observed ultrasonic softening. Within the RPA, we obtain the ferro-orbital order below TQ which induces the tetragonal-orthorhombic structural transition at Ts = TQ, together with the stripe-type antiferromagnetic order below TN. Near the phase transitions, the system shows the s++ wave superconductivity due to the orbital fluctuation for a large g case with TQ > TN, while the s+- wave due to the magnetic fluctuation for a small g case with TQ TN.

FFLO Excitonic State in the Three-Chain Hubbard Model for Ta2NiSe5

The three-chain Hubbard model for Ta2NiSe5, known as a candidate material for an excitonic insulator, is investigated over the wide range of the energy gap D between the twofold degenerate conduction bands and the nondegenerate valence band including both semiconducting (D > 0) and semimetallic (D < 0) cases. In the semimetallic case, the difference in the band degeneracy inevitably causes the imbalance of each Fermi wavenumber, resulting in a remarkable excitonic state characterized by the condensation of excitons with finite center-of-mass momentum q, the so-called Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) excitonic state. With decreasing D corresponding to increasing pressure, the obtained excitonic phase diagram shows a crossover from BEC (\(D \gtrsim 0\)) to BCS (\(D \lesssim 0\)) regime, and then shows a distinct phase transition at a certain critical value Dc (< 0) from the uniform (q = 0) to the FFLO (q ≠ 0) excitonic state, as expected to be observed in Ta2NiSe5 under high pressure.

Structural Transition of Li2RuO3 Induced by Molecular-Orbit Formation

A pseudo honeycomb system Li 2 RuO 3 exhibits a second-order-like transition at temperature T = T c ∼540 K to a low- T nonmagnetic phase with a significant lattice distortion forming Ru–Ru pairs. For this system, we have calculated the band structure, using the generalized gradient approximation (GGA) in both the high- and low- T phases, and found that the results of the calculation can naturally explain the insulating behavior observed in the low- T phase. The detailed characters of the Ru 4 d t 2g bands obtained by the tight-binding fit to the calculated dispersion curves show clear evidence that the structural transition is driven by the formation of the Ru–Ru molecular orbit, as proposed in our previous experimental studies.

Critical behavior near the metal-insulator transition in the one-dimensional extended Hubbard model at quarter filling

The one-dimensional extended Hubbard model with both the on-site

Effect of Magnetic Field on the Superconducting Phase in the Electron-Doped Metallic Double-Chain Compound Pr2Ba4Cu7O15-δ

U

Metal–Insulator Transition and Superconductivity in the Two-Orbital Hubbard–Holstein Model for Iron-Based Superconductors

and the nearest neighbor

Phase diagram and dynamic response functions of the Holstein–Hubbard model

V

Hole-s± State Induced by Coexisting Ferro- and Antiferromagnetic and Antiferro-orbital Fluctuations in Iron Pnictides

interactions at quarter filling is studied by using a novel finite size scaling. We diagonalize finite size systems numerically and calculate the Luttinger-liquid parameter

Dynamical Mean-Field Study on the Superconductivity Mediated by Spin and Orbital Fluctuations in the Five-Orbital Hubbard Model for Iron Pnictides

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