New experiments are reshaping our understanding of ultrathin superconductors—materials that can conduct electricity without resistance. For years, two layered compounds, niobium diselenide (NbSe₂) and tantalum disulfide (TaS₂), were widely modeled as “single-gap” superconductors when reduced to just a few atomic layers. That picture implied a simple underlying order parameter describing how electrons pair up.
A new study from the Hebrew University of Jerusalem challenges that assumption. Using ultra-sensitive tunneling spectroscopy, researchers probed the superconducting energy spectrum with high resolution, searching for subtle deviations from the expected single-gap behavior. The measurements revealed features that could not be reconciled with conventional single-order descriptions.
Instead, the data point to an unexpected mechanism: the materials host two superconducting states that are strongly interacting. Rather than acting independently, these two orders become coupled so effectively that their combined signatures mimic what looks like one clean superconducting gap. In other words, the system “disguises” its complexity, producing an experimental fingerprint similar to a simpler superconductor.
The team applied a more sophisticated two-band superconductivity framework to match both the detailed spectral line shape and how the superconducting state evolves in the presence of magnetic fields. This modeling clarified why earlier theories struggled to reproduce the full form of the energy spectrum, even when they captured some aspects of the response.
The results were consistent across both compounds, suggesting the phenomenon is not an isolated anomaly of NbSe₂. In TaS₂, the same hidden two-order structure emerges, implying a broader principle for superconductivity in certain layered transition-metal dichalcogenides.
The study also raises a tantalizing implication for the bulk form of NbSe₂. While the thin samples show two coupled superconducting orders, the thicker material may contain three interacting superconducting orders, implying an even richer hierarchy of electronic states beyond the minimal two-gap picture.
Beyond resolving a long-standing puzzle, the work provides a practical guide for interpreting superconducting spectra in materials where multiple orders can masquerade as one. This matters for the design of future superconducting devices, especially in regimes where precise control of electronic behavior is crucial.
As superconductivity moves toward applications in quantum technologies and ultra-efficient electronics, the ability to accurately identify hidden order parameters could improve material engineering. Understanding what electrons actually “do” inside these compounds may be the next step toward building superconductors with predictable, tunable performance.
Subject of Research:
Article Title: Two-Band Superconductivity in Few-Layer NbSe₂ and TaS₂
News Publication Date:
Web References: http://dx.doi.org/10.1103/p836-tdgw
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Image Credits: Avigail Ben Eliyahu
Keywords: superconductivity, quantum matter, condensed matter physics, superconducting energy gap, spectroscopy, tunneling spectroscopy, NbSe₂, TaS₂, two-band superconductivity, quantum computing
