Raman spectroscopy of K _x Co_2-y Se₂ single crystals near the ferromagnet-paramagnet transition

Polarized Raman scattering spectra of the K _x Co_2-y Se₂ single crystals reveal the presence of two phonon modes, assigned as of the A _1g and B _1g symmetry. The absence of additional modes excludes the possibility of vacancy ordering, unlike in K _x Fe_2-y Se₂. The ferromagnetic (FM) phase transition at [Formula: see text] K leaves a clear fingerprint on the temperature dependence of the Raman mode energy and linewidth. For [Formula: see text] the temperature dependence looks conventional, driven by the thermal expansion and anharmonicity. The Raman modes are rather broad due to the electron-phonon coupling increased by the disorder and spin fluctuation effects. In the FM phase the phonon frequency of both modes increases, while an opposite trend is seen in their linewidth: the A _1g mode narrows in the FM phase, whereas the B _1g mode broadens. We argue that the large asymmetry and anomalous frequency shift of the B _1g mode is due to the coupling of spin fluctuations and vibration. Our density functional theory (DFT) calculations for the phonon frequencies agree rather well with the Raman measurements, with some discrepancy being expected since the DFT calculations neglect the spin fluctuations.