Pressure effects and orbital characters in cuprate and carbon-based superconductors

Abstract

Pressure effect is overviewed for the cuprates and carbon-based superconductors, with an emphasis on how their orbital characters are modified by pressure. For the high-Tc cuprates, we start from an observation for ambient pressure that, on top of the main orbital (dx2-y2), a hybridization with the second (dz2) orbital around the Fermi energy significantly affects Tc in the spin-fluctuation mediated pairing, where the hybridization is dominated by material parameters. We can then show that applying pressures along a, b axes enhances Tc while a c axis pressure suppresses Tc, where not only the dz2 hybridization but also Cu(4s) hybridization exert an effect. For the multi-layer cuprates, inter-layer pair hopping is suggested to be important, which may contribute to pressure effect. Pressure effect is also interesting in a recently discovered aromatic family of superconductors (picene, etc). There, we have again multi-band systems, which in this case derive from different molecular orbitals. The Fermi surface is an intriguing composite of different pockets/sheets having different dimensionalities arising from anisotropic transfers between the molecular orbitals, and pressure effects should be an important probe of these.