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Abstract:
We report the first determination of the in-plane complex optical conductivity of 1111 high-T-c superconducting iron oxypnictide single crystals PrFeAs(O,F) and thin films SmFeAs(O,F) by means of conventional and microfocused infrared spectroscopy, ellipsometry, and time-domain THz transmission spectroscopy. A strong itinerant contribution is found to exhibit a dramatic difference in coherence between the crystal and the film. Using extensive temperature-dependent measurements of THz transmission, we identify a previously undetected 2.5-meV collective mode in the optical conductivity of SmFeAs(O,F), which is strongly suppressed at T-c and experiences an anomalous T-linear softening and narrowing below T* approximate to 110 K >> T-c. The suppression of the infrared absorption in the superconducting state reveals a large optical superconducting gap with a similar gap ratio 2 Delta/k(B)T(c) approximate to 7 in both materials, indicating strong pairing.
