Charge ordering and magneto-polarons in Na$_{0.82}$CoO$_2$
  Using spectral ellipsometry, we have measured the dielectric function of a Na$_{0.82(2)}$CoO$_2$ crystal that exhibits bulk antiferromagnetism with T$_{N}$=19.8 K. We identify two prominent transitions as a function of temperature. The first one at 280 K involves marked changes of the electronic and the lattice response that are indicative of charge ordering in the CoO$_{2}$ layers. The second transition coincides with T$_{N}$=19.8 K and reveals a sizeable spin-charge coupling. The data are discussed in terms of charge ordering and formation of magneto-polarons due to a charge-induced spin-state transition of adjacent Co$^{3+}$ ions. 
C. Bernhard, A.V. Boris, N.N. Kovaleva, G. Khaliullin, A. Pimenov, L.   Yu, D.P. Chen, C.T. Lin, and B. Keimer