Low Level Second Harmonic Detection System

A second harmonic detection system suitable for detailed study of current-voltage characteristics, particularly tunneling characteristics of superconductor and semiconductor junctions, is described. The system is capable of detecting second harmonic voltage changes as low as 10−9 V which allows the fundamental signal level to be ≤kT at a junction temperature of 1°K. A selection-rejection impedance transformation reactance network which is the heart of the system is described. The quantitative relationships between second harmonic voltage and second derivative of current with respect to voltage are given. The capabilities of the system are illustrated by a photograph of an X-Y recorder tracing of second harmonic voltage vs dc bias voltage for an Al-I-Pb tunnel junction.

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