Measuring the Debye Energy in Superconductors via two Electron Photoemission Spectroscopy

We demonstrate theoretically that double angle resolved photoemission spectroscopy (2eARPES) can directly probe the existence of Cooper pairs away from the Fermi surface, and can thus provide insight into the characteristic energy scale around the Fermi surface, the Debye energy, in which electrons are bound into Cooper pairs. To this end, we compute the photoelectron counting rate $P^{(2)}$ in two different types of unconventional superconductors, a $d_{x^2-y^2}$-wave superconductor, and a topological superconductor with a broken time-reversal symmetry. We show that $P^{(2)}$ provides insight into the relative strength of intra- and inter-band pairing in multi-band systems, as well as into the spin polarization of the bands.