IQIM Postdoctoral and Graduate Student Seminar

Abstract: Josephson junctions in semiconductors with strong spin-orbit interaction are a promising platform for engineering topological superconductivity. Proposals to achieve this include proximitizing the quantum (spin) hall edge state of a material, and reaching a topological regime by creating a junction with a superconducting phase difference of π. First, we study the edge states using superconducting quantum interference measurements of Josephson junctions in InAs quantum wells and InSb flakes. In the trivial regime, we observe a SQUID pattern already, demonstrating superconducting edge transport through trivial edge states. More strikingly, we find SQUID oscillations with a flux periodicity of h/e (not h/2e), which is considered a hallmark of the topological regime.  We argue that crossed Andreev states gives rise to the h/e periodicity in the trivial regime and confirm this by modelling our junctions using tight binding calculations. In a second series of experiments we investigate inducing a π-junction. This can be done via the Zeeman effect by applying a magnetic field, or by controlling superconducting phase bias across the junction, as well as a combination of both. We do not find evidence for a 0-π transition in DC SQUID devices made in InAs quantum wells, because the Zeeman energy is too low compared to the Thouless energy. However, in single Josephson junctions in InSb quantum wells, a Zeeman induced 0-π transition is observed. The transition is tunable with both the Zeeman energy via the magnetic field, and Thouless energy via the electron density. This observation paves the way for engineering topological superconductivity in the InSb quantum wells.