Quantized spin pumping in topological ferromagnetic-superconducting nanowires

TL;DR

This paper investigates quantized spin pumping in topological nanowires with Majorana zero modes, showing that certain observables are correlated and quantized only in the topologically nontrivial phase, providing a way to identify Majorana modes.

Contribution

It demonstrates that spin pumping is quantized in topological phases and establishes correlations between conductance, entropy, and spin pumping as signatures of Majorana zero modes.

Findings

Quantized spin pumping occurs in topologically nontrivial nanowires.

Correlated peaks in conductance, entropy, and spin pumping indicate Majorana modes.

Uncorrelated observables in trivial phases help distinguish Majorana from quasi-Majorana states.

Abstract

Semiconducting nanowires with strong spin-orbit coupling in the presence of induced superconductivity and ferromagnetism can support Majorana zero modes. We study the pumping due to the precession of the magnetization in single-subband nanowires and show that spin pumping is robustly quantized when the hybrid nanowire is in the topologically nontrivial phase, whereas charge pumping is not quantized. Moreover, there exists one-to-one correspondence between the quantized conductance, entropy change and spin pumping in long topologically nontrivial nanowires but these observables are uncorrelated in the case of accidental zero-energy Andreev bound states in the trivial phase. Thus, we conclude that observation of correlated and quantized peaks in the conductance, entropy change and spin pumping would provide strong evidence of Majorana zero modes, and we elaborate how topological Majorana zero modes can be distinguished from quasi-Majorana modes potentially created by a smooth tunnel barrier at the lead-nanowire interface. Finally, we discuss peculiar interference effects affecting the spin pumping in short nanowires at very low energies.