Abstract
We report the first determination of the in-plane complex optical conductivity of 1111 high- 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 and experiences an anomalous -linear softening and narrowing below . The suppression of the infrared absorption in the superconducting state reveals a large optical superconducting gap with a similar gap ratio in both materials, indicating strong pairing.
- Received 4 July 2017
DOI:https://doi.org/10.1103/PhysRevLett.120.087001
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