Theory for doping trends in titanium oxypnictide superconductors

A family of titanium oxypnictide materials BaTi2Pn2O (Pn=pnictogen) becomes superconducting when a charge and/or spin-density wave is suppressed. With hole doping, isovalent doping and pressure, a whole range of tuning parameters is available. We investigate how charge doping controls the superconducting transition temperature Tc. To this end, we use experimental crystal structure data to determine the electronic structure and Fermi surface evolution along the doping path. We show that a naive approach to calculating Tc via the density of states at the Fermi level and the McMillan formula systematically fails to yield the observed Tc variation. On the other hand, spin-fluctuation theory pairing calculations allow us to consistently explain the Tc increase with doping. All alkali-doped materials Ba1-xAxTi2Sb2O (A=Na,K,andRb) are described by a sign-changing s-wave order parameter. Susceptibilities also reveal that the physics of the materials is controlled by a single Ti 3d orbital.