Kondo physics from quasiparticle poisoning in Majorana devices

TL;DR

This paper analyzes how quasiparticle poisoning affects Majorana devices, revealing new Kondo effects and stability issues in multi-lead systems through theoretical and renormalization group methods.

Contribution

It introduces a low-energy theory for quasiparticle poisoning in Majorana systems and uncovers the impact on topological Kondo effects with multiple leads.

Findings

Quasiparticle poisoning induces a spin-1/2 Kondo effect for two leads.

Poisoning destabilizes the topological Kondo effect for more than two leads.

For three leads, the system becomes trivial with decoupled leads.

Abstract

We present a theoretical analysis of quasiparticle poisoning in Coulomb-blockaded Majorana fermion systems tunnel-coupled to normal-conducting leads. Taking into account finite-energy quasiparticles, we derive the effective low-energy theory and present a renormalization group analysis. We find qualitatively new effects when a quasiparticle state with very low energy is localized near a tunnel contact. For $M=2$ attached leads, such "dangerous" quasiparticle poisoning processes cause a spin $S=1/2$ single-channel Kondo effect, which can be detected through a characteristic zero-bias anomaly conductance peak in all Coulomb blockade valleys. For more than two attached leads, the topological Kondo effect of the unpoisoned system becomes unstable. A strong-coupling bosonization analysis indicates that at low energy the poisoned lead is effectively decoupled and hence, for $M>3$, the topological Kondo fixed point re-emerges, though now it involves only $M-1$ leads. As a consequence, for $M=3$, the low-energy fixed point becomes trivial corresponding to decoupled leads.