Probing the energy gap of high-temperature cuprate superconductors by resonant inelastic x-ray scattering

The determination of the symmetry of the energy gap is crucial for research on the microscopic mechanisms of unconventional superconductivity. Here, we demonstrate experimentally that high-resolution resonant inelastic X-ray scattering at the Cu L 3 edge can serve as a momentum-resolved, bulk-sensitive probe of the superconducting gap. We studied two optimally doped cuprates Bi 2 Sr 2 CaCu 2 O 8+ δ and Bi 2 Sr 2 Ca 2 Cu 3 O 10+ δ , in which we observe a strongly momentum dependent reduction of the spectral weight upon entering the superconducting state, with a maximum for momenta connecting antinodal regions of the Fermi surface. Based on a comparison with the calculated charge susceptibility and electronic Raman scattering data, we interpret our observation as a renormalization of the non-local charge susceptibility due to the superconducting gap opening. Our data demonstrate the methodological potential of resonant inelastic X-ray scattering as a versatile probe of the energy gap of high-temperature superconductors, including buried interfaces in heterostructures which are inaccessible to angle-resolved photoemission spectroscopy.