Single-particle excitation gap in La_2-xSr_xCuO₄ studied by high-resolution angle-resolved photoemission

We report angle-resolved photoemission spectroscopy on optimally doped high-T_c superconductor La_1.85Sr_0.15CuO₄ (T_c = 37 K) to elucidate the character of low-energy excitations in the superconducting state. We find a clear evidence for the emergence of a sharp superconducting quasiparticle peak below T_c together with opening of a superconducting gap at off-nodal region, while no signature of a quasiparticle peak is observed at (π, 0). From the comparison of spectral lineshape and energy dispersion between the nodal and the off-nodal region, we conclude that there are two characteristic energy scales (70 and 20 meV) in the superconducting state of optimally doped La_1.85Sr_0.15CuO₄.