Scaling of the critical temperature with the Fermi temperature in diborides

The experimental determination of the scaling of the superconducting critical temperature (Tc) vs the Fermi temperature (Tf) of the holes in the boron s subband is presented. The Fermi level has been tuned near the "shape resonance" i.e., the two- to three-dimensional crossover of the Fermi surface of the boron s subband by changing the Al/Mg content in Al1-x Mgx B2. The product kfy0 of the Fermi wave vector (kf) times the superconducting Pippard coherence length (yo), that is a measure of the pairing strength, remains constant, kfy0=90 for x > 0.66. This high-Tc phase occurs in the boron superlattice under a tensile microstrain in the range 3%< e <6%.