Zeeman Tunability of Andreev Bound States in van der Waals Tunnel Barriers

Abstract

Quantum dots proximity-coupled to superconductors are attractive research platforms due to the intricate interplay between the single-electron nature of the dot and the many body nature of the superconducting state. These have been studied mostly using nanowires and carbon nanotubes, which allow a combination of tunability and proximity. Here we report a new type of quantum dot which allows proximity to a broad range of superconducting systems. The dots are realized as embedded defects within semiconducting tunnel barriers in van-der-Waals layers. By placing such layers on top of thin NbSe 2 , we can probe the Andreev bound state spectra of such dots up to high in-plane magnetic fields without observing effects of a diminishing superconducting gap. As tunnel junctions defined on NbSe 2 have a hard gap, we can map the sub-gap spectra without background related to the rest of the junction. We find that the proximitized defect states invariably have a singlet ground state, manifest in the Zeeman splitting of the sub-gap excitation. We also find, in some cases, bound states which converge to zero energy and remain there. We discuss the role of the spin-orbit 1 arXiv:1906.01215v1 [cond-mat.supr-con] 4 Jun 2019 term, present both in the barrier and the superconductor, in the realization of such topologically trivial zero-energy states.

Spectroscopy of NbSe2 Using Energy-Tunable Defect-Embedded Quantum Dots

“…The Zeeman split features exhibit an energy split E Z = ±gµ B B with Landé factor g ≈ 1.7. Recalling that strongly coupled defect dots exhibit g factors ranging between 1.3 and 2.017 , we suggest that the weakly coupled QD observed here originates from similar type of defect.…”

Quantum-dot assisted spectroscopy of degeneracy-lifted Landau levels in graphene

“…The origin of such large Zeeman splits is puzzling. In atomic defects g ≈ 2 11 , so the split is hence likely to originate in the graphene layer. Quantum Hall states are known to exhibit strong enhancements of the g factors, associated with exchange coupling 45 , 46 .…”

“…In addition, it should sustain a high bias without deforming local potentials. Here we show that these seemingly contradicting requirements are fulfilled by resonant tunneling through quantum dots (QDs) bound to atomic defects within van der Waals tunnel barriers 10 , 11 . Graphene-based tunnel junctions are highly parallel 12 , and dot energies are capacitively tuned by the planar electrode, thus avoiding local charging effects due to applied bias.…”

Majorana Bound States Induced by Antiferromagnetic Skyrmion Textures

“…A trilayer of Fe 1/3 NiS 2 |Ir|NiSe 2 would allow injection or deletion of AFM skyrmions in the magnetic layer, stabilized by the heavy metal, which induces an effective mag- netic exchange interaction within the NiSe 2 superconducting layer. Because the latter can be back gated [60], the topological phase space of the heterostructure, which we predict to be dense, can be scanned or tuned to the topologically nontrivial regime.…”