Topological Actinide Superconductivity (TOPAS)

• Applicant: Toni Helm, Ph.D.
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 524386141

Topological superconductors provide a potential route to future quantum technologies. In this project, we intend to reveal signatures of the topological superconductivity in candidate actinide compounds. Most of these compounds possess intriguing phase diagrams with multiple superconducting phases and high-field phase transitions. As we discuss in the introductory part these compounds are among the most likely candidates for spin-triplet superconductivity. To date unambiguous evidence for topological superconductivity in bulk materials is still lacking. We plan to conduct electrical- and thermal-transport, as well as torque-magnetometry experiments with precision-cut samples. One of the key assets in this project is the microfabrication by means of focused-ion-beam assisted cutting. This approach offers the potential to scale dimensions down to the sub-micron regime with nanometer precision. In doing so, we expect to significantly improve signal-to-noise ratios. It furthermore enables us to tailor microstructures with features particularly suitable for our proposed experiments. Recent improvements in crystal growth quality in combination with improved absolute detection signals will help us to study the high-field phase diagrams and potentially quantum oscillations for these compounds. Our goal here is to learn more about their electronic band structure and its relation to the superconducting ground states. We furthermore plan to study the thermal conductivity and thermal Hall effect in order to reveal signatures of topological surface states in these compounds. Thermal excitations in superconductors are characterized by an energy gap. Therefore, topological properties will depend on the superconducting order-parameter symmetries. Topological properties of the elementary energy excitations in the superconductor can enforce the presence of gapless (zero energy) modes at the surface. However, the presence of the condensate of Cooper pairs prevents the system from being a bulk electrical insulator, whereas it contributes nothing to its thermal properties. All fully gaped superconductors can be considered as thermal insulators at temperatures far below TSC. Therefore, thermal transport is sensitive to topological surface states. Hence, a large part of the project will be devoted to thermal transport experiments. FIB shaping of the crystals will be of vital importance, as it enables optimal surface-to-volume ratios to study the influence of surface excitations at very low temperatures. We also want to develop a thermal-Hall-effect probe in order to hunt for topological signatures recently predicted by theory. If successful, our thermal experiments in combination with micron-size samples will open new paths for the investigation of topological superconductors.

• DFG Programme: Research Grants
• International Connection: France, Japan
• Cooperation Partners: Jean-Pascal Brison, Ph.D.; Professor Motoi Kimata, Ph.D.