Related papers: Anion height as a controlling parameter for the su…
A phase diagram is drawn in a parameter space of the nearly half-filled single band two-dimensional Hubbard model with U/t, t'/t and n as the parameters, U, t, t' and n being the on-site interaction, the nearest and second nearest neighbor…
We propose orbital fluctuations in a multi-band ground state as the superconducting pairing mechanism in the new iron-based materials. We develop a general SU(4) theoretical framework for studying a two-orbital model and discuss a number of…
The newly discovered iron-based superconductors have attracted lots of interests, and the corresponding theoretical studies suggest that the system should have six bands. In this paper, we study the multiband effects on the conductivity…
In order to understand the material dependence of $T_c$ within the single-layered cuprates, we study a two-orbital model that considers both $d_{x^2-y^2}$ and $d_{z^2}$ orbitals. We reveal that a hybridization of $d_{z^2}$ on the Fermi…
The nature of the normal state and the mechanism of superconductivity in two families of high-temperature superconductors, cuprates and pnictides, remain a matter of intense discussions. According to band-structure calculations, confirmed…
In the cuprate and iron-pnictide systems, valence changes induce high-temperature superconductivity while the local structural chemistry and local spin order both independently generate the attractive interactions responsible for the high…
The origin of uniaxial and hydrostatic pressure effects on $T_c$ in the single-layered cuprate superconductors is theoretically explored. A two-orbital model, derived from first principles and analyzed with the fluctuation exchange…
In order to explore the reason why the single-layered cuprates, La$_{2-x}$(Sr/Ba)$_x$CuO$_4$ ($T_c\simeq$ 40K) and HgBa$_2$CuO$_{4+\delta}$ ($T_c\simeq$ 90K), have such a significant difference in $T_c$, we study a two-orbital model that…
We study the effect of the lattice structure on the spin-fluctuation mediated superconductivity in the iron pnictides adopting the five-band models of several virtual lattice structures of LaFeAsO as well as actual materials such as NdFeAsO…
Although iron-based superconductors are multi-orbital systems with complicated band structures, we demonstrate that the low energy physics which is responsible for high-$T_c$ superconductivity is essentially governed by one effective…
Iron-based superconductors have been found to exhibit an intimate interplay of orbital, spin, and lattice degrees of freedom, dramatically affecting their low-energy electronic properties, including superconductivity. Albeit the precise…
Superconductivity in iron pnictides is studied by using a two-orbital Hubbard model in the large U limit. The Coulomb repulsion induces an orbital-dependent pairing between charge carriers. The pairing is found mainly from the scattering…
Like high Tc cuprates, the newly discovered iron based superconductors lie in close proximity to a magnetically ordered parent phase. However, while the magnetic order in parent cuprates is known to derive from a spin-spin local…
Although the microscopic origin of the superconductivity in high Tc copper oxides remains the subject of active inquiry, several of their electronic characteristics are well established as universal to all the known materials, forming the…
The discovery of superconductivity at 80 K under high pressure in La$_3$Ni$_2$O$_7$ presents the groundbreaking confirmation that high-$T_c$ superconductivity is a property of strongly correlated materials beyond cuprates. We use density…
We study the pairing mechanism in iron pnictide superconductors based on the five-orbital Hubbard-Holstein model. Due to Fe-ion oscillations, the s-wave superconducting (SC) state without sign reversal (s_{++}-wave state) is induced by…
Superconductivity in a novel class of layered materials, Ti-based pnictide oxides, was recently discovered. These compounds have attracted interest since they combine features of copper oxide and iron pnictide superconductors. Here the…
We explore multiband effects on d-wave superconducting instabilities driven by electron-electron interactions. Our models on the two-dimensional square lattice consist of a main band with an extended Fermi surface and predominant weight…
In the 35 years since the discovery of cuprate superconductors, we have not yet reached a unified understanding of their properties, including their material dependence of the superconducting transition temperature $T_{\text{c}}$. The…
We study superconductivity in the Hubbard model on various quasi-one-dimensional lattices with coexisting wide and narrow bands originating from multiple sites within a unit cell, where each site corresponds to a single orbital. The systems…