Three-orbital study on the orbital distillation effect in the high Tc cuprates

Our recent study has revealed that the mixture of the dz2 orbital component into the Fermi surface suppresses Tc in the cuprates such as La2CuO4. We have also shown that applying hydrostatic pressure enhances Tc due to smaller mixing of the Cu4s component. We call these the "orbital distillation" effect. In our previous study, the 4s orbital was taken into account through the hoppings in the dx2-y2 sector, but here we consider a model in which of the dx2-y2, dz2 and 4s orbitals are all considered explicitly. The present study reinforces our conclusion that smaller 4s hybridization further enhances Tc.Comment: 4 pages, 2 figures, submitted as a proceeding of ISS2012(Tokyo

Theoretical study of the chemical pressure effect on Tc in the cuprate superconductors

Application of physical pressure on the cuprate superconductors often results in an enhancement of Tc. Motivated by this fact, we study the chemical pressure effect on the single-layered La2CuO4 and HgBa2CuO4 starting from the two-orbital Hubbard model deduced from a first-principles calculation. It is shown that the chemical pressure effects induced by La-site substitution in La2CuO4 or Hg-site substitution in HgBa2CuO4 are not expected to be effective for raising Tc

Three-orbital study on the orbital distillation effect in the high Tc cuprates

Our recent study has revealed that the mixture of the dz2 orbital component into the Fermi surface suppresses Tc in the cuprates such as La2CuO4. We have also shown that applying hydrostatic pressure enhances Tc due to smaller mixing of the Cu4s component. We call these the “orbital distillation” effect. In our previous study, the 4s orbital was taken into account through the hoppings in the dx2-y2 sector, but here we consider a model in which of the dx2-y2, dz2 and 4s orbitals are all considered explicitly. The present study reinforces our conclusion that smaller 4s hybridization further enhances Tc

Sensitivity of the superconducting state and magnetic susceptibility to key aspects of electronic structure in ferropnictides

Experiments on the iron-pnictide superconductors appear to show some materials where the ground state is fully gapped, and others where low-energy excitations dominate, possibly indicative of gap nodes. Within the framework of a 5-orbital spin fluctuation theory for these systems, we discuss how changes in the doping, the electronic structure or interaction parameters can tune the system from a fully gapped to nodal sign-changing gap with s-wave ($A_{1g}$) symmetry ($s^\pm$). In particular we focus on the role of the hole pocket at the $(\pi,\pi)$ point of the unfolded Brillouin zone identified as crucial to the pairing by Kuroki {\it et al.}, and show that its presence leads to additional nesting of hole and electron pockets which stabilizes the isotropic $s^\pm$ state. The pocket's contribution to the pairing can be tuned by doping, surface effects, and by changes in interaction parameters, which we examine. Analytic expressions for orbital pairing vertices calculated within the RPA fluctuation exchange approximation allow us to draw connections between aspects of electronic structure, interaction parameters, and the form of the superconducting gap

Theoretical Study of the Chemical Pressure Effect on Tc in the Cuprate Superconductors

AbstractApplication of physical pressure on the cuprate superconductors often results in an enhancement of Tc. Motivated by this fact, we study the chemical pressure effect on the single-layered La2CuO4 and HgBa2CuO4 starting from the two-orbital Hubbard model deduced from a first-principles calculation. It is shown that the chemical pressure effects induced by La-site substitution in La2CuO4 or Hg-site substitution in HgBa2CuO4 are not expected to be effective for raising Tc