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The controlled tunability of superconductivity in low-dimensional materials may enable new quantum devices. Particularly in triplet or topological superconductors, tunneling devices such as Josephson junctions, etc., can demonstrate exotic functionalities. The tunnel barrier, an insulating or normal material layer separating two superconductors, is a key component for the junctions. Thin layers of NbSe2 have been shown as a superconductor with strong spin orbit coupling, which can give rise to topological superconductivity if driven by a large magnetic exchange field. Here we demonstrate the superconductor−insulator transitions in epitaxially grown few-layer NbSe2 with wafer-scale uniformity on insulating substrates. We provide the electrical transport, Raman spectroscopy, cross-sectional transmission electron microscopy, and X-ray diffraction characterizations of the insulating phase. We show that the superconductor−insulator transition is driven by strain, which also causes characteristic energy shifts of the Raman modes. Our observation paves the way for high-quality heterojunction tunnel barriers to be seamlessly built into epitaxial NbSe2 itself, thereby enabling highly scalable tunneling devices for superconductor-based quantum electronics.
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This content will become publicly available on September 1, 2026
Andreev bound states in magnetic topological insulator Josephson junctions
We study Andreev bound states in the presence of a magnetic moment in a ferromagnetic topological insulator in superconductor/magnetic topological insulator/superconductor Josephson junctions. We analytically find zero energy states for out-of-plane and in-plane directions of the magnetic moment. In the case of the out-of-plane magnetic moment, the energy is independent of the scattering angle. If both magnetic and nonmagnetic scattering mechanisms are considered, the zero energy state requires the scattering angle to the electrode to be zero as in the case of Majorana fermions. In the presence of an in-plane magnetic moment, the energy band always exhibits a nonvanishing gap if the magnetic moment has a nonzero component, i.e., there are no zero energy states. Here we assume that the electrons tunnel in the direction. If the magnetic moment is aligned with the tunneling direction, the zero energy states always exist and are independent of the scattering angle. Contrary to the Majorana fermion case, the phase shift between two superconductor electrodes is not. This phase difference depends on the system parameters such as the Fermi velocity, the barrier potential magnitude, the exchange coupling between localized and delocalized electrons, and the component of the magnetic moment. We find an anomalous Josepheson current when the magnetic moment has a component in the direction, where the current is nonzero despite. This is due to the violation of time reversal and chiral symmetries in the Josepheson junction. This leads to the observation of the Josephson Diode effect as well. For large scattering magnitudes, we find that the transmission coefficient approaches one at larger barrier magnitudes. This is the main reason why in superconductor/magnetic topological insulator/superconductor Josephson junctions critical current is much higher than in superconductor/normal metal/superconductor junctions. This effect is similar in origin to Klein Tunneling for relativistic Dirac electrons. In the case of nonmagnetic and out-of-plane magnetic scatterings, the current vanishes when the barrier amplitudes are approximately equal and large. This effect cannot be explained by the relativistic nature of the Dirac equation and is specific to the model. We also study temperature dependencies for in- and out-of-plane magnetic moments. We find that current at high temperatures is significantly smaller than at low temperatures. The current approaches a constant value at low temperatures, at approximately. This value depends on the other system parameters. The existence of new zero energy states in magnetic topological insulators in superconductor/magnetic topological insulator/superconductor Josephson junctions opens new opportunities in quantum computing because of the presence of the additional symmetry with respect to the scattering angle.
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- Award ID(s):
- 2228841
- PAR ID:
- 10628387
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Physica B: Condensed Matter
- Volume:
- 713
- Issue:
- C
- ISSN:
- 0921-4526
- Page Range / eLocation ID:
- 417296
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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