Spin Transport in Ferromagnet-InSb Nanowire Quantum Devices

Abstract

Signatures of Majorana zero modes (MZMs), which are the building blocks for fault-tolerant topological quantum computing, have been observed in semiconductor nanowires (NW) with strong spin-orbital-interaction (SOI), such as InSb and InAs NWs with proximity-induced superconductivity. Realizing topological superconductivity and MZMs in this most widely-studied platform also requires eliminating spin degeneracy, which is realized by applying a magnetic field to induce a helical gap. However, the applied field can adversely impact the induced superconducting state in the NWs and also places geometric restrictions on the device, which can affect scaling of future MZM-based quantum registers. These challenges could be circumvented by integrating magnetic elements with the NWs. With this motivation, in this work we report the first experimental investigation of spin transport across InSb NWs, which are enabled by devices with ferromagnetic (FM) contacts. We observe signatures of spin polarization and spin-dependent transport in the quasi-one-dimensional ballistic regime. Moreover, we show that electrostatic gating tunes the observed magnetic signal and also reveals a transport regime where the device acts as a spin filter. These results open an avenue towards developing MZM devices in which spin degeneracy is lifted locally, without the need of an applied magnetic field. They also provide a path for realizing spin-based devices that leverage spin-orbital states in quantum wires.

Spin-helical detection in a semiconductor quantum device with ferromagnetic contacts

“…near the contacts. Fabry-Pérot interference can severely limit or even prevent the detection of helical gaps using normal (non-FM) leads by limiting the visibility of conductance plateaus or the ability to distinguish conductance fingerprints of helical states from interferencerelated conductance modulation [9,[14][15][16][17]. Below, we demonstrate that these challenges can be greatly mitigated by using FM contacts, which can substantially improve the contrast between helical features and quantum interference.…”

“…In 1D quantum systems with partially-reflecting ends the interference of propagating and reflected particle waves leads to quantum interference, analogous to that of photons in a laser cavity. Such Fabry-Pérot intereference is common for electron waves in mesoscopic systems, such as nanotube-and nanowirebased quantum devices [9,[32][33][34][35], where the electrostatic potential can be spatially dependent in the neighborhood of metal/semiconductor interfaces, leading to partial reflection of the electron wavefunctions, e.g. near the contacts.…”

“…Semiconductor nanowire-superconductor hybrid systems are a highly promising materials platform for investigating Majorana zero modes (MZMs)[1-8], as well as spin transport in quantum channels [9,10]. Because MZMs arise in oddparity topological superconductors, realizing them in hybrid semiconductor nanowire-superconductor devices requires removing the spin degeneracy to achieve an effective odd-parity state.…”

“…Normal metal contacts are also not suitable for studying the rich spin texture that makes the helical state relevant for realizing MZMs and for investigating other quantum spin-based effects. Recently, there has been growing interest in integrating ferromagnetic (FM) elements with high SO coupling materials to realize topological states [18][19][20] and to demonstrate quantum transport effects with spin-polarized currents, including spin-filtering in the regime of few quantum modes and possible spin-dependent signatures of the helical state [9,10]. However, an approach for reliably detecting spin-helical states and exploring their spin texture is currently lacking.…”

“…Recent experimental work [9,10] has demonstrated that spin-filtering can be achieved in strong SO nanowire devices with magnetic contacts, however a comprehensive understanding and description of the spin transport picture in such devices is still missing. Below, we show that FM contacts can provide access to key information about the helical state and its spin texture, which cannot be obtained in devices with normal contacts.…”