Raman evidence for the superconducting gap and spin-phonon coupling in the superconductor Ca(Fe_0.95Co_0.05)₂As₂

Abstract

Inelastic light scattering studies on a single crystal of electron-doped Ca(Fe_0.95Co_0.05)₂As₂ superconductor, covering the tetragonal-to-orthorhombic structural transition as well as the magnetic transition at T_SM ~140 K and the superconducting transition temperature T_c ~23 K, reveal evidence for superconductivity-induced phonon renormalization. In particular, the phonon mode near 260 cm^-1 shows hardening below T_c, signaling its coupling with the superconducting gap. All three Raman active phonon modes show anomalous temperature dependence between room temperature and T_c, 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 T_SM. Using first-principles density functional theory based calculations, we show that the low temperature phase (T_c < T < T_SM) exhibits short-ranged stripe antiferromagnetic ordering, and estimate the spin-phonon couplings that are responsible for these phonon anomalies.