Phonon anomalies in pure and underdoped R1-xKxFe2As2 (R=Ba, Sr) investigated by Raman light scattering

We present a detailed temperature-dependent Raman light scattering study of optical phonons in Ba1-xKxFe2As2 (x similar to 0.28, superconducting T-c similar to 29 K), Sr1-xKxFe2As2 (x similar to 0.15, T-c similar to 29 K), and nonsuperconducting BaFe2As2 single crystals. In all samples we observe a strong continuous narrowing of the Raman-active Fe and As vibrations upon cooling below the spin-density wave transition T-s. We attribute this effect to the opening of the spin-density wave gap. The electron-phonon linewidths inferred from these data greatly exceed the predictions of ab initio density-functional calculations without spin polarization, which may imply that local magnetic moments survive well above T-s. A first-order structural transition accompanying the spin-density wave transition induces discontinuous jumps in the phonon frequencies. These anomalies are increasingly suppressed for higher potassium concentrations. We also observe subtle phonon anomalies at the superconducting transition temperature T-c with a behavior qualitatively similar to that in the cuprate superconductors.