Measuring Debye energy in two electron photoemission spectroscopy

Two electron photoemission spectroscopy (2e-ARPES) has been shown to be able to probe momentum and spin structure of superconducting condensate. We further demonstrate theoretically that 2e-ARPES 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. In this work, we compute the photoelectron counting rate P⁽²⁾ in two different types of unconventional superconductors, a d_x2-y2-wave superconductor and a topological superconductor with a broken time-reversal symmetry, which 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.