Temperature and Magnetic Field Dependence of the Subharmonic Structure in Pb-Pb Superconducting Tunneling

Peaks occur in the $I-V$ characteristics of lead-lead-oxide-lead superconducting tunnel junctions at submultiples $\frac{2\Delta }{n}$ of the full energy gap $2\Delta$. This "subharmonic structure" has been studied in detail for its dependence on temperature and magnetic field. Its position varies with temperature identically as the energy gap does. The widths of the subharmonic peaks are independent of temperature. Magnetic fields are found to reduce the amplitudes and increase the widths of the structure. Evidence is presented that none of the present theories explains the phenomenon satisfactorily.