Yuki Yanagi

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.

Cooperative effects of Coulomb and electron-phonon interactions in the two-dimensional 16-band d − p model for iron-based superconductors

We study the electronic states and the superconductivity in the two-dimensional 16-band d-p model coupled with A1g, B1g and Eg local phonons and obtain the rich phase diagram including the magnetic, charge and orbital ordered phases on the parameter plane of the Coulomb and electron-phonon interactions. When the electron-phonon interaction is dominant, the charge fluctuations induce the s++ wave superconductivity, while when the Coulomb interaction is dominant, the magnetic fluctuations induce the s+- wave superconductivity. Remarkably, the orbital fluctuations are enhanced due to the cooperative effects of the Coulomb and electron-phonon interactions and induce the s++ wave and the nodal s+- wave superconductivities.

Local Correlation Effects on the s±- and s++-Wave Superconductivities Mediated by Magnetic and Orbital Fluctuations in the Five-Orbital Hubbard Model for Iron Pnictides

We investigate the electronic state and the superconductivity in the five-orbital Hubbard model for iron pnictides by using the dynamical mean-field theory in conjunction with the Eliashberg equation. The renormalization factor exhibits significant orbital dependence resulting in the large change in the band dispersion as observed in recent ARPES experiments. The critical interactions towards the magnetic, orbital and superconducting instabilities are suppressed as compared with those from the random phase approximation (RPA) due to local correlation effects. Remarkably, the \(s_{++}\)-pairing phase due to the orbital fluctuation is largely expanded relative to the RPA result, while the \(s_{\pm}\)-pairing phase due to the magnetic fluctuation is reduced.

Dimensional Reduction and Odd-Frequency Pairing of the Checkerboard-Lattice Hubbard Model at 1/4-Filling

The ferromagnetism of the checkerboard-lattice Hubbard model at quarter filling is one of the few exact ferromagnetic ground states known in the family of Hubbard models. When the nearest neighbor hopping, t 1 , is negligible compared with the second neighbor one, t 2 , the system reduces to a collection of Hubbard chains. We find that the 1D character is surprisingly robust as long as t 1 < t 2 . This phenomenon of dimensional reduction due to the geometrical frustration leads to peculiar magnetic orders with 1D character for intermediate U and is responsible for odd-frequency superconducting states close to the magnetic boundary.

SU(3) Dirac electrons in the15-depleted square-lattice Hubbard model at14filling

We investigate the magnetic and metal-insulator (M-I) phase diagram of the

Manipulating the magnetoelectric effect: Essence learned from Co4Nb2O9

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

-depleted square-lattice Hubbard model at

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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Continuous Mott transition in a two-dimensional Hubbard model

filling by the mean-field approximation. There exist three magnetic phases of nonmagnetic (N), antiferromagnetic (AF), and ferromagnetic (F) types, each realized for the large intrasquare hopping

Two types of s-wave pairing due to magnetic and orbital fluctuations in the two-dimensional 16-band d-p model for iron-based superconductors

{t}_{1}