Low frequency noise in ballistic Nb/Ag point contacts

TL;DR

This study investigates low frequency noise in ballistic Nb/Ag point contacts, revealing how noise characteristics vary with contact resistance and how two-level fluctuations can affect superconductivity.

Contribution

It provides new insights into the noise behavior and fluctuation mechanisms in ballistic superconductor-normal metal point contacts.

Findings

Voltage dependence of noise varies with contact resistance.

High bias voltages induce two-level fluctuations.

Two-level fluctuators may cause depairing of superconductivity.

Abstract

We report on the low frequency noise in the ballistic point-contacts between a silver tip and a niobium foil. The ballistic nature of the point-contacts is confirmed by Andreev reflection spectroscopy at low bias voltage with the Nb foil cooled below its superconducting transition temperature ($T_c$). We find that the voltage dependence of the low frequency noise differs for different point-contacts with varying contact resistances. At high bias voltages, random two level fluctuations appear and dominate over the background $1/f$ noise. From analysis of the Andreev reflection spectra, we show that such two level fluctuators may give rise to depairing of the superconducting order parameter.