Hallmark Signatures of Electronic Pairing in Two-Photon Two-Electron Coincidence Angle-Resolved Photoemission Spectroscopy

Abstract

Understanding strongly correlated quantum materials remains a central challenge in condensed matter physics and materials science. While angle-resolved photoemission spectroscopy (ARPES) has become an indispensable probe of single-quasiparticle excitations, it accesses electronic correlations only indirectly. Here we show that unlike one-photon in, two-electrons out coincidence ARPES ($\gamma\!\rightarrow\!2e$ 2eARPES), the two-photon in, two-electron out $2\gamma\!\rightarrow\!2e$ 2eARPES provides a direct and unambiguous probe of electronic pairing. We establish this on general theoretical grounds and substantiate it through large-scale numerical simulations of strongly correlated models with both paired and unpaired ground states. The key result is a model-independent separation in the $(\omega_1,\omega_2)$ plane of the two photoelectrons' energies, between signal from electrons emitted from the \emph{same} pair and signal from electrons emitted from \emph{different} pairs; this follows from energy conservation alone and is independent of any material-specific assumptions. Our findings demonstrate that $2\gamma\!\rightarrow\!2e$ 2eARPES can identify pairing and extract the pair binding energy as well as the energy of the 'glue' boson without any sophisticated data analysis or complementary measurements.