Superconductivity of Tantalum, Niobium and Lanthanum Studied by Electron Tunneling: Problems of Surface Contamination

The properties of superconducting transition metal tunnel junctions are sensitive to the methods of junction preparation. The derivatives of a tantalum‐silver junction made by a sputtering method clearly show much poorer gap and phonon induced characteristics than a junction made on an outgassed surface. Analysis of the latter gives α2(ω)F(ω) with phonon peaks at energies in agreement with neutron scattering, $μ* = 0.11$ and $λ = 0.69$ in good agreement with calculated values. It is concluded that tantalum is an electron‐phonon coupled superconductor.

The data from niobium‐gold and lanthanum‐aluminum junctions are analyzed and presented. Even though the niobium and lanthanum junctions are the best ones ever made using these materials, they still possess sufficient surface contamination to affect the gap characteristic and weaken the phonon‐induced structures in the tunneling characteristics. As a result, the Coulomb pseudopotentials $μ*$ for both Nb and La are found to be −0.04, a significant deviation from the $μ* = 0.1 to 0.15$ range typical of most superconductors. The electron phonon coupling strengths $λ$ are 0.58 for Nb, and 0.66 for La, compared to 0.82 and 0.75 expected on the basis of their T_c and θ_D. However, at present we believe these disagreements are the result of studying junctions with contaminated transition metal surfaces rather than evidence for a second coupling mechanism of superconductivity.