arXiv subjects

Youichi Yamakawa

Publications and source records attributed to Youichi Yamakawa.

77 records · Page 5Linked to original sources

arXiv2010

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

We study superconductivity in the two-dimensional 16-band d-p model extracted from a tight-binding fit to the band structure of LaFeAsO, using the random phase approximation. When the intraorbital repulsion U is larger than the interorbital one U', an extended s-wave (s+--wave) pairing with sign reversal of order parameter is mediated by antiferromagnetic spin fluctuations, while when U U'.

cond-mat.supr-con↗

arXiv2009

Quadrupole Susceptibility and Elastic Softening due to a Vacancy in Silicon Crystal

We investigate the electronic states around a single vacancy in silicon crystal by using the Green's function approach. The triply degenerate vacancy states within the band gap are found to be extended over a large distance $\sim20 {\rm Å}$ from the vacancy site and contribute to the reciprocal temperature dependence of the quadrupole susceptibility resulting in the elastic softening at low temperture. The Curie constant of the quadrupole susceptibility for the trigonal mode ($O_{yz},O_{zx},O_{xy}$) is largely enhanced as compared to that for the tetragonal mode ($O_{2}^{0},O_{2}^{2}$). The obtained results are consistent with the recent ultrasonic experiments in silicon crystal down to 20 mK. We also calculate the dipole and octupole susceptibilities and find that the octupole susceptibilities are extremely enhannced for a specific mode.

cond-mat.other↗

arXiv2009

Structural Transition of Li2RuO3 Induced by Molecular-Orbit Formation

A pseudo honeycomb system Li2RuO3 exhibits a second-order-like transition at temperature T=Tc=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 4d t2g 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-orbits, as proposed in our previous experimental studies.

cond-mat.str-el↗

arXiv2008

Magnetism and Superconductivity in the Two-Dimensional 16 Band d-p Model for Iron-Based Superconductors

The electronic states of the Fe2As2 plane in iron-based superconductors are investigated on the basis of the two-dimensional 16-band d-p model which includes the Coulomb interaction on a Fe site: the intra- and inter-orbital direct terms U and U', the Hund's coupling J and the pair-transfer J'. Using the random phase approximation (RPA), we obtain the magnetic phase diagram including the stripe and the incommensurate order on the U'-J plane. We also solve the superconducting gap equation within the RPA and find that, for large J, the most favorable pairing symmetry is extended s-wave whose order parameter changes its sign between the hole pockets and the electron pockets, while it is dxy-wave for small J.

cond-mat.str-el↗

arXiv2008

Successive Phase Transitions in Na_{0.5}CoO_{2} based on the Triangular Lattice d-p Model

We investigate the electronic states of the CoO_2 plane in the layered cobalt oxides Na_{0.5}CoO_{2} using the realistic 11 band d-p model on a two-dimensional triangular lattice. Effects of the Coulomb interaction on a Co site: the intra- and inter-orbital direct terms U and U', the exchange coupling J and the pair-transfer J', are treated within the Hartree-Fock approximation. It is found that the metallic antiferromagnetism takes place below T_{c1} and, in addition, the orbital order accompanied by the small charge order takes place below T_{c2} (< T_{c1}) where the system becomes insulator. The obtained results are consistent with the successive phase transitions observed in Na_{0.5}CoO_{2}.

cond-mat.str-el↗