Optical evidence for the proximity to a spin-density-wave metallic state in <InlineEquation ID="Equ1"> <EquationSource Format="TEX">$\mathsf{Na_{0.7}CoO_2}$</EquationSource> </InlineEquation>

We present the optical properties of Na<Subscript>0.7</Subscript>CoO<Subscript>2</Subscript> single crystals, measured over a broad spectral range as a function of temperature (T). The capability to cover the energy range from the far-infrared up to the ultraviolet allows us to perform reliable Kramers-Kronig transformation, in order to obtain the absorption spectrum (i.e., the complex optical conductivity). To the complex optical conductivity we apply the generalized Drude model, extracting the frequency dependence of the scattering rate (<InlineEquation ID="Equ2"> <EquationSource Format="TEX">$\Gamma$</EquationSource> </InlineEquation>) and effective mass (m <Superscript>*</Superscript>) of the itinerant charge carriers. We find that <InlineEquation ID="Equ3"> <EquationSource Format="TEX">$\Gamma(\omega)\sim \omega$</EquationSource> </InlineEquation> at low temperatures and for <InlineEquation ID="Equ4"> <EquationSource Format="TEX">$\omega > T$</EquationSource> </InlineEquation>. This suggests that Na<Subscript>0.7</Subscript>CoO<Subscript>2</Subscript> is at the verge of a spin-density-wave metallic phase. Copyright Springer-Verlag Berlin/Heidelberg 2004