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.
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