Topological surface superconductivity in the Weyl semimetal t-PtBi2

• Applicants: Professor Dr. Bernd Büchner; Professor Dr. Christian Hemker-Heß
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 566479091

Topological superconductivity implies a plethora of novel physical phenomena. In particular, at surfaces and at edges exotic states are expected. This includes so-called Majorana fermions, which are considered particularly relevant for new concepts in quantum computing. Topological superconductivity is rare, and previous experiments typically concern complex materials systems at sub-Kelvin temperatures. In this project, we want to build upon recent experimental results for the Weyl semimetal t-PtBi2 (t=trigonal), which provide evidence for an unusual surface superconductivity at temperatures significantly above 5 K as well as for Weyl topology. t-PtBi2 is therefore a promising candidate for intrinsic topological superconductivity and, furthermore, for topological superconductivity at temperatures higher than that of previous considered systems. Yet, many aspects of the surface superconductivity and also of the Weyl fermiology remain unresolved. Hence, it is the goal of this project to clarify the nature of the superconductivity and of the topological electrons in t-PtBi2, and to rationalize their interplay upon doping. We expect that thereby the presence of intrinsic topological superconductivity in t-PtBi2 can ultimately be resolved. In our proposed work, we want to synthesize single crystals of pristine and doped t-PtBi2, and we want to investigate the superconducting and topological properties of its electrons. Methodically, after general characterization of the crystals (e.g. by electrical transport, magnetic susceptibility, specific heat measurements), a particular focus will be on Nernst effect and scanning tunneling microscopy/spectroscopy (STM/STS) measurements. Both probes feature sensitivity on superconducting as well as topological properties of the electrons in the bulk and at the surface, respectively. More specifically, we want to use the Nernst effect to detect superconducting fluctuations, and through identifying the anomalous Nernst effect (ANE) to probe the Weyl fermiology of the samples. STM/STS is planned to be used to measure the local density of states at the surface of t-PtBi2. We thus expect, on the one hand, information on the superconducting gap, including the possible signature of Majorana modes within the gap. On the other hand, we want to scrutinize the quasiparticle interference of the surface electrons in order to detect and to better understand the topological properties of the electrons, in particular those of the Fermi arcs. The doping experiments primarily aim at manipulating the crystal structure such that the broken inversion symmetry and thereby the Weyl fermiology vanish with concomitant enhancement of bulk superconductivity. We expect thereby pertinent insights into the interplay of superconductivity and topology of the electrons.

• DFG Programme: Research Grants