Electron-boson mode coupling and the pseudogap of Na${}_x$CoO${}_2$ by infrared spectroscopy

Abstract

We report the in-plane optical measurements on Na${}_x$CoO${}_2$ with 0.18$\le$x$\le$0.92. The crystal growth and characterization were described in detail. The spectral weight increases monotonically with decreasing x. For sample with the lowest Na content x$\sim$0.18, the optical conductivity was strongly suppressed below 2200 $c{m}^{-1}$ at low temperature. The suppression becomes weaker with increasing Na content, and disappears in charge-ordered sample with x$\sim$0.5. At higher Na contents, similar suppression appears again but locates at higher energy scale near 3300 $c{m}^{-1}$. Our analysis indicates that those spectral features are dominated by a combination of electrons coupling to a bosonic mode and a pseudogap-like phenomenon. We suggest that the pseudogap-like phenomenon is purely a band structure effect. The infrared activated phonon modes were discussed in relation with the structural geometries.

arXiv commentrevised version 8 pages, 7 figures, to be published in Phys. Rev. B

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• Phonon softening and "forbidden" mode observed by Raman scattering in Na0.5CoO2
• Charge order and phonon renormalizations: Possible implications for cobaltates
• Angular-dependent Magnetoresistance Oscillations in Na${}_{0.48}$CoO${}_2$ Single Crystal
• One-particle spectral properties of the t-J-$V$ model on the triangular lattice near charge order
• Temperature-driven sodium-ion dynamical-to-static crossover in the zig-zag ordered phase of Na${}_{0.5}$CoO${}_2$