Collapse of the Josephson emission in a carbon nanotube junction in the Kondo regime

TL;DR

This study investigates the high-frequency emission of a carbon nanotube Josephson junction in the Kondo regime, revealing a suppression of AC Josephson emission linked to a transition in the junction's quantum ground state.

Contribution

It provides the first experimental observation of the collapse of Josephson emission in a Kondo regime and compares results with NRG theory to explain the underlying quantum transition.

Findings

AC Josephson emission is suppressed in the Kondo regime when supercurrent is enhanced.

The suppression is linked to a transition from singlet to doublet ground state.

Results agree with numerical renormalization group (NRG) theoretical predictions.

Abstract

We probe the high frequency emission of a carbon nanotube based Josephson junction and compare it to its DC Josephson current. The AC emission is probed by coupling the carbon nanotube to an on-chip detector (a Superconductor-Insulator-Superconductor junction), via a coplanar waveguide resonator. The measurement of the photo-assisted current of the detector gives direct access to the signal emitted by the carbon nanotube. We focus on the gate regions that exhibit Kondo features in the normal state and demonstrate that when the DC supercurrent is enhanced by the Kondo effect, the AC Josephson effect is strongly reduced. This result is compared to NRG theory and is attributed to a transition between the singlet ground state and the doublet excited state which is enabled only when the junction is driven out-of-equilibrium by a voltage bias.