A groundbreaking discovery in superconductivity is occurring with a material known as platinum-bismuth-two (PtBi2), which is challenging long-held beliefs among physicists. Researchers from the IFW Dresden and the Cluster of Excellence ct.qmat have revealed that, despite its seemingly unremarkable shiny gray appearance, the internal behavior of electrons within PtBi2 is astonishingly unique and unprecedented.
In previous research conducted in 2024, the team found that superconductivity in PtBi2 is restricted to just the top and bottom surfaces of the material, allowing for the pairing of electrons that enables them to flow without any resistance. Their most recent findings, however, uncover even more intriguing phenomena. The mechanism by which these electrons pair is unlike anything previously observed in other superconductors. Additionally, the edges of the superconducting surfaces are home to rare Majorana particles, which are considered potential components for creating reliable quantum bits (qubits) in future quantum computing technologies.
Understanding How PtBi2 Achieves Topological Superconductivity
To grasp the extraordinary properties of PtBi2, we can break down its behavior into three essential components.
Firstly, specific electrons are confined to travel exclusively along the top and bottom surfaces of the crystal. This confinement is a result of a topological characteristic of PtBi2, stemming from the interactions between electrons and the material’s orderly atomic arrangement. Such topological traits are notably robust; they remain unchanged unless the entire material's symmetry is disrupted, either by altering the shape of the crystal or by applying an external electromagnetic field.
What is particularly remarkable about PtBi2 is that the electrons on the top surface are always paired with corresponding electrons on the bottom surface—this holds true regardless of the crystal’s thickness. If you were to slice the crystal in half, the newly exposed surfaces would instantly develop the same surface-bound electrons as before.
A Superconducting Surface with a Conventional Core
The second phase of this phenomenon occurs at low temperatures. Here, the surface-confined electrons begin to form pairs, enabling them to move freely without resistance. In stark contrast, the electrons located within the bulk of the material do not participate in this pairing and continue to act like typical electrons.
This results in a distinctive configuration described by researchers as a 'superconducting sandwich.' The outer surfaces exhibit perfect electrical conductivity, while the interior remains a standard metal. Because the superconductivity is derived from surface electrons that are protected topologically, PtBi2 is classified as a topological superconductor.
Only a handful of materials are believed to exhibit intrinsic topological superconductivity, and until now, none have consistently provided strong experimental support. PtBi2 emerges as one of the most compelling examples in this category.
An Unseen Electron Pairing Configuration
The final aspect of this discovery comes from highly detailed measurements conducted in Dr. Sergey Borisenko's laboratory at the Leibniz Institute for Solid State and Materials Research (IFW Dresden). These experiments demonstrated that not all surface electrons contribute equally to the superconducting phenomenon.
Surprisingly, electrons traveling in six specific, evenly spaced directions along the surface do not engage in any pairing whatsoever. This distinct pattern is a reflection of the three-fold rotational symmetry inherent in the atomic structure of PtBi2's surface.
In traditional superconductors, electrons typically pair regardless of their direction of movement. Some unconventional superconductors, such as the well-known cuprates operating at relatively high temperatures, exhibit directional pairing characterized by four-fold symmetry. However, PtBi2 stands out as the first superconductor identified to have a pairing pattern constrained by a six-fold symmetry.
"This is unprecedented. Not only is PtBi2 a topological superconductor, but the manner in which the electrons pair to enable superconductivity is unlike anything seen in prior superconductors," notes Borisenko. "We are still trying to understand the origins of this pairing."
Crystal Edges That Capture Majorana Particles
The research also affirms that PtBi2 presents a novel and practical avenue for generating Majorana particles, which have been the subject of intense interest in condensed matter physics for some time.
"Our calculations indicate that the topological superconductivity inherent in PtBi2 naturally gives rise to Majorana particles that are confined along the edges of the material. We could even engineer step edges in the crystal to produce as many Majoranas as needed," explains Prof. Jeroen van den Brink, Director of the IFW Institute for Theoretical Solid State Physics and lead investigator of the Würzburg-Dresden Cluster of Excellence ct.qmat.
Majorana particles exist in pairs that collectively function as a single electron, yet individually exhibit fundamentally different behaviors. This concept of effectively splitting an electron is vital to the development of topological quantum computing, which aims to create qubits that are much more resilient against interference and errors.
Harnessing Majoranas for Future Quantum Technologies
With the unusual superconductive properties of PtBi2 and the identification of edge-bound Majorana particles, researchers are now focusing on how to control these phenomena. One strategy involves thinning the material, which could modify the non-superconducting interior, potentially changing it from a conductive metal to an insulator. This transformation could prevent regular electrons from disrupting the Majoranas utilized as qubits.
Another method under investigation is the application of a magnetic field. By adjusting the energy levels of the electrons, a magnetic field could potentially relocate Majorana particles from the edges of the crystal to its corners, opening up new possibilities for utilizing PtBi2 as a foundation for upcoming quantum technologies.
