Andreev and Majorana bound states in single and double quantum dot structures

TL;DR

This study numerically investigates how Majorana and Andreev bound states emerge and coexist in single and double quantum dot systems coupled to different types of superconductors, revealing distinct behaviors in trivial and topological phases.

Contribution

It demonstrates the conditions under which Majorana and Andreev bound states can coexist in coupled quantum dots, using a recursive Green's function approach for exact local density of states calculations.

Findings

Majorana and Andreev states appear separately in single quantum dots.

In double dots, Majorana and Andreev states can coexist depending on coupling and phase.

Majorana states remain localized to one dot in the molecular regime.

Abstract

We present a numerical study of the emergence of Majorana and Andreev bound states in a system composed by two quantum dots, in which one of then is coupled to a conventional superconductor, SC1, and the other connects to a topological superconductor, SC2. By controlling the interdot coupling we can drive the system from a two single (uncoupled) quantum dots to a double (coupled) dot system configurations. We employ a recursive Green's function technique that provides us with numerically exact results for the local density of states of the system. We first show that in the uncoupled dot configuration (single dot behavior) the Majorana and the Andreev bound states appear in an individual dot in two completely distinct regimes. Therefore, they cannot coexist in the single quantum dot system. We then study the coexistence of these states in the coupled double-dot configuration. In this situation we show that in the trivial phase of the SC2, the Andreev states are bound to an individual quantum dot in the atomic regime (weak interdot coupling) or extended over the entire molecule in the molecular regime (strong interdot coupling). More interesting features is actually seen in the topological phase of the SC2. In this case, in the atomic limit, the Andreev states appear bound to one of the quantum dots while a Majorana zero mode appears in the other one. In the molecular regime, on the other hand, the Andreev bound states take over the entire molecule while the Majorana state remains always bound to one of the quantum dots.