Measurement of a Sign-Changing Two-Gap Superconducting Phase in Electron-Doped Ba(Fe_(1-x)CO_x)_2As_2 Single Crystals Using Scanning Tunneling Spectroscopy

摘要

Scanning tunneling spectroscopic studies of Ba(Fe_(1-x)Co_x)_2As_2 (x = 0.06, 0.12) single crystals reveal direct evidence for predominantly two-gap superconductivity. These gaps decrease with increasing temperature and vanish above the superconducting transition T_c.. The two-gap nature and the slightly doping- and energy-dependent quasiparticle scattering interferences near the wave vectors ( ± π, 0) and (0, ±π) are consistent with sign-changing i-wave superconductivity. The excess zero-bias conductance and the large gap-to-T_c ratios suggest dominant unitary impurity scattering.