Raman spectroscopy of K_xCo_{2-y}Se_2 single crystals near the ferromagnet-paramagnet transition

TL;DR

This study investigates the phonon modes in K_xCo_{2-y}Se_2 single crystals near the ferromagnetic transition using Raman spectroscopy, revealing how magnetic order influences lattice vibrations and electron-phonon interactions.

Contribution

It provides the first detailed Raman analysis of K_xCo_{2-y}Se_2, linking phonon behavior to magnetic phase transition and comparing experimental results with DFT calculations.

Findings

Two phonon modes identified as A_{1g} and B_{1g}.

Ferromagnetic transition affects phonon frequencies and linewidths.

DFT calculations align with experimental data, highlighting spin fluctuation effects.

Abstract

Polarized Raman scattering spectra of the K_xCo_{2-y}Se_2 single crystals reveal the presence of two phonon modes, assigned as of the A_{1g} and B_{1g} symmetry. Absence of additional modes excludes the possibility of vacancy ordering, unlike in K_xFe_{2-y}Se_2. The ferromagnetic (FM) phase transition at T_c\approx 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.