Charge of a quasiparticle in a superconductor

TL;DR

This paper investigates the charge transport in SIS Josephson junctions, revealing quantized partitioned quasiparticle charges and a notable dip near the superconducting gap, advancing understanding of quasiparticle behavior in superconducting devices.

Contribution

It provides experimental evidence of charge quantization in quasiparticles and identifies a unique charge dip near the superconducting gap, supported by numerical simulations.

Findings

Quantized partitioned quasiparticle charge observed (n=1-4)

Reproducible dip in charge near the gap at q≈0.6

Numerical simulations support experimental results

Abstract

Non-linear charge transport in SIS Josephson junctions has a unique signature in the shuttled charge quantum between the two superconductors. In the zero-bias limit Cooper pairs, each with twice the electron charge, carry the Josephson current. An applied bias $V_{SD}$ leads to multiple Andreev reflections (MAR), which in the limit of weak tunneling probability should lead to integer multiples of the electron charge $ne$ traversing the junction, with $n$ integer larger than $2{\Delta}/eV_{SD}$ and ${\Delta}$ the superconducting order parameter. Exceptionally, just above the gap, $eV_{SD}>2{\Delta}$, with Andreev reflections suppressed, one would expect the current to be carried by partitioned quasiparticles; each with energy dependent charge, being a superposition of an electron and a hole. Employing shot noise measurements in an SIS junction induced in an InAs nanowire (with noise proportional to the partitioned charge), we first observed quantization of the partitioned charge $q=e^*/e=n$, with $n=1-4$; thus reaffirming the validity of our charge interpretation. Concentrating next on the bias region $eV_{SD}{\approx}2{\Delta}$, we found a reproducible and clear dip in the extracted charge to $q{\approx}0.6$, which, after excluding other possibilities, we attribute to the partitioned quasiparticle charge. Such dip is supported by numerical simulations of our SIS structure.