Energy- and momentum-resolved Cooper-pair emission from a cuprate superconductor

Abstract

Superconductivity arises from Cooper pairing, but conventional photoemission probes electrons individually rather than as pairs. Here we report the observation of Cooper-pair emission from an optimally doped cuprate superconductor using two-electron angle-resolved photoemission (2e-ARPES). Below the superconducting transition, we observe a distinct two-electron emission peak at zero pair binding energy and zero pair momentum that disappears in the normal state, revealing the characteristic fingerprint of a Cooper pair. Unexpectedly, the joint energy distribution broadens markedly along the relative-energy axis as temperature decreases, while the total energy width remains nearly unchanged. This anomalous behavior cannot be explained by instrumental resolution, static disorder or thermal broadening, and instead points to scattering processes involving collective excitations of the superconductor. Our results establish 2e-ARPES as a probe of both Cooper-pair emission and the many-body dynamics of the superconducting state.