Magnetism, disorder and non-Fermi liquid behavior in a Na$_{0.78}$CoO$_{2}$ single crystal*

In recent years, significant interest has been devoted to high Na-doped cobaltates, in which the competition between geometric frustration, strong electronic correlations, and magnetic interactions leads to disorder, either correlated or non-correlated, and a variety of ground states with possible QPTs are expected. Specific heat down to 100 mK and the ac-susceptibility at several frequencies and temperatures, down to 40 mK and in fields up to 10 T, of a layered Na$_{0.78}$CoO$_{2}$ single crystal have been measured. The results will be discussed in terms of disorder induced non-Fermi liquid behavior, related to the spin dynamics of the ferromagnetic nano-domains coexisting with antiferromagnetically correlated regions in the system.