Tunable reentrant Kondo effect in quantum dots coupled to metal-superconducting hybrid reservoirs

TL;DR

This paper explores the reentrant Kondo effect in quantum dots coupled to hybrid metal-superconductor reservoirs, revealing its fragility and conditions for observation, and providing a new physical context for understanding this phenomenon.

Contribution

It introduces a new physical context for the reentrant Kondo effect in superconducting quantum dots and analyzes its fragility related to quantum phase transitions and contact symmetry.

Findings

Reentrant Kondo effect identified in superconducting quantum dots.

Fragility of the effect due to quantum phase transitions.

Experimental feasibility suggests possible observation with current technology.

Abstract

We elaborate on the recently introduced concept of reentrant Kondo effect in quantum impurities/dots coupled to hybrid metal-semiconductor contacts [G.~Diniz {\em et al.}, Phys. Rev. B {\bf 101}, 125115 (2020)]. By noticing the equivalence of the originally suggested semiconducting arrangement to an analogous three-terminal quantum dot setup with a normal and two phase-biased superconducting leads introduced in our recent work [Phys. Rev. B {\bf 103}, 035419 (2021)], we put this effect into a new physical context, which enables us a fresh look on the problem. First, we identify the superconducting counterpart of the reentrant Kondo effect and, consequently, reveal its fragility with respect to an underlying doublet-singlet quantum phase transition induced by the particle-hole asymmetry of the reservoir densities of states. This is pertinent (even if previously unnoticed) also in the original as well as extended semiconducting setups, where it puts stringent conditions on the symmetry of the contact density of states. Furthermore, we analyze experimental feasibility of observing the reentrant Kondo effect in its superconducting realization concluding that even present day experiments might see the onset of the reentrant behavior, but fully developed Kondo features cannot be reached due to the required vast separation of energy/temperature scales.