Photoemission study of the superconducting-gap symmetry in electron-doped high-T_c superconductors

To study the superconducting-gap symmetry in electron-doped HTSC, we performed a high-resolution ARPES on Pr_0.89LaCe_0.11CuO₄ (optimally-doped, T_c = 26 K). Obtained momentum dependence of the superconducting gap is basically consistent with the d_{x2 - y2}-wave symmetry, but obviously deviates from the monotonic form of the d_{x2 - y2} gap function. The maximum superconducting gap is not located at the zone boundary as expected from a simple d_{x2 - y2}-wave, but observed at the so-called hot spot where the antiferromagnetic spin fluctuation strongly couples to the electrons on the Fermi surface. This ARPES result suggests that the spin-correlation plays an essential role in the pairing mechanism of electron-doped HTSC.