The scaling of the critical temperature with the Fermi temperature in diborides

The experimental determination of the scaling of the superconducting critical temperature ( T c ) vs the Fermi temperature ( T f ) of the holes in the boron σ 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 σ subband by changing the Al/Mg content in Al 1 - x Mg x B 2 . The product k f ξ 0 of the Fermi wave vector ( k f ) times the superconducting Pippard coherence length ( ξ 0 ), that is a measure of the pairing strength, remains constant, kF ξ0 = 90 for x >0.66. This high-Tc phase occurs in the boron superlattice under a tensile microstrain in the range 3%< ɛ <6%.