title: Raman spectroscopy of KxCo2-ySe2single crystals near the ferromagnet-paramagnet transition.
abstract: Polarized Raman scattering spectra of the KxCo2-ySe2single crystals reveal the presence of two phonon modes, assigned as of the A1gand B1gsymmetry. The absence of additional modes excludes the possibility of vacancy ordering, unlike in KxFe2-ySe2. 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 A1gmode narrows in the FM phase, whereas the B1gmode broadens. We argue that the large asymmetry and anomalous frequency shift of the B1gmode 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.
authors.affiliation: Center for Solid State Physics and New Materials, Institute of Physics Belgrade, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia.
authors.forename: M
authors.initials: M
authors.lastname: Opačić
authors.forename: N
authors.initials: N
authors.lastname: Lazarević
authors.forename: M M
authors.initials: MM
authors.lastname: Radonjić
authors.forename: M
authors.initials: M
authors.lastname: Šćepanović
authors.forename: Hyejin
authors.initials: H
authors.lastname: Ryu
authors.forename: Aifeng
authors.initials: A
authors.lastname: Wang
authors.forename: D
authors.initials: D
authors.lastname: Tanasković
authors.forename: C
authors.initials: C
authors.lastname: Petrovic
authors.forename: Z V
authors.initials: ZV
authors.lastname: Popović