Evidence of a topological gap under EuS nanowires coupled to a superconducting platform with strong spin-orbit coupling

We present our progress on nano point-contact Andreev spectroscopy (AS) measurements in EuS nanowires coupled to an epitaxial superconductor with surface states hosting strong Rashba spin-orbit (SOC) coupling. In this hybrid system, topological superconductivity has been predicted when a Zeeman field drives the system to open up a mini-gap in the sub-gap states formed by the quantum confinement of Bogoliubov quasiparticles. Using highly transparent nano-scale point contacts fabricated at the ends of the EuS nanowires, we develop AS probes that are sensitive and capable of distinguishing any in-gap quasiparticle states from mini-gap openings. In high-resolution AS measurements at milli-Kelvin temperatures, we observe evidence of quasiparticle distributions within the bulk superconducting gap and the opening of additional mini-gap when the Zeeman field enhances. The mini-gap is robust against an external magnetic field as large as several Tesla and demonstrates oscillating behavior. We also observe sensitive dependence of the mini-gap opening corresponding to switching events of magnetic domains. This observation could underline the predicted transition into a state with a topological gap. We will also present our studies of conductance fluctuations as a function of energy within the mini-gap for the search of zero-energy quasiparticles. Our results pave the wave for future correlated measurements on the noise generated by the conductance fluctuations and potentially unveil any non-local quantum phenomena in this system.