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 0.5K and in magnetic fields up to 14T, in NaxCoO2samples with a Na content ranging from x∼0.5 to 0.82. Two distinct regimes are observed: for x from about 0.64 to x∼0.75 the specific heat is strongly enhanced, with a pronounced upturn of Cp/T below about 10K, reaching 47mJ/(molK2). This enhancement is suppressed in a magnetic field indicative of strong low-energy spin fluctuations. At higher Na content, the fluctuations are reduced and μSR data confirm the SDW ground state below 22K and the much reduced heat capacity is field independent