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.
Electrons moving in six specific, evenly spaced directions on the surface refuse to pair up at all. This unusual pattern reflects the three-fold rotational symmetry of how atoms are arranged on the surface of PtBi2.
In conventional superconductors, electrons pair regardless of the direction in which they travel. Some unconventional superconductors, including the well-known cuprates that operate at relatively high temperatures, show directional pairing with four-fold symmetry. PtBi2 is the first known superconductor where pairing is restricted in a six-fold symmetric pattern.
“We have never seen this before. Not only is PtBi2 a topological superconductor, but the electron pairing that drives this superconductivity is different from all other superconductors we know of,” says Borisenko. “We don’t yet understand how this pairing comes about.”
Crystal Edges That Trap Majorana Particles
The study also confirms that PtBi2 provides a new and practical route to producing Majorana particles, which have long been sought in condensed matter physics.
“Our computations demonstrate that the topological superconductivity in PtBi2 automatically creates Majorana particles that are trapped along the edges of the material. In practice, we could artificially make step edges in the crystal, to create as many Majoranas as we want,” explains Prof. Jeroen van den Brink, Director of the IFW Institute for Theoretical Solid State Physics and principal investigator of the Würzburg-Dresden Cluster of Excellence ct.qmat.
Majorana particles come in pairs that together behave like a single electron, but individually act in fundamentally different ways. This idea of effectively splitting an electron is central to topological quantum computing, an approach designed to create qubits that are far more resistant to noise and errors.
Controlling Majoranas for Future Quantum Devices
With PtBi2‘s unusual superconductivity and edge-bound Majorana particles now identified, researchers are turning their attention to controlling these effects. One strategy involves thinning the material, which would alter the non-superconducting interior. This could transform it from a conducting metal into an insulator, preventing ordinary electrons from interfering with the Majoranas used as qubits.
Another approach involves applying a magnetic field. By shifting the energy levels of the electrons, a magnetic field could potentially move Majorana particles from the edges of the crystal to its corners. These capabilities would represent important steps toward using PtBi2 as a platform for future quantum technologies.
