Observation of strong electron pairing on bands without Fermi surfaces   in LiFe1-xCoxAs
  In conventional BCS superconductors, the quantum condensation of superconducting electron pairs is understood as a Fermi surface (FS) instability, in which the low-energy electrons are paired by attractive interactions. Whether this explanation is still valid in high-Tc superconductors such as cuprates and iron-based superconductors remains an open question. In particular, a fundamentally different picture of the electron pairs, which are believed to be formed locally by repulsive interactions, may prevail. Here we report a high-resolution angle-resolved photoemission spectroscopy study on LiFe1-xCoxAs. We reveal a large and robust superconducting (SC) gap on a band sinking below the Fermi energy upon Co substitution. The observed FS-free SC order is also the largest over the momentum space, which rules out a proximity effect origin and indicates that the SC order parameter is not tied to the FS as a result of a FS instability. 
H. Miao, T. Qian, X. Shi, P. Richard, T. K. Kim, M. Hoesch, L. Y.   Xing, X. C. Wang, C. Q. Jin, J. P. Hu and H. Ding
cond-mat.supr-con