Raman evidence for the superconducting gap and spin–phonon coupling in the superconductor Ca(Fe0.95Co0.05)2As2

Inelastic light scattering studies on a single crystal of electron-doped Ca(Fe0.95Co0.05)2As2 superconductor, covering the tetragonal-to-orthorhombic structural transition as well as the magnetic transition at TSM ∼ 140 K and the superconducting transition temperature Tc ∼ 23 K, reveal evidence for superconductivity-induced phonon renormalization. In particular, the phonon mode near 260 cm−1 shows hardening below Tc, signaling its coupling with the superconducting gap. All three Raman active phonon modes show anomalous temperature dependence between room temperature and Tc, i.e. the phonon frequency decreases with lowering temperature. Further, the frequency of one of the modes shows a sudden change in temperature dependence at TSM. Using first-principles density functional theory based calculations, we show that the low temperature phase (Tc < T < TSM) exhibits short-ranged stripe antiferromagnetic ordering, and estimate the spin–phonon couplings that are responsible for these phonon anomalies. (Some figures in this article are in colour only in the electronic version)