Possible quantum criticality in NaxCoO2

A (T,H) phase diagram of metallic NaxCoO2 is proposed. At low Na content (x approximate to 0.36), the magnetic susceptibility diverges with a power law T-eta (eta < 1) and shows (T,H) scaling, indicating the proximity to a magnetic quantum phase transition. A magnetic field suppression of the low-temperature specific heat was observed for these compositions. At high Na content (x approximate to 0.69) the susceptibility does not diverge, but renormalizes and becomes strongly field-dependent at low temperatures, forming a heavy Fermi liquid.