Direct observation of the superconducting gap of Sr2RuO4

Abstract

We report the direct observation of the superconducting gap of Sr2RuO4, by means of tunnelling spectroscopy. The tunneling conductance parallel to the c axis was measured using a scanning tunneling microscope operating in a dilution refrigerator. Our results show a suppression in the density of states of Sr2RuO4 which disappears at temperatures higher than Tc or in the presence of a magnetic field larger than the critical field. This demonstrates that superconductivity persists up to the crystal surface and allows us to infer a lower limit of 8.0 for 2Δmax/kTc in the bulk. We attribute this large value to a strong in-plane anisotropy of the gap.