Quantum Effects in Small-Capacitance Single Josephson Junctions

TL;DR

This study investigates quantum effects in small-capacitance Josephson junctions, demonstrating Coulomb blockade and negative differential resistance influenced by environmental impedance, with experimental results supported by numerical theory.

Contribution

It provides experimental evidence of quantum phenomena in Josephson junctions and compares these with numerical models, highlighting environmental control effects.

Findings

Observation of Coulomb blockade of Cooper-pair tunneling

Detection of negative differential resistance

Qualitative agreement between experiments and numerical calculations

Abstract

We have measured the current-voltage (I-V) characteristics of small-capacitance single Josephson junctions at low temperatures (T=0.02-0.6 K), where the strength of the coupling between the single junction and the electromagnetic environment was controlled with one-dimensional arrays of dc SQUIDs. The single-junction I-V curve is sensitive to the impedance of the environment, which can be tuned IN SITU. We have observed Coulomb blockade of Cooper-pair tunneling and even a region of negative differential resistance, when the zero-bias resistance R_0' of the SQUID arrays is much higher than the quantum resistance R_K = h/e^2 = 26 kohm. The negative differential resistance is evidence of coherent single-Cooper-pair tunneling within the theory of current-biased single Josephson junctions. Based on the theory, we have calculated the I-V curves numerically in order to compare with the experimental ones at R_0' >> R_K. The numerical calculation agrees with the experiments qualitatively. We also discuss the R_0' dependence of the single-Josephson-junction I-V curve in terms of the superconductor-insulator transition driven by changing the coupling to the environment.