Enhancement of the Kondo effect in a quantum dot formed in a full-shell nanowire

We analyze results of a recent experiment [D. Razmadze, E. C. T. O'Farrell, P. Krogstrup, and C. M. Marcus, Phys. Rev. Lett. 125, 116803 (2020)0031-900710.1103/PhysRevLett.125.116803] on transport through a quantum dot between two full-shell nanowires and show that the observed effects are caused by the Kondo effect enhancement due to a nontrivial geometry (magnetic flux in a full-shell nanowire) rather than the presence of Majorana bound states. Moreover, we propose that such a setup presents a unique and convenient system to study the competition between superconductivity and the Kondo effect and has significant advantages in comparison to other known approaches, as the important parameter is controlled by the magnetic flux through the full-shell nanowire, which can be significantly varied with small changes of magnetic field, and does not require additional gates. This competition is of fundamental interest as it results in a quantum phase transition between an unscreened doublet and a many-body Kondo singlet ground state of the system.