Study of Superconductors by Electron Tunneling

If a small potential difference is applied between two metals separated by a thin insulating film, a current will flow due to the quantum mechanical tunnel effect. For both metals in the normal state the current-voltage characteristic is linear, for one of the metals in the superconducting state the current voltage characteristic becomes nonlinear, and for both metals in the superconductive state even a negative-resistance region is obtained. From these changes in the current voltage characteristics, the change in the electron density of states when a metal goes from its normal to its superconductive state can be inferred. By using this technique we have found the energy gap in metal films 1000-3000 A thick at 1^ to be 2₏₁= (2. 680. 06) 10^-3 ev, 2ₒ₍= (1. 110. 03) 10^-3 ev, 2₈₍= (1. 050. 03) 10^-3 ev, and 2₀₋= (0. 320. 03) 10^-3 ev. The variation of the gap width with temperature is found to agree closely with the Bardeen-Cooper-Schrieffer theory. Furthermore, the energy gap in these films has been found to depend upon the applied magnetic field, decreasing with increasing field.