Coexistence of a pseudogap and a superconducting gap for the high-T_c superconductor La_(2-x)Sr_xCuO_4 studied by angle-resolved photoemission spectroscopy

摘要

The relationship between the superconducting gap and the pseudogap has been the subject of controversies. In order to clarify this issue, we have studied the superconducting gap and pseudogap of the high-T_c superconductor La_(2-x)Sr_xCuO_4 (x = 0.10,14) by angle-resolved photoemission spectroscopy (ARPES). Through the analysis of the ARPES spectra above and below T_c, we have identified a superconducting coherence peak even in the antinodal region on top of the pseudogap of a larger energy scale. The superconducting peak energy nearly follows the pure d-wave form. The d-wave order parameter Δ_0 [defined by Δ(k) = Δ_0(cos k_xa - cos k_ya) ] for x = 0.10 and 0.14 are nearly the same, Δ_0 ～ 12-14 meV, leading to strong coupling 2Δ_0/k_BT_c ～ 10. The present result indicates that the pseudogap and the superconducting gap are distinct phenomena and can be described by the "two-gap" scenario.