A new superconductor breaks rules physicists thought were fixed
A shiny gray crystal called platinum-bismuth-two hides an electronic world unlike anything scientists have seen before. Researchers discovered that only the crystal’s outer surfaces become superconducting—allowing electrons to flow with zero resistance—while the interior remains ordinary metal. Even stranger, the electrons on the surface pair up in a highly unusual pattern that breaks all known rules of superconductivity.
Something unexpected is happening inside a material called platinum-bismuth-two (PtBi2). According to a new study from researchers at IFW Dresden and the Cluster of Excellence ct.qmat, this shiny gray crystal may look ordinary, but the electrons inside it behave in ways scientists have never observed before.
In earlier work published in 2024, the team showed that only the top and bottom surfaces of PtBi2 become superconducting, meaning electrons can pair up and flow without resistance. Their latest results reveal something even more surprising. The way these electrons pair is unlike any known superconductor. Even more intriguing, the edges surrounding these superconducting surfaces naturally host elusive Majorana particles, which are considered promising building blocks for fault-tolerant quantum bits (qubits) in future quantum computers.
How PtBi2 Becomes a Topological Superconductor
The unusual behavior of PtBi2 can be understood by breaking it into three key steps.
To begin with, certain electrons are confined strictly to the top and bottom surfaces of the crystal. This happens because of a topological property of PtBi2 that arises from how electrons interact with the material's orderly atomic structure. Topological properties are remarkably stable. They do not change unless the symmetry of the entire material is altered, either by reshaping the crystal itself or by applying an electromagnetic field.
What makes PtBi2 especially striking is that the electrons bound to the top surface are always matched by corresponding electrons on the bottom surface, regardless of how thick the crystal is. If the crystal were sliced in half, the newly exposed surfaces would immediately develop the same surface-bound electrons.
A Superconducting Surface With a Normal Interior
The second step occurs at low temperatures. The electrons confined to the surfaces begin to pair up, allowing them to move without resistance. Meanwhile, electrons inside the bulk of the material do not join this pairing and continue to behave like ordinary electrons.
This creates an unusual structure that researchers describe as a natural superconductor sandwich. The outer surfaces conduct electricity perfectly, while the interior remains a normal metal. Because the superconductivity comes from topologically protected surface electrons, PtBi2 qualifies as a topological superconductor.
Only a small number of materials are believed to host intrinsic topological superconductivity. So far, none of those candidates has been backed by consistently strong experimental evidence. PtBi2 now stands out as one of the most convincing examples yet.
A Never-Before-Seen Pattern of Electron Pairing
The final piece of the puzzle comes from exceptionally high-resolution measurements performed in Dr. Sergey Borisenko's lab at the Leibniz Institute for Solid State and Materials Research (IFW Dresden). These experiments showed that not all surface electrons participate equally in superconductivity.