Direct observation of the superconducting gap of Sr₂RuO₄

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 T_c 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 2Δ_max/kT_c in the bulk. We attribute this large value to a strong in-plane anisotropy of the gap.