Raman spectroscopy of K x Co2−y Se2 single crystals near the ferromagnet–paramagnet transition

Polarized Raman scattering spectra of the KxCo2-ySe2 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 KxFe2-ySe2. The ferromagnetic (FM) phase transition at T-c approximate to 74 K leaves a clear fingerprint on the temperature dependence of the Raman mode energy and linewidth. For T > T-c 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.