Observation of Momentum-Confined In-Gap Impurity State in Ba_{0.6}K_{0.4}Fe_{2}As_{2}: Evidence for Antiphase s_{±} Pairing

• Main Authors: P. Zhang, P. Richard, T. Qian, X. Shi, J. Ma, L.-K. Zeng, X.-P. Wang, E. Rienks, C.-L. Zhang, Pengcheng Dai, Y.-Z. You, Z.-Y. Weng, X.-X. Wu, J. P. Hu, H. Ding
• Format: Article
• Language: English
• Published: American Physical Society 2014-07-01
• Series: Physical Review X
• Online Access: http://doi.org/10.1103/PhysRevX.4.031001

We report the observation by angle-resolved photoemission spectroscopy of an impurity state located inside the superconducting gap of Ba_{0.6}K_{0.4}Fe_{2}As_{2} and vanishing above the superconducting critical temperature, for which the spectral weight is confined in momentum space near the Fermi wave-vector positions. We demonstrate, supported by theoretical simulations, that this in-gap state originates from weak scattering between bands with opposite sign of the superconducting-gap phase. This weak scattering, likely due to off-plane nonmagnetic (Ba, K) disorder, occurs mostly among neighboring Fermi surfaces, suggesting that the superconducting-gap phase changes sign within holelike (and electronlike) bands. Our results impose severe restrictions on the models promoted to explain high-temperature superconductivity in these materials.