0-Pi quantum transition in a carbon nanotube Josephson junction: Universal phase dependence and orbital degeneracy

TL;DR

This paper investigates 0-pi quantum phase transitions in carbon nanotube Josephson junctions, revealing universal phase behavior and distinct mechanisms in single and two-level regimes driven by orbital degeneracy and electron correlations.

Contribution

It provides experimental insights into the phase dependence and orbital effects in 0-pi transitions within carbon nanotube Josephson junctions, highlighting universal and distinct transition mechanisms.

Findings

Universal phase diagram near orbital degeneracy

Discontinuous level-crossing transition in single-level regime

Continuous transition in two-level regime

Abstract

In a quantum dot hybrid superconducting junction, the behavior of the supercurrent is dominated by Coulomb blockade physics, which determines the magnetic state of the dot. In particular, in a single level quantum dot singly occupied, the sign of the supercurrent can be reversed, giving rise to a pi-junction. This 0-pi transition, corresponding to a singlet-doublet transition, is then driven by the gate voltage or by the superconducting phase in the case of strong competition between the superconducting proximity effect and Kondo correlations. In a two-level quantum dot, such as a clean carbon nanotube, 0-pi transitions exist as well but, because more cotunneling processes are allowed, are not necessarily associated to a magnetic state transition of the dot. In this proceeding, after a review of 0-pi transitions in Josephson junctions, we present measurements of current-phase relation in a clean carbon nanotube quantum dot, in the single and two-level regimes. In the single level regime, close to orbital degeneracy and in a regime of strong competition between local electronic correlations and superconducting proximity effect, we find that the phase diagram of the phase-dependent transition is a universal characteristic of a discontinuous level-crossing quantum transition at zero temperature. In the case where the two levels are involved, the nanotube Josephson current exhibits a continuous 0-pi transition, independent of the superconducting phase, revealing a different physical mechanism of the transition.