s-Wave-Like excitation in the superconducting state of electron-doped cuprates with d-wave pairing
An intrinsic physical mechanism, based on the doping evolution of the Fermi surface (FS), is explored to reconcile the contradictory experimental results on the superconducting (SC) pairing symmetry in electron-doped cuprates. It is argued that the FS pocket around $(π/2,π/2)$ has not yet formed until doping reaches about the optimal value. Therefore, in the underdoped regime, even if the SC order parameter is $d$ wave which vanishes along the line $k_x=k_y$, the quasiparticle excitation gap is still finite and looks s-wave-like due to the absence of the FS across that line. This makes it possible, with d-wave SC pairing, to understand those experiments which evidenced the s-wave quasiparticle excitation. An explicit theory with consideration of both antiferromagnetic and SC orders is implemented to exhibit the FS evolution from underdoping to overdoping and associated with it the variations of the quasiparticle property, electronic density of states, and low temperature dependences of the physical quantities heat capacity and superfluid density.