Exploring topological superconductivity in PtBi₂ with hybrid circuits
Topological superconductors are materials in which the superconducting state is itself topologically nontrivial. They are fascinating from a fundamental perspective and may also host Majorana states that could be useful for protected quantum devices. Despite extensive efforts, clear experimental examples remain rare. One central difficulty is that the relevant superconductivity may be confined to a material’s surface or edge, while the bulk remains normal or develops a different superconducting state. Distinguishing these contributions is particularly challenging using conventional transport measurements.
A promising material in this context is trigonal PtBi₂, a layered van der Waals Weyl semimetal. Photoemission experiments have reported superconducting Fermi arcs on its surface indicating nodal superconducting order [1,2], while recent tunneling measurements found distinct bulk and surface gaps [3,4]. PtBi₂ therefore offers an unusual opportunity to explore the coexistence of surface and bulk superconductivity, how these states are related, and whether the surface superconductivity is topological.
In this project, we will integrate exfoliated PtBi₂ flakes into superconducting microwave resonators. Changes in the resonator frequency and loss provide a contactless probe of the material's superfluid response and dissipation. By comparing devices with different flake thicknesses and circuit geometries, and measuring their response as a function of temperature, magnetic field, and microwave power, we will investigate the distinct electrodynamic signatures of the various superconducting states.
Our group recently demonstrated this approach by resolving an unconventional gap structure in flakes of the kagome superconductor CsV₃Sb₅ [5]. We have also fabricated a first PtBi₂ hybrid device, which shows a clear change in the resonance below about 0.3 K, consistent with bulk superconductivity, and a weaker feature that persists to higher temperature. These preliminary observations provide the starting point for the PhD project. The student will develop new device geometries and systematic measurements to determine the origin of the two regimes.
The project is a collaboration between the Quantum Information for Quantum Materials (QIQM) group at the MPI-CPfS and the Institute for Solid State Research (IFF) at IFW Dresden, combining expertise in hybrid superconducting circuits and topological materials. The student will gain experience in microwave-circuit design, nanofabrication, van der Waals assembly, dilution-refrigerator measurements, microwave spectroscopy, and superconducting electrodynamics.
