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°K to be $2{\epsilon }_{\mathrm{Pb}}=(2.68\mathrm{}\pm 0.06)\times {10}^{-3\mathrm{}}$ ev, $2{\epsilon }_{\mathrm{Sn}}=(1.11\mathrm{}\pm 0.03)\times {10}^{-3\mathrm{}}$ ev, $2{\epsilon }_{\mathrm{In}}=(1.05\mathrm{}\pm 0.03)\times {10}^{-3\mathrm{}}$ ev, and $2{\epsilon }_{\mathrm{Al}}=(0.32\mathrm{}\pm 0.03)\times {10}^{-3\mathrm{}}$ ev.