Scientists Find Two Superconducting States Hiding as One

Superconductors -- materials that carry electric current with zero energy loss -- have been studied by physicists for decades, since a detailed understanding of how their electrons behave could eventually help researchers design superconducting materials and devices with greater precision for technologies such as quantum computers, ultra-efficient electronics, and advanced medical systems. Niobium diselenide (NbSe2), one of the best-studied examples, seemed straightforward once reduced to just a few atomic layers: experiments appeared to show a single energy gap, the property that reflects how electrons pair up so they can move without electrical resistance.

But researchers had reason to suspect that this simple picture was incomplete. Using highly sensitive tunneling spectroscopy measurements, a team at the Racah Institute of Physics and the Center for Nanoscience and Nanotechnology at the Hebrew University of Jerusalem found that NbSe2 does not behave like a straightforward superconductor governed by a single order -- instead, it contains two different superconducting orders that interact so strongly they appear as one. The same concealed behavior turned up in a closely related material, tantalum disulfide (TaS2). "It's a bit like listening to what sounds like a single singer, only to discover it's actually a perfectly synchronized duet," the researchers said. The study, led by PhD student Shahar Simon and MSc student Maya Klang under the guidance of Prof. Oded Millo and Prof. Hadar Steinberg, was published in the journal Physical Review Letters.

The two-order picture explains a puzzle that had gone unresolved for years: traditional theories could not fully reproduce the detailed shape of the superconducting energy spectrum seen in earlier experiments, but a model built around two distinct superconducting orders matched the measurements far more accurately -- including how the materials behave when magnetic fields are applied.

The findings also point to even more complexity in thicker samples: bulk NbSe2 may contain three interacting superconducting orders, suggesting that superconductivity in these materials is richer than previously recognized.