Stripe correlations in Na$_{0.75}$CoO$_2$
  We unambiguously demonstrate, based on high-energy x-ray diffraction data and LDA calculations, that sodium-density stripes are formed in Na0.75CoO2 at low temperatures and rule out the previously proposed Na-ordering models. The LDA calculations prove, that the sodium-density stripes lead to a sizeable dip in the density of the Co-states at the Fermi level, pointing to band structure effects as a driving force for the stripe formation. This indicates that the sodium ordering is connected to stripe-like charge correlations within the CoO2 layers, leading to an astonishing similarity between the doped cuprates and the NaxCoO2 compounds. 
J. Geck, M. v. Zimmermann, H. Berger, S.V. Borisenko, H. Eschrig, K.   Koepernik, M. Knupfer, and B. B\"uchner