Evidence of a pseudogap for superconducting iron-pnictide Ba _{0. 6+δ}K _{0. 4?δ}Fe ₂As ₂ single crystals from optical conductivity measurements

We report the observation of pseudogap (PG) behavior in optical conductivity in an underdoped Ba _0.6+δK _0.4?δFe ₂As ₂ (T _c ^onsetc≈ 36 K) single crystal far above T _c, up to 100K (≈ 3T _c). Wide separation of the energy scales of the magnetic and superconducting (SC) correlations in the Ba _0.6+δK _0.4?δFe ₂As ₂ sample enabled us to establish that the PG structures observed in optical conductivity in the range of 50-150 cm ^?1 are uniquely driven by the SC correlation, and the magnetic correlation coexists in the far separated range of about 700 cm?1. Theoretical calculations, based on the preformed Cooper pair model, provided an excellent description of the temperature evolution of the SC correlation in optical conductivity data from below to above T _c.