Two strongly coupled superconducting states can disguise themselves as a single energy gap in ultrathin materials.
Two ultrathin superconductors appeared to have a simple internal structure, but closer measurements revealed something more complex. Instead of relying on one superconducting state, each material contains two strongly coupled states whose combined behavior makes them look like a single one.
The finding resolves a persistent mystery surrounding these materials and provides a more accurate picture of how their superconductivity works. That understanding could eventually help researchers develop improved materials for quantum computers, ultra-efficient electronics, advanced sensors, and other superconducting technologies.
Superconductors can carry electrical current without losing energy, making them promising for a wide range of future devices. Physicists have studied these materials for decades, but even familiar examples can conceal unexpected behavior.
Niobium diselenide (NbSe₂) is among the most extensively investigated superconductors. When reduced to only a few atomic layers, experiments appeared to show that it had one superconducting energy gap, a defining feature that reflects how electrons pair together and move without electrical resistance.
Two hidden states appear as one
The apparent simplicity raised questions that existing theories could not fully answer. Shahar Simon, a Ph.D. student, and Maya Klang, an M.Sc. student, led the study under the guidance of Prof. Oded Millo and Prof. Hadar Steinberg of the Racah Institute of Physics and the Center for Nanoscience and Nanotechnology at the Hebrew University of Jerusalem. The findings were published in Physical Review Letters.
Using highly sensitive tunneling spectroscopy, which probes the electronic states inside a material, the researchers found that ultrathin NbSe₂ does not behave like a superconductor governed by a single order. It instead contains two distinct superconducting orders that interact so strongly that measurements make them appear to be one.
The researchers observed the same concealed behavior in TaS₂, a closely related superconducting material.
“It’s a bit like listening to what sounds like a single singer, only to discover it’s actually a perfectly synchronized duet,” said the researchers.
A richer model solves the puzzle
Traditional theories had been unable to reproduce the precise shape of the materials’ superconducting energy spectrum. The researchers instead used a more advanced model that included two separate but strongly interacting superconducting orders.
That model accurately accounted for both the spectroscopy results and the way the materials changed when exposed to magnetic fields. The agreement helped explain why earlier experiments had produced measurements that looked simple even though the underlying superconductivity was not.
Bulk material may hide three states
The results also indicate that thicker, bulk NbSe₂ may contain three interacting superconducting orders rather than two. This suggests that the material’s superconductivity becomes even more intricate beyond the ultrathin form.
Recognizing this hidden structure could give scientists greater control when designing superconducting materials and devices. As research advances toward quantum computers and ultra-efficient electronics, accurately understanding how electrons organize and interact inside these materials will be essential.
Reference: “Two-Band Superconductivity in Few-Layer NbSe2 and TaS2” by Shahar Simon, Maya Klang, Oded Millo and Hadar Steinberg, 29 June 2026, Physical Review Letters.
DOI: 10.1103/p836-tdgw
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