Emergence of a negative charging energy in a metallic dot capacitively coupled to a superconducting island

TL;DR

This paper investigates a hybrid metallic-superconducting system where charge fluctuations induce a negative charging energy in the metallic dot, revealing novel electrostatic behavior with potential nanoelectronic applications.

Contribution

It introduces the concept of negative charging energy in a metallic dot due to capacitive coupling with a superconducting island, a phenomenon not previously reported.

Findings

Charge fluctuations in the superconducting island overscreen the Coulomb repulsion.

The metallic dot exhibits positive and negative steps in its charging curve.

Negative differential capacitance is observed as the offset charge varies.

Abstract

We consider the hybrid setup formed by a metallic dot, capacitively coupled to a superconducting island S connected to a bulk superconductor by a Josephson junction. Charge fluctuations in S act as a dynamical gate and overscreen the electronic repulsion in the metallic dot, producing an attractive interaction between two additional electrons. As the offset charge of the metallic dot is increased, the dot charging curve shows positive steps ($+2e$) followed by negative ones ($-e$) signaling the occurrence of a negative differential capacitance. A proposal for experimental detection is given, and potential applications in nanoelectronics are mentioned.