Phonon anomalies in pure and underdopedR1−xKxFe2As2(R=Ba, Sr) investigated by Raman light scattering

Abstract

We present a detailed temperature-dependent Raman light scattering study of optical phonons in ${\text{Ba}}_{1\ensuremath{-}x}{\text{K}}_{x}{\text{Fe}}_{2}{\text{As}}_{2}$ ($x\ensuremath{\sim}0.28$, superconducting ${T}_{c}\ensuremath{\sim}29\text{ }\text{K}$), ${\text{Sr}}_{1\ensuremath{-}x}{\text{K}}_{x}{\text{Fe}}_{2}{\text{As}}_{2}$ ($x\ensuremath{\sim}0.15$, ${T}_{c}\ensuremath{\sim}29\text{ }\text{K}$), and nonsuperconducting ${\text{BaFe}}_{2}{\text{As}}_{2}$ 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.