## SciPost Submission Page

# Reproducing topological properties with quasi-Majorana states

### by A. Vuik, B. Nijholt, A. R. Akhmerov, M. Wimmer

#### - Published as SciPost Phys. 7, 061 (2019)

### Submission summary

As Contributors: | Anton Akhmerov · Adriaan Vuik · Michael Wimmer |

Arxiv Link: | https://arxiv.org/abs/1806.02801v3 (pdf) |

Date accepted: | 2019-10-28 |

Date submitted: | 2019-10-14 |

Submitted by: | Vuik, Adriaan |

Submitted to: | SciPost Physics |

Discipline: | Physics |

Subject area: | Condensed Matter Physics - Theory |

Approaches: | Theoretical, Computational |

### Abstract

Andreev bound states in hybrid superconductor-semiconductor devices can have near-zero energy in the topologically trivial regime as long as the confinement potential is sufficiently smooth. These quasi-Majorana states show zero-bias conductance features in a topologically trivial phase, mimicking spatially separated topological Majorana states. We show that in addition to the suppressed coupling between the quasi-Majorana states, also the coupling of these states across a tunnel barrier to the outside is exponentially different for increasing magnetic field. As a consequence, quasi-Majorana states mimic most of the proposed Majorana signatures: quantized zero-bias peaks, the $4\pi$ Josephson effect, and the tunneling spectrum in presence of a normal quantum dot. We identify a quantized conductance dip instead of a peak in the open regime as a distinguishing feature of true Majorana states in addition to having a bulk topological transition. Because braiding schemes rely only on the ability to couple to individual Majorana states, the exponential control over coupling strengths allows to also use quasi-Majorana states for braiding. Therefore, while the appearance of quasi-Majorana states complicates the observation of topological Majorana states, it opens an alternative route towards braiding of non-Abelian anyons and protected quantum computation.

### Ontology / Topics

See full Ontology or Topics database.Published as SciPost Phys. 7, 061 (2019)

### List of changes

We thank the referee for the favorable assessment of our manuscript. We have addressed

the requests for minor clarifications raised by the referee:

1.) Indeed, the smoothness requirement is not absolute. We now refer explicitly to

previous works that have found near-zero energy states with potentials involving

kinks or abrupt changes. We have also clarified that it is indeed the smooth potential

slope of the tunnel barrier that is responsible for the appearance of quasi-Majorana

states in all of our systems, and that the quantum dot in system Fig. 2(b) serves

only as a probe.

2.) We have now clarified that with "spin" we mean the expectation value of

sigma_x.

Furthermore, note that quasi-Majorana states are actually favored by

weak spin-orbit interaction, as shown in Fig. 2(c) and (d). For this reason,

we focus on the case of weak spin-orbit interaction, where the transverse

part of the spin-orbit coupling is of negligible influence. In fact, in the

3D model of Sec. VI we do include the transverse part explicitly. For the weak

spin-orbit strengths considered there, quasi-Majorana states appear. We have

extended the discussion in this regard in the paper.

3.) We have now clarified that we compare the separation of the quasi-Majorana

states to the coherence length \xi. Furthermore, we have added to Fig. 4 the spectrum

corresponding to the wave functions as suggested by the referee.

4.) In relation to the questions raised by the referee, we would like to point out that

Fig. 8 was computed using our microscopic model, i.e. the contributions of above-gap

states are explicitly included. We have clarified this aspect in the text.

Furthermore, we have now added at the end of Section V an extended discussion on how

robust the different features of the conductance shown in Fig. 8 are.

5.) We have clarified that the coupling elements t in Eq. (12) and (14) are in units of sqrt(energy).

With respect to quasi-Majoranas allowing for topological operations, we would like to

point out that in Sec. VIII we do discuss the fact that quasi-Majoranas themselves are

not topologically protected. To avoid confusion, we have renamed topological quantum

computation with quasi-Majorana states to protected quantum computation.

With respect to the occurrence of quasi-Majoranas at smaller fields, we have now

added the clarification that this indeed holds for a given chemical potential.