Direct observation of the superconducting gap of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$

We report the direct observation of the superconducting gap of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4,$ 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 ${\mathrm{Sr}}_2{\mathrm{RuO}}_4,$ 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 $2{\Delta }_{\max }{/kT}_c$ in the bulk. We attribute this large value to a strong in-plane anisotropy of the gap.