Origin of subgap states in normal-insulator-superconductor van der Waals heterostructures

Abstract

Superconductivity in van der Waals materials, such as NbSe${}_2$ and TaS${}_2$, is fundamentally novel due to the effects of dimensionality, crystal symmetries, and strong spin-orbit coupling. In this work we perform tunnel spectroscopy on NbSe${}_2$ by utilizing MoS${}_2$ or hexagonal Boron Nitride (hBN) as a tunnel barrier. We observe subgap excitations and probe their origin by studying various heterostructure designs. We show that the edge of NbSe${}_2$ hosts many defect states, which strongly couple to the superconductor and form Andreev bound states. Furthermore, by isolating the NbSe${}_2$ edge we show that the subgap states are ubiquitous in MoS${}_2$ tunnel barriers, but absent in hBN tunnel barriers, suggesting defects in MoS${}_2$ as their origin. Their magnetic nature reveals a singlet or a doublet type ground state and based on nearly vanishing g-factors or avoided-crossing of subgap excitations we highlight the role of strong spin-orbit coupling.

arXiv comment16 pages, 7 figures. Includes Supporting Information

References in corpus (30)

• Majorana Fermions and a Topological Phase Transition in Semiconductor-Superconductor Heterostructures
• Helical liquids and Majorana bound states in quantum wires
• Evidence of Ising pairing in superconducting NbSe${}_2$ atomic layers
• Hopping Transport through Defect-induced Localized States in Molybdenum Disulfide
• Two Dimensional Ising Superconductivity in Gated MoS${}_2$
• Towards Intrinsic Charge Transport in Monolayer Molybdenum Disulfide by Defect and Interface Engineering
• Spin-resolved Andreev levels and parity crossings in hybrid superconductor-semiconductor nanostructures
• Tuning Ising superconductivity with layer and spin-orbit coupling in two-dimensional transition-metal dichalcogenides
• Topological superconductivity in a van der Waals heterostructure
• A Quantum Dot in the Kondo Regime Coupled to Superconductors
• Direct Measurement of the Spin-Orbit Interaction in a Two-Electron InAs Nanowire Quantum Dot
• Josephson and Andreev transport through quantum dots
• Long range coherent magnetic bound states in superconductors
• Self-consistent description of Andreev bound states in Josephson quantum dot devices
• Near-unity Cooper pair splitting efficiency
• Spectral properties of locally correlated electrons in a BCS superconductor
• Enhanced Superconductivity and Suppression of Charge-density Wave Order in 2H-TaS${}_2$ in the Two-dimensional Limit
• Magnetic Field Driven Nodal Topological Superconductivity in Monolayer Transition Metal Dichalcogenides
• Coupled Yu-Shiba-Rusinov states in molecular dimers on NbSe${}_2$
• Unusual suppression of the superconducting energy gap and critical temperature in atomically thin NbSe2
• Spectroscopy of bulk and few-layer superconducting NbSe${}_2$ with van der Waals tunnel junctions
• Evidence for defect-mediated tunneling in hexagonal boron nitride-based junctions
• Electric field tunable superconductor-semiconductor coupling in Majorana nanowires
• Non-local spectroscopy of Andreev bound states
• Robust parity-mixed superconductivity in disordered monolayer transition metal dichalcogenides
• Yu-Shiba-Rusinov states in the charge-density modulated superconductor NbSe2
• Crystalline Nodal Topological Superconductivity and Bogolyubov Fermi Surfaces in Monolayer NbSe${}_2$
• Hybridization of sub-gap states in one-dimensional superconductor/semiconductor Coulomb islands
• Zeeman tunability of Andreev bound states in van-der-Waals tunnel barriers
• Spectroscopy of $NbS{e}_2$ using Energy-Tunable Defect-Embedded Quantum Dots