Phonon softening and "forbidden" mode observed by Raman scattering in Na0.5CoO2

Abstract

Polarized Raman scattering measurements have been performed on Na0.5CoO2 single crystal from 8 to 305 K. Both the A1g and E1g phonon modes show a softening below Tc1 ~ 83 K. Additionally, the A1g phonon mode, which is forbidden in the scattering geometry of cross polarization for the triangular CoO2 layers, appears below Tc1. In contrast, the metal-insulator transition at Tc2 ~ 46 K has only secondary effect on the Raman spectra. The phonon softening and the ``forbidden'' Raman intensity follow closely magnetic order parameter and the gap function at the Fermi surface, indicating that the distortion of CoO6 octahedra at Tc1, instead of the Na ordering at ~350 K, is the relevant structural component of the 83 K phase transition.

arXiv comment5 pages, 4 figures

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• Enhanced low energy fluctuations and increasing out-of-plane coherence in vacancy ordered NaxCoO2
• Temperature-driven sodium-ion dynamical-to-static crossover in the zig-zag ordered phase of Na${}_{0.5}$CoO${}_2$