Superconductivity in Hf₅Sb_3-xRu_x: Are Ru and Sb a Critical Charge-Transfer Pair for Superconductivity?

Sperconductivity remains unpredictable for new compounds because, from the physics perspective, it results from instabilities in a compound's electronic system that are delicately balanced with other factors such as electron-lattice coupling or magnetism. In contrast to the quantitative k space or Fermi surface view often employed in the hunt for superconductivity, the concept of critical charge transfer pairs is clearly a qualitative, real space view of what can give rise to superconductivity. However, this can be an equally important and productive viewpoint, especially from the perspective of the synthetic chemistry. Specific heat measurements are a reliable indication of the presence of bulk superconductivity when combined with resistivity and susceptibility measurements due to the change in bulk thermodynamic properties at the superconducting transition.