Cooper Pair Transport in a Resistor-Biased Josephson Junction Array

TL;DR

This study investigates the dc transport and Bloch oscillations in long arrays of small Al Josephson junctions biased with on-chip resistors, revealing autonomous oscillations and a large effective capacitance.

Contribution

It demonstrates the presence of autonomous Bloch oscillations in Josephson junction arrays and measures an effective capacitance much larger than the geometric one.

Findings

Observation of high-frequency Bloch oscillations (~1 GHz)

Identification of a large Coulomb threshold for current

Effective Bloch capacitance exceeds geometric capacitance

Abstract

The dc transport properties of long arrays of small Al Josephson junctions, biased through on-chip Cr resistors, are studied. The IV-characteristics show a large Coulomb threshold for current as well as negative-slope regions indicating the regime of autonomous Bloch oscillations up to rather high frequencies of f = I/2e ~1 GHz, comparable to those reported by other groups for single junctions. On the other hand, a small depth of the back-bending implies a low duty cycle and a broad spectrum of the oscillations, which we attribute to the insufficiently high impedance of the bias resistors. A self-sustained switching process at a small bias current is used to study the statistics of the switching voltages and to determine the effective Bloch capacitance which was found to considerably exceed the geometric junction capacitance.