Sord(xy) Wave Superconducting Gap Created from the Electron–Phonon Coupled States in Underdoped La2-xSrxCuO4

Abstract

In La 2- x Sr x CuO 4 the superconducting gap energy in the B 2g Raman spectra is almost constant from the optimum doping to near the insulator–metal (I–M) transition, indicating that the gap symmetry is s or d ( x y ), because the Fermi arc shrinks into the nodal direction of the d ( x 2 - y 2 ) gap. The reason of the different result from other experiments comes from the dual components of the low-energy electronic states around (π/2, π/2), the resonant peak relating to the I–M transition and the polaron states. The superconducting coherent peak is found to be created only in the polaron states in the underdoped phase.