Josephson current via spin and orbital states of a tunable double quantum dot

TL;DR

This study investigates supercurrent transport in a tunable double quantum dot Josephson junction, revealing how spin states and hybridization influence supercurrent behavior and ground state transitions.

Contribution

It demonstrates the control of supercurrent via spin and orbital states in a double quantum dot, highlighting the effects of hybridization and symmetry on supercurrent suppression and emergence.

Findings

Supercurrent depends on the spin ground state of the quantum dots.

Supercurrent is suppressed in the spin-triplet ground state at zero magnetic field.

A supercurrent appears when the ground state transitions to a spin-singlet with increased hybridization.

Abstract

Supercurrent transport is experimentally studied in a Josephson junction hosting a double quantum dot (DQD) with tunable symmetries. The QDs are parallel-coupled to two superconducting contacts and can be tuned between strong inter-dot hybridization and a ring geometry where hybridization is suppressed. In both cases, we observe supercurrents when the two interacting orbitals are either empty or filled with spins, or a combination. However, when each QD hosts an unpaired spin, the supercurrent depends on the spin ground state. It is strongly suppressed for the ring geometry with a spin-triplet ground state at zero external magnetic field. By increasing the QD hybridization, we find that a supercurrent appears when the ground state changes to spin-singlet. In general, supercurrents are suppressed in cases of spin doublet ground state, but an exception occurs at orbital degeneracy when the system hosts one additional spin, as opposed to three, pointing to a broken particle-hole symmetry.