Momentum Dependence Of Superconducting Gap, Strong-coupling Dispersion Kink, And Tightly Bound Cooper Pairs In The High-t_c (sr,ba)_(1-x)(k,na)_xfe_2as_2 Superconductors

摘要

We present a systematic angle-resolved photoemission spectroscopic study of the high-Tc superconductor class (Sr/Ba)_(1-x)K_xFe_2As_2. By utilizing a photon-energy-modulation contrast and scattering geometry we report the Fermi surface and the momentum dependence of the superconducting gap, Δ(k~→). A prominent quasiparticle dispersion kink reflecting strong scattering processes is observed in a binding-energy range of 25-55 meV in the superconducting state, and the coherence length or the extent of the Cooper pair wave function is found to be about 20 A, which is uncharacteristic of a superconducting phase realized by the BCS-phonon-retardation mechanism. The observed 40 ± 15 meV kink likely reflects contributions from the frustrated spin excitations in a J_1-J_2 magnetic background and scattering from the soft phonons. Results taken collectively provide direct clues to the nature of the pairing potential including an internal phase-shift factor in the superconducting order parameter which leads to a Brillouin zone node in a strong-coupling setting.