Topological superconductivity in semiconductor-superconductor-magnetic insulator heterostructures

Abstract

Hybrid superconductor-semiconductor heterostructures are promising platforms for realizing topological superconductors and exploring Majorana bound states physics. Motivated by recent experimental progress, we theoretically study how magnetic insulators offer an alternative to the use of external magnetic fields for reaching the topological regime. We consider different setups, where: (1) the magnetic insulator induces an exchange field in the superconductor, which leads to a splitting in the semiconductor by proximity effect, and (2) the magnetic insulator acts as a spin-filter tunnel barrier between the superconductor and the semiconductor. We show that the spin splitting in the superconductor alone cannot induce a topological transition in the semiconductor. To overcome this limitation, we propose to use a spin-filter barrier which enhances the magnetic exchange and provides a mechanism for a topological phase transition. Moreover, the spin-dependent tunneling introduces a strong dependence on the band alignment, which can be crucial in quantum-confined systems. This mechanism opens up a route towards networks of topological wires with less constraints on device geometry as compared to previous devices which require external magnetic fields.

Evidence for spin-polarized bound states in semiconductor-superconductor-ferromagnetic insulator islands

“…1(a). Tunneling spectroscopy into the ends of long grounded hybrid wires [10] showed signatures consistent with topological superconductivity, as recently investigated theoretically [29][30][31][32][33][34].…”

Topological superconductivity in tripartite superconductor-ferromagnet-semiconductor nanowires

“…As shown schematically in Fig. 1, the relevant region of the nanowire includes all three possible interfaces between the three constituents, N, SC, and F. To elucidate and differentiate the mechanisms by which topological superconductivity can emerge in this structure, we focus on the three paradigmatic stackings SC-N-F, N-SC-F, and N-F-SC (see also [23]). Our approach treats these stackings within a microscopic wave-function approach, but neglects band bending effects (which, however, would be the underlying reason why the intersection region of the three constituents is most relevant for the emergence of topological superconductivity [21,22,24]).…”

“…Another experiment [17] grows Au wires on top of a superconducting substrate (V) and covers them by a EuS layer. Motivated by these experiments, we study the emergence of topological superconductivity in such tripartite nanowires, which combine a semiconducting or metallic core (N) with epitaxial superconducting (SC) and ferromagnetic (F) layers, from a theoretical perspective, complementing a series of concurrent studies [21][22][23][24].…”

Topological superconductivity in nanowires proximate to a diffusive superconductor-magnetic insulator bilayer

“…These experiments have inspired several related theoretical investigations [21][22][23][24][25][26]. For basic reasons [25] it is impossible to achieve topological superconductivity solely by proximity to a spin-split conventional superconductor.…”