Observation of superconductivity-induced leading-edge gap in Sr-doped ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ thin films

Abstract

The discovery of high-temperature superconductivity in pressurized bulk ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ has ignited significant interest in nickelate superconductors. Unlike cuprates, where superconductivity predominantly originates from the $3d_{x^2-y^2}$ orbital, nickelates exhibit additional complexities involving contributions from the $3d_{z^2}$ orbital, prompting fundamental questions about their pairing mechanisms. Despite recent progress in stabilizing superconductivity in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ thin films at ambient pressure, direct spectroscopic evidence of the superconducting gap opening remains elusive. Here, we present an in-situ angle-resolved photoemission spectroscopy study of Sr-doped superconducting ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ thin films. Fermi surface mapping reveals Ni-$3d_{x^2-y^2}$-derived $\alpha$ and $\beta$ pockets, with orbital fillings of 0.11$\pm$0.02 electrons and 0.66$\pm$0.03 holes per Ni, respectively, resulting in a total of 0.45$\pm$0.04 electrons for each Ni. These bands exhibit moderate electron correlations, characterized by a band renormalization factor of 3-4. Notably, both $\alpha$ and $\beta$ bands exhibit leading-edge shifts across the superconducting transition, with gap magnitude of ~1-2 meV at Fermi momenta along the Brillouin zone diagonal and slightly away from the zone diagonal, deviating from the conventional $d_{x^2-y^2}$-wave gap structure. Additionally, the Ni-$3d_{z^2}$-derived $\gamma$ band lies ~75 meV below the Fermi level, indicating a $3d_{x^2-y^2}$-dominated fermiology in this compound.

18 pages and 5 figures