System for Observing Small Nonlinearities in Tunnel Junctions

Abstract

A system using a combination of harmonic detection and bridge techniques for the measurement of dV/dI and d²V/dI² of superconducting tunnel junctions having resistances ranging from a few ohms to several thousand ohms is described. These quantities are of fundamental interest in the study of the density of electron states and phonon spectra of superconductors. This system is capable of determining σ=(dV/dI)_n/(dV/dI)_s, the relative dynamic conductance, where dV/dI is the dynamic resistance of the junction in the normal (n) and superconducting (s) state to within a few parts in 10⁵. This high resolution is achieved using very small modulation levels of 60 μV rms (kT at 1°K=86 μV rms) or less. Finally, circuitry for obtaining dI/dV in the region near the energy gap using extremely low modulation levels (5 to 10 μV rms) and capable of resolving the negative resistance region is also presented. Typical data obtained with this system are shown.