Raman scattering study of electron-doped Pr_xCa _1-xFe₂As₂ superconductors

Temperature-dependent polarized Raman spectra of electron-doped superconducting Pr_xCa_1-xFe₂As₂ (0.12) single crystals are reported. All four even-parity phonons allowed by symmetry are identified. The phonon mode of B_1g symmetry at 222 cm^-1, which is associated with the c-axis motion of Fe ions, is found to exhibit an anomalous frequency hardening at low temperatures, that signals nonvanishing electron-phonon coupling in the superconducting state and implies that the superconducting gap magnitude 2Δ_c<27 meV.