Nonmonotonic gap in the coexisting antiferromagnetic and superconducting state for electron-doped cuprate superconductors

We argue that the experimentally observed nonmonotonic gap in electron-doped cuprates at optimal doping is the lowest quasiparticle excitation energy in the coexisting antiferromagnetic (AF) and superconducting (SC) state. The idea is implemented by studying the coexistence of AF and SC orders with the t-t'-t''-J model. Although the pairing gap itself is assumed to be the simplest $d$ wave which is monotonic, we have found that the quasiparticle excitation gap in the coexisting state is nonmonotonic, with the maxima around the hot spots where the Fermi surface is missing due to the AF gap. Within the same framework of the coexisting state the spectral function is also calculated at optimal doping. The obtained results are all consistent with experiments.