Alternative paths to realize Majorana Fermions in Superconductor-Ferromagnet Heterostructures

Abstract

A fundamental obstacle for achieving quantum computation is local decoherence. One way to circumvent this problem rests on the concepts of topological quantum computation using non-local information storage, for example on pairs of Majorana fermions (MFs). The arguably most promising way to generate MFs relies at present on spin-triplet p-wave states of superconductors (SC), which are not abundant in nature, unfortunately. Thus, proposals for their engineering in devices, usually via proximity effect from a conventional SC into materials with strong spin-orbit coupling (SOC), are intensively investigated nowadays. Here we take an alternative path, exploiting the different connections between fields based on a quartet coupling rule for fields introduced by one of us, we demonstrate that, for instance, coexisting Zeeman field with a charge current would provide the conditions to induce p-wave pairing in the presence of singlet superconductivity. This opens new avenues for the engineering of robust MFs in various, not necessarily (quasi-)one-dimensional, superconductor-ferromagnet heterostructures, including such motivated by recent pioneering experiments that report MFs, in particular, without the need of any exotic materials or special structures of intrinsic SOC.

Majorana Zero Modes in Ferromagnetic Wires without Spin-Orbit Coupling

“…The magnetisations of the two ferromagnetic insulators point in opposite directions and both are perpendicular to the magnetisation of the ferromagnetic metallic wire. Majorana zero modes emerge localised at the edges of the FM, due to the synergy between the externally applied supercurrents with the emergent in the SCs exchange fields [27]. We remark that several works have demonstrated both theoretically [28,29] and experimentally [30][31][32] the emergence of exchange fields in conventional SCs proximised by FIs.…”

Superconducting triplet pairings and anisotropic magnetoresistance effects in ferromagnet/superconductor/ferromagnet double-barrier junctions

“…While early works focused, e.g., on the conductance of ferromagnet/superconductor point contacts [20,21], * Corresponding author: andreas.costa@physik.uni-regensburg.de thereby demonstrating that Andreev reflections impact transport in a unique way from which the ferromagnet's spin polarization can be experimentally extracted, more intricate junction setups are being explored nowadays. Magnetic Josephson junctions are particularly promising candidates to investigate unprecedented transport anomalies, covering current-reversing 0-π transitions [22,23] that could form the elementary two-level system for quantum computing, substantially enhanced current magnetoanisotropies [24][25][26][27], the potential appearance of Majorana states [28][29][30][31][32][33][34][35][36], as well as the possibility to efficiently generate long-range spinpolarized triplet-Cooper pair supercurrents [18,37]. Such triplet pairings are typically induced in s-wave superconductors proximitized by (strongly spin-polarized) ferromagnets either in the presence of noncollinearly magnetized interfacial domains [38][39][40][41][42][43][44][45][46][47][48][49] or spin-orbit coupling (SOC) [26,50,51].…”

Surface step states and Majorana end states in profiled topological insulator thin films

“…with TI nanowires on SC substrate [22][23][24][25][26][27] which may need the application of a magnetic flux through the wire. In alternative devices the wire has to be itself ferromagnetic [28] or heterostructures with ferromagnetic layers are used [24]. In the present scenario no magnetic field has to be applied, a properly chosen step of the TI profile on the substrate is sufficient to create the possibility for Majorana states at the step ends.…”