Surface-only superconductor is the strangest of its kind

(Press-News.org) Something strange goes on inside the material platinum-bismuth-two (PtBi₂). A new study by researchers at IFW Dresden and the Cluster of Excellence ct.qmat demonstrates that while PtBi₂ may look like a typical shiny grey crystal, electrons moving through it do some things never seen before.

In 2024, the research team demonstrated that the top and bottom surfaces of the material superconduct, meaning electrons pair up and move without resistance. Now, they reveal that this pairing works differently from any superconductor we have seen before. Enticingly, the edges around the superconducting surfaces hold long-sought-after Majorana particles, which may be used as fault-tolerant quantum bits (qubits) in quantum computers.

Three steps to a unique topological superconductor

We can break down PtBi₂’s strange superconductivity in three steps.

First, some electrons are confined to the top and bottom surfaces of the material. This is a so-called ‘topological’ property of PtBi₂, caused by interactions between the electrons and the neatly arranged atoms in the crystalline material. Importantly, topological properties are robust: they can’t change unless you change the symmetry of the whole material, either by changing the entire crystal structure, or by applying an electromagnetic field.

In PtBi₂, the electrons confined to the top surface are complemented by those bound to the bottom surface, no matter how many layers of atoms lie between these surfaces. And if you would cut the crystal in half, the new top and bottom surfaces automatically also host complementary electrons confined to the surface.

Second, these surface-bound electrons pair up at low temperatures, allowing them to move without any resistance. The rest of the electrons don’t pair up, and keep behaving like normal electrons do. This makes PtBi₂ a natural superconductor sandwich, with superconducting top and bottom surfaces and a normal metallic interior.

The topological nature of the surface electrons make PtBi₂ a topological superconductor. There are only a handful of other candidate materials thought to have intrinsic topological superconductivity, and to date none of these is supported by convincingly consistent or conclusive experimental evidence.

Finally, new uniquely high-resolution measurements from Dr. Sergey Borisenko’s lab at the Leibniz Institute for Solid State and Materials Research (IFW Dresden) reveal that not all surface-bound electrons pair up equally. Remarkably, surface electrons moving along six symmetrical directions resolutely refuse to pair up. These directions reflect the three-fold rotation symmetry of how the atoms are arranged in the material’s surface.

In normal superconductors, all electrons pair up regardless of what direction they move in. Some unconventional superconductors, like the cuprate materials famous for becoming superconducting at higher temperatures, have a more restricted pairing with a four-fold rotation symmetry. PtBi₂ is the first superconductor showing restricted pairing with a six-fold rotation symmetry.

“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.”

Edges trap elusive Majorana particles

The new study also confirms that PtBi₂ offers a new way to produce long-sought-after Majorana particles.

“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,” notes 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.

A pair of Majorana particles acts as a single electron, but individually they behave very differently. This concept of ‘split electrons’ is the foundation for topological quantum computing, which aims to build more stable qubits. The separation of Majorana particle pairs protects them against noise and errors.

Now that PtBi₂’s unique superconductivity and related Majorana particles have been found, a next step is to control them. For example, thinning the material down will change the non-superconducting ‘sandwich filling’, potentially turning it from a conducting metal into an insulator. This also means that the non-superconducting electrons cannot interfere with the use of the Majoranas as qubits. Alternatively, applying a magnetic field will shift the electron energy levels, and could, for example, cause the Majorana particles to move from the edges to the corners of the material.

Publication

Topological nodal i-wave superconductivity in PtBi₂. S. Changdar, O. Suvorov, A. Kuibarov, S. Thirupathaiah, G. Shipunov, S. Aswartham, S. Wurmehl, I. Kovalchuk, K. Koepernik, C. Timm, B. Büchner, I. Cosma Fulga, S. Borisenko, J. van den Brink. Nature (2025), DOI: 10.1038/s41586-025-09712-6 (LINK: https://www.nature.com/articles/s41586-025-09712-6) (arXiv)

Leibniz Institute for Solid State and Materials Research Dresden

The Leibniz Institute for Solid State and Materials Research Dresden (IFW Dresden) is an independent, non-university research institute and member of the Leibniz Association. Around 500 employees from more than 35 nations investigate the physics and chemistry of solids and materials to develop new functionalities for quantum materials, 2D materials and technologies for energy applications. In five institutes, an interdisciplinary team from experimental physics, theoretical solid-state physics, chemistry, materials research and electrical engineering links basic research with application-oriented work.

Cluster of Excellence ct.qmat

The Cluster of Excellence ct.qmat – Complexity and Topology in Quantum Matter has been jointly run by Julius-Maximilians-Universität Würzburg and Technische Universität Dresden since 2019. Nearly 400 scientists from more than 30 countries and from four continents study topological quantum materials that reveal surprising phenomena under extreme conditions such as ultra-low temperatures, high pressure, or strong magnetic fields. ct.qmat is funded through the German Excellence Strategy of the Federal and State Governments and is the only Cluster of Excellence to be based in two different federal states.

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