Masters Defense: Alexandros Vekris – University of Copenhagen

Masters Defense: Alexandros Vekris

Supercurrent and Screening of Spins in a Double Quantum Dot Josephson Junction

In my thesis, I studied a lateral double quantum dot (DQD) system defined in a one-dimensional nanowire coupled to superconducting electrodes. The goal of this project was to investigate the supercurrent behaviour through this Josephson junction, as well as experiment with screening effects induced by increased coupling between the quantum dots (QDs) and the leads.

It was found that in the honeycomb regime, on which the couplings are much weaker than the charging energies of the QDs, the occupation number of the dots directly affects the sign of the supercurrent flowing through the junction. That is because even and odd electrons in the DQD correspond to singlet and doublet ground state of the system respectively. The ground state dictates whether the phase of the supercurrent acquires a π, changing its sign to negative.

Increasing the coupling to the leads with the aim of reaching the intermediate coupling regime, triggered screening of S=1/2 states of the DQD. We demonstrate intermediate steps of the charge stability diagrams with increasing coupling, indicating screening of a doublet ground state to a Yu-Shiba-Rusinov (YSR) singlet. Furthermore we investigated screening of the singlet (1,1) charge sector, by screening its excited triplet ground state, to a doublet YSR. Lastly, we increased the coupling to both superconducting leads and studied simultaneous screening of both QDs. We traced these transitions by analysing the supercurrent through these charge states and observing the 0 to π transitions.

To conclude, these results point in the better understanding of how superconductors behave in close proximity to magnetic impurities, as well as aid in the better visualisation of supercurrent through a DQD system.