Two Bogoliubov quasiparticles trapped by a spin in a hybrid superconductor-semiconductor nanowire*

The binding of a single Bogoliubov quasiparticle (BQ) to a spin produces a singlet many-body state. Binding a second BQ to this spin would seem an impossible task, as the total spin is already zero.

 

We show experimentally that, for approximately equal binding energies, the spin can be bound to two BQs at the same time to form a many-body doublet state. This state is a close cousin of the overscreened state (OS) in the metallic two-channel Kondo model, albeit of different nature [1]. We measure excitations from/to the OS as subgap differential conductance resonances in the “three-electron” sector of an InAs nanowire hosting a semiconductor quantum dot coupled to two nominally identical superconducting islands [2]. A partially decoupled doublet and a bound singlet are also observed in this way for other charge occupations.

 

From a careful study of ~20 quantum dot shells displaying the OS state as the ground state, we find that, surprisingly, this state becomes unstable against the two-electron singlet as the binding energies of the quantum dot to the two islands are symmetrically increased. The OS is recovered with a magnetic field, encoding the spin correlations of the system.

 

[1] Potok, R. M. et al. (2007). Nature, 446

[2] Estrada Saldaña, J. C. et al. (2021). arXiv, 2101.10794.