Evidence of a pseudogap for superconducting iron-pnictide Ba0.6+δK0.4-δFe2As2 single crystals from optical conductivity measurements

We report the observation of pseudogap (PG) behavior in optical conductivity in an underdoped Ba0.6+δK0.4-δFe2As2 (Tonsetc ≈ 36 K) single crystal far above Tc, up to 100 K (≈3Tc). Wide separation of the energy scales of the magnetic and superconducting (SC) correlations in the Ba0.6+δK0.4-δFe2As2 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 Tc. (paper)