S or d(xy) wave superconducting gap created from the electron-phonon coupled states in underdoped La2-xSrxCuO4

Abstract

The present Raman scattering experiment in La2-xSrxCuO4 disclosed that the low-energy conductive electronic states at (π/2, π/2) in the underdoped phase are composed of dual electronic states of the central resonant peak and the electron-phonon coupled polaron states. Both states develop at the lowest-energy end of the B2g electron - two-magnon coupled high-energy peak. The narrow resonant peak is enhanced at the insulator-metal transition, but it is insensitive to the superconducting transition indicating no relation to the superconductivity. On the other hand the polaron peak changes into the superconducting coherent peak (SCP) exactly below Tc. The gap energy determined from the SCP is almost constant from the optimum doping to near the insulator-metal transition, indicating that the gap symmetry is s or d(xy), because the Fermi arc with the large electronic density of states shrinks into the nodal direction of the d(x2 - y2) gap. In the overdoped phase the low-energy conductive electronic states are created at (π, 0) by decreasing the peak energy of the B1g electron - two-magnon coupled states. The gap symmetry in the overdoped phase is consistent with d(x2 - y2).