Lucia Steinke – Low Temperature Experiment Candidate Seminar

Superconductivity at quantum critical points and in proximity with topological boundary states

Collective quantum effects can be enhanced when two quantum phenomena combine in one material: in quantum critical systems, the critical fluctuations of a quantum phase transition at zero temperature can mediate superconducting pairing and lead to enhanced transition temperatures, or even be required to observe superconductivity at all. In the first part of my talk, I will show how we studied this connection between quantum criticality and superconductivity at ultra-low temperatures, and how we solved experimental challenges encountered at these extreme conditions.

In the second half of my talk, I will discuss a potentially new route to topological boundary states, similar to quantum Hall edges, found on the surface of the half-Heusler compound HfNiSn. These states appear to originate from strong electronic correlations, and they form even in the absence of external magnetic fields. Oscillations in the magnetoresistance suggest quantum interference with coherence lengths up to 1 ?m, at only moderately low temperatures up to 80 K. The combination of quantum Hall edges or similar chiral one-dimensional states and superconductors is particularly attractive, as such junctions are expected to host the elusive Majorana fermions that seem a promising choice for future qubits, and the chiral nature of quantum Hall edge states could enable braiding operations between them, providing a possible platform for topological quantum computing. Our first tests of metal deposition on HfNiSn single crystals show promising results, where proximity to superconducting tin or niobium leads to conductance steps close to the quantized value of ~ 0.5 e2/h expected for Majorana fermions. We further observe a clear disruption of quantum interference patterns at the superconducting transition, and magnetoresistance features associated with the critical field that can be traced up to approximately 80 K.