Inversion of Two-Band Superconductivity at the Critical Electron Doping of (Mg,Al)B2

Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg_(1-x)Al_xB₂. A simultaneous decrease of EELS intensity from σ-band hole states and the magnitude of the σ gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x≈0.33 as inferred from the unit cell volume. However, superconductivity was not quenched completely, but persisted with T_c<7K up to about x≈55. Only the π band had detectable density of states for 0.33≲x≲0.55, implying an inversion of the two-band hierarchy of MgB₂ in that regime. Since π-band superconductivity is active in other materials such as intercalated graphite, implications for new materials with high T_c are discussed.