PtBi Exhibits Uniform Superconducting Gaps at Atomic Level, Enabling Majorana Zero Modes

Superconductors continue to attract intense research interest due to their potential for hosting exotic quasiparticles with applications in robust quantum computing, and recent evidence suggests the Weyl semimetal PtBi may exhibit intrinsic superconductivity. Xiaochun Huang from the University of Würzburg, Lingxiao Zhao from the Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area, and Sebastian Schimmel from Bergische University Wuppertal, alongside their colleagues, now demonstrate remarkably uniform superconducting gaps within PtBi at the atomic level, resolving previous uncertainties about the material’s superconducting properties. This uniformity allows the team to observe previously hidden low-energy states, and theoretical modelling confirms an unusual anisotropic chiral pairing symmetry responsible for these states. The discovery of a substantial, non-trivial superconducting gap, coupled with easily accessible surface states, positions PtBi as a particularly promising material for both fundamental investigations into superconductivity and the search for elusive Majorana modes.

Majorana Zero Modes and Topological Superconductivity

Topological superconductors represent a compelling pathway towards realising Majorana zero modes, quasiparticles exhibiting unique topological protection and holding significant promise for fault-tolerant quantum computation. Unlike conventional particles, Majorana zero modes are their own antiparticles, a property stemming from their distinct quantum mechanical nature, and this self-annihilation resistance makes them exceptionally stable against decoherence, a major obstacle in building practical quantum computers. Recent research increasingly suggests the possibility of intrinsic topological superconductivity in various materials, sparking intense investigation into their fundamental properties and potential applications. However, definitively confirming the existence of Majorana zero modes remains a substantial challenge, as distinguishing them from other zero-energy states arising from disorder or conventional superconductivity proves difficult, requiring sophisticated experimental techniques and theoretical modelling.

The search for materials exhibiting topological superconductivity encompasses several approaches, including proximity-induced superconductivity, where a conventional superconductor induces superconductivity in a topological insulator, and the exploration of unconventional superconductors possessing inherent topological features. Topological insulators are materials that behave as insulators in their interior but conduct electricity on their surface due to topologically protected surface states, and combining these with conventional superconductors can create the conditions necessary for Majorana mode formation. Achieving robust topological superconductivity at experimentally accessible temperatures and magnetic fields remains a key goal, as many candidate materials require cryogenic conditions or high magnetic fields for observation. Furthermore, understanding how topological superconductivity interacts with other correlated electronic phenomena, such as magnetism and charge density waves, collective electronic behaviours that can significantly alter material properties, is crucial for designing and controlling Majorana-based quantum devices. This work addresses these challenges by investigating a novel material system with strong spin-orbit coupling, an interaction between an electron’s spin and its motion, and unconventional superconducting properties, aiming to establish a clear pathway towards realising and manipulating Majorana zero modes.

Detailed STM Spectroscopy of PtBi2 Superconductivity

This supplemental information provides detailed supporting evidence and methodology for the main findings presented in the primary research paper, demonstrating the robustness of observed phenomena and ensuring transparency for other researchers. Scanning tunnelling microscopy (STM) spectroscopy, the primary technique employed, allows for the probing of the electronic structure of materials at the atomic scale, revealing details about the density of states and the presence of superconducting gaps. The data analysis procedures, including the fitting of experimental spectra to theoretical models, are meticulously described, enabling independent verification of the results. This detailed presentation of methodology is crucial for reproducibility and fosters collaboration within the scientific community. The content is organised around a series of measurements and analyses, each contributing to a comprehensive understanding of the superconductivity in PtBi2, a bismuth-platinum alloy exhibiting promising superconducting characteristics.

Measurements demonstrate the spatial homogeneity of the superconducting gap across hundreds of nanometers on the PtBi2 surface, confirming that superconductivity is a bulk property of the material, rather than a surface effect. High-resolution STM imaging, combined with spatially resolved spectroscopy, reinforces this homogeneity at the atomic scale, revealing a consistent superconducting gap across the measured region. Overlaid spectra from multiple measurements consistently show the emergence of negative differential conductance, a characteristic signature of Andreev bound states, states formed at the interface between a superconductor and a normal metal, providing visual evidence of the reproducibility of the findings. Extended measurements of in-gap states, states appearing within the superconducting gap, confirm their consistent presence across the measured region, suggesting the potential for topological protection. Detailed theoretical modelling, incorporating the superconducting density of states, the number of available electronic states at a given energy, and the behaviour of in-gap states, accurately reproduces the experimental data, supporting the conclusion that the superconductivity in PtBi2 is chiral and anisotropic, a rare and exotic state of matter where the superconducting order parameter has a specific spatial orientation. This comprehensive analysis provides strong evidence for the robustness, spatial homogeneity, and unique characteristics of the superconductivity in PtBi2, offering a detailed understanding of the underlying physics and providing a solid foundation for further investigation into its potential for hosting Majorana zero modes.

Theoretical Considerations and Future Directions

The observed characteristics of PtBi2 superconductivity strongly suggest the presence of unconventional pairing mechanisms, deviating from the standard Bardeen-Cooper-Schrieffer (BCS) theory that describes conventional superconductivity. BCS theory relies on phonon-mediated electron pairing, but the observed anisotropy and chiral nature of the superconducting order parameter in PtBi2 indicate a more complex pairing scenario, potentially involving spin fluctuations or other electronic correlations. Further theoretical modelling, incorporating these factors, is crucial for a complete understanding of the pairing mechanism and its implications for topological superconductivity. Investigating the influence of external parameters, such as magnetic fields and pressure, on the superconducting properties of PtBi2 could reveal further insights into the underlying physics and potentially enhance the topological protection of Majorana zero modes.

Future research directions include the fabrication of nanoscale devices based on PtBi2, such as nanowires or heterostructures, to explore the possibility of inducing topological superconductivity and manipulating Majorana zero modes. Probing the edge states of these devices using advanced spectroscopic techniques could provide direct evidence for the existence of Majorana modes and their topological protection. Furthermore, exploring the potential of PtBi2 as a platform for building fault-tolerant quantum circuits requires careful consideration of decoherence mechanisms and the development of strategies for mitigating their effects. Combining theoretical modelling with experimental investigations will be essential for advancing our understanding of topological superconductivity and realising the full potential of Majorana zero modes for quantum computation and other advanced technologies.