NaxCoO2: Enhanced low-energy excitations of electrons on a 2D triangular lattice

To elucidate the low-energy excitation spectrum of correlated electrons on a 2D triangular lattice, we have studied the electrical resistance and specific heat down to 0.5 K and in magnetic fields up to 14 T, in NaxCoO2 samples with a Na content ranging from x \approx 0.5 to 0.82. Two distinct regimes are observed: for x from about 0.6 to x \approx 0.75 the specific heat is strongly enhanced, with a pronounced upturn of C/T below about 10 K, reaching 47 mJ/(mol K^2). This enhancement is suppressed in a magnetic field indicative of strong low-energy spin fluctuations. At higher Na content the fluctuations are reduced and mu-SR data confirm the SDW ground state below 22 K and the much reduced heat capacity is field independent.