Dynamic competition between spin density wave order and   superconductivity in underdoped $Ba_{1-x}K_xFe_2As_2$
  An intriguing aspect of unconventional superconductivity is that it always appears in the vicinity of other competing phases, whose suppression brings the full emergence of superconductivity. In the iron-pnictides, these competing phases are marked by a tetragonal-to-orthorhombic structural transition and a collinear spin-density-wave (SDW) transition. There has been macroscopic evidence for competition between these phases and superconductivity as the magnitude of both the orthorhombicity and magnetic moment are suppressed in the superconducting state. Here, using angle-resolved photoemission spectroscopy on detwinned underdoped $Ba_{1-x}K_xFe_2As_2$, we observe a coexistence of both the SDW gap and superconducting gap in the same electronic structure. Furthermore, our data reveal that following the onset of superconductivity, the SDW gap decreases in magnitude and shifts in a direction consistent with a reduction of the orbital anisotropy. This observation provides direct spectroscopic evidence for the dynamic competition between superconductivity and both SDW and electronic nematic orders in these materials. 
M. Yi, Y. Zhang, Z.-K. Liu, X. Ding, J.-H. Chu, A. F. Kemper, N.   Plonka, B. Moritz, M. Hashimoto, S.-K. Mo, Z. Hussain, T....
cond-mat.supr-con