[Paper Review] Evidence for anisotropic spin-triplet Andreev reflection at the 2D van der Waals ferromagnet/superconductor interface

Ranran Cai, Yunyan Yao+12 more|arXiv (Cornell University)|Nov 2, 2021

Topological Materials and PhenomenaPhysics and Astronomy45 references39 citations

TL;DR

This study provides experimental evidence for anisotropic spin-triplet Andreev reflection at a 2D van der Waals ferromagnet/superconductor (Fe₀.₂₉TaS₂/NbN) interface, driven by Rashba spin-orbit coupling. The observed large magnetoresistance with strong angular dependence on magnetic field direction—peaking when magnetization is out-of-plane—confirms spin-triplet superconductivity and non-collinear spin-mixing at the interface, offering a platform for topological quantum computing and spintronic devices.

ABSTRACT

Fundamental symmetry breaking and relativistic spin-orbit coupling give rise to fascinating phenomena in quantum materials. Of particular interest are the interfaces between ferromagnets and common s-wave superconductors, where the emergent spin-orbit fields support elusive spin-triplet superconductivity, crucial for superconducting spintronics and topologically-protected Majorana bound states. Here, we report the observation of large magnetoresistances at the interface between a quasi-two-dimensional van der Waals ferromagnet Fe0.29TaS2 and a conventional s-wave superconductor NbN, which provides the possible experimental evidence for the spin triplet Andreev reflection and induced spin-triplet superconductivity at ferromagnet/superconductor interface arising from Rashba spin-orbit coupling. The temperature, voltage, and interfacial barrier dependences of the magnetoresistance further support the induced spin-triplet superconductivity and spin-triplet Andreev reflection. This discovery, together with the impressive advances in two-dimensional van der Waals ferromagnets, opens an important opportunity to design and probe superconducting interfaces with exotic properties.

Motivation & Objective

• To demonstrate the existence of spin-triplet Andreev reflection at a 2D van der Waals ferromagnet/superconductor interface.
• To establish that Rashba spin-orbit coupling at the interface enables anisotropic spin-triplet pairing and non-collinear spin-mixing.
• To rule out vortex-induced effects as the origin of observed magnetoresistance in type-II superconducting NbN.
• To provide experimental evidence for induced spin-triplet superconductivity in a platform free from lattice-matching constraints.
• To explore the feasibility of realizing topological Majorana bound states and dissipationless spin currents in 2D quantum materials.

Proposed method

• Fabricated three-terminal heterostructures using exfoliated Fe₀.₂₉TaS₂ (2D vdW ferromagnet) and NbN (s-wave superconductor) with Pt normal metal contacts.
• Measured interfacial resistance using a three-terminal geometry under variable magnetic field angles (Θyz) at T = 2 K.
• Employed anomalous Hall effect to characterize the Curie temperature (~90 K) and magnetic easy axis (perpendicular to plane) of Fe₀.₂₉TaS₂.
• Conducted control experiments using Al/NbN heterostructures to isolate vortex contributions, with RJS-matched devices for comparison.
• Analyzed bias and temperature dependence of dI/dV curves to assess interfacial transparency and Andreev reflection strength.
• Used theoretical modeling of spin-orbit-coupled pairing (Rashba + Dresselhaus) to derive angular dependence of spin-triplet Andreev reflection, predicting suppression when spin quantization axis aligns with in-plane wave vector.

Experimental results

Research questions

• RQ1 Can anisotropic spin-triplet Andreev reflection be experimentally observed at a 2D van der Waals FM/SC interface with Rashba spin-orbit coupling?
• RQ2 Does the observed magnetoresistance depend on the relative orientation between the ferromagnet's magnetization and the interfacial spin-orbit field?
• RQ3 What is the contribution of vortices in the type-II superconductor NbN to the measured magnetoresistance?
• RQ4 How does the interfacial transparency (barrier strength) affect the visibility of spin-triplet Andreev reflection?
• RQ5 Can the angular dependence of the magnetoresistance be explained by spin-triplet pairing induced via spin-orbit coupling?

Key findings

• A large magnetoresistance of ~20% was observed at T = 2 K when the magnetic field was rotated in the yz plane, with maximum resistance at Θyz = 0° (magnetization out-of-plane).
• The magnetoresistance ratio MR(Θyz) = [R(Θyz) − R(0°)] / R(0°) × 100% reached up to ~20% at Θyz = 90°, indicating strong anisotropy.
• The normal-state resistance at T = 20 K showed negligible variation with magnetic field angle, ruling out geometric or strain effects.
• Control experiments with Al/NbN heterostructures showed only ~3% MR within noise level, excluding vortex-induced effects as the dominant mechanism.
• Zero-bias conductance at T = 2 K was ~80% of the normal-state value (GN), indicating a modest interfacial barrier strength, consistent with significant Andreev reflection contribution.
• Theoretical modeling confirmed that spin-triplet Andreev reflection vanishes when the electron’s in-plane wave vector is perpendicular to the spin-quantization axis, explaining the observed angular dependence.