s or d(xy) Wave Superconducting Gap Created from the Electron-Phonon Coupled States in Underdoped La2-xSrxCuO4

In La_2-xSr_xCuO₄ 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(xy), because the Fermi arc shrinks into the nodal direction of the d(x²-y²) 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.