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

Abstract

We study semiconductor nanowires coupled to a bilayer of a disordered superconductor and a magnetic insulator, motivated by recent experiments reporting possible Majorana-zero-mode signatures in related architectures. Specifically, we pursue a quasiclassical Usadel equation approach that treats superconductivity in the bilayer self-consistently in the presence of spin-orbit scattering, magnetic-impurity scattering, and Zeeman splitting induced by both the magnetic insulator and a supplemental applied field. Within this framework we explore prospects for engineering topological superconductivity in a nanowire proximate to the bilayer. We find that a magnetic-insulator-induced Zeeman splitting, mediated through the superconductor alone, cannot induce a topological phase since the destruction of superconductivity (i.e., Clogston limit) preempts the required regime in which the nanowire's Zeeman energy exceeds the induced pairing strength. However, this Zeeman splitting does reduce the critical applied field needed to access the topological phase transition, with fields antiparallel to the magnetization of the magnetic insulator having an optimal effect. Finally, we show that magnetic-impurity scattering degrades the topological phase, and spin-orbit scattering, if present in the superconductor, pushes the Clogston limit to higher fields yet simultaneously increases the critical applied field strength.

Optimizing the topological properties of stacking semiconductor-ferromagnet-superconductor heterostructures

“…As pointed out in Refs. [20][21][22][23], the relative positioning of the ferromagnetic and superconducting layers on top of the nanowire could affect the electrostatic potential profile inside the semiconductor as well as the total strength of the induced Zeeman spin splitting in a delicate manner. Additionally, it has also been shown via a phenomenological model that topological superconductivity can appear in a hybrid system when the ferromagnetic film is inserted as a spin-filtering barrier separating the semiconductor and superconductor layers [24,25].…”

Electronic properties of InAs/EuS/Al hybrid nanowires

“…We note that there is concurrent work on the effects of electrostatics in these hybrid systems [51]. Also, there are concurrent efforts to go beyond the effective model as used in our work and do a self-consistent treatment of proximity effect between EuS and Al when the shells overlap [52]. When the spin-orbit and spin-flip scattering processes at the Al/EuS interface can be neglected, the self-consistent model of the Al layer reproduces the model used in the current work.…”

Conductance Spectroscopy of Majorana Zero Modes in Superconductor-Magnetic Insulating Nanowire Hybrid Systems

“…With the aforementioned on one hand, the basic Rashba wire setup has further drawbacks which includes the requirement of large magnetic fields that could potentially destroy superconductivity [37][38][39][40] apart from the practicalities of precise magnetic field alignment [41]. Recently, efforts are being made towards realizing * bm@ee.iitb.ac.in topological superconductivity with zero external magnetic fields by using proximity effects from magnetic insulators (MI) [42][43][44][45][46][47][48][49][50][51][52]. Recent experimental [43] and theoretical works [44][45][46][47][48]51] featuring this setup indicate that at very low external magnetic fields, or even zero external magnetic fields, a topological MZM phase can emerge.…”

“…Recently, efforts are being made towards realizing * bm@ee.iitb.ac.in topological superconductivity with zero external magnetic fields by using proximity effects from magnetic insulators (MI) [42][43][44][45][46][47][48][49][50][51][52]. Recent experimental [43] and theoretical works [44][45][46][47][48]51] featuring this setup indicate that at very low external magnetic fields, or even zero external magnetic fields, a topological MZM phase can emerge. The object of this paper is hence to provide an in-depth analysis of the transport signatures of MZMs in these structures, particularly focusing on the local and non-local conductance spectra in both pristine and disordered nanowires.…”