Origin of Subgap States in Normal-Insulator-Superconductor van der Waals Heterostructures

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Materials

Background:

• Superconductivity in van der Waals (vdW) materials exhibits unique properties influenced by dimensionality, crystal symmetries, and spin-orbit coupling.
• Investigating novel superconducting phenomena in 2D materials is crucial for advancing quantum technologies.

Purpose of the Study:

• To investigate the origin of subgap excitations in niobium diselenide (NbSe2) superconductor using tunnel spectroscopy.
• To explore the role of defects and material interfaces in the superconducting properties of vdW heterostructures.

Main Methods:

• Fabrication of NbSe2-based heterostructures utilizing molybdenum disulfide (MoS2) or hexagonal boron nitride (hBN) as tunnel barriers.
• Performing low-temperature tunnel spectroscopy to probe electronic states within the superconductor.
• Systematic variation of heterostructure designs to isolate and identify the source of observed phenomena.

Main Results:

• Observation of subgap excitations in NbSe2, attributed to defect states at the material's edge.
• These edge defect states couple strongly to the superconductor, forming Andreev bound states.
• Subgap states were prevalent with MoS2 barriers but absent with hBN barriers, indicating MoS2 defects as the likely origin.
• Magnetic properties of these states suggest singlet or doublet ground states, influenced by strong spin-orbit coupling.

Conclusions:

• Edge defect states in NbSe2 play a significant role in forming Andreev bound states, impacting superconducting properties.
• The choice of tunnel barrier material (MoS2 vs. hBN) is critical, with MoS2 defects being a primary source of observed subgap excitations.
• The study highlights the interplay of defects, spin-orbit coupling, and superconductivity in 2D vdW materials.