Raman spectroscopy of KxCo2-ySe2 single crystals near the ferromagnet-paramagnet transition
- Raman spectroscopy of KxCo2-ySe2 single crystals near the ferromagnet-paramagnet transition
- 류혜진; Marko Opacic; Nenad Lazarevic; Milos Radonjic; Maja Scepanovic; Aifeng Wang; Darko Tanaskovic; Cedomir Petrovic; Zoran Popovic
- Issue Date
- Journal of physics, Condensed matter : an Institute of Physics journal
- VOL 28-485401-8
- Polarized Raman scattering spectra of the KxCo2− ySe2 single crystals reveal the presence of
two phonon modes, assigned as of the A1g and B1g symmetry. The absence of additional modes excludes the possibility of vacancy ordering, unlike in KxFe2− ySe2. The ferromagnetic (FM) phase transition at Tc ≈ 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 A1g mode narrows in the FM phase, whereas the B1g mode broadens. We argue that the large asymmetry and anomalous frequency shift of the B1g 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.
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