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.