Superconducting Behavior of Thin Films and Small Particles of Aluminum

Tunneling measurements have been made of the transition temperatures of films of aluminum in the thickness range 400-20 Å. The highest $T_c$ observed was 5.7 °K in agreement with other measurements. Softening of the average phonon frequency by low-frequency surface phonons does not provide sufficient enhancement to explain the observed results. We therefore conclude that disorder within the metal itself is the operative mechanism for enhancement. Films of ∼20-Å thickness show no evidence of an energy gap, but they do show a magnetic effect which we interpret in terms of paramagnetic impurity, associated with the highly disordered metaloxide interface. We suggest that it is these paramagnetic ions that act as pair breakers and suppress the transition temperature in ultrathin films.