Formula omitted]: Enhanced low-energy excitations of electrons on a 2d triangular lattice

Abstract

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 Na x CoO 2 samples 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 C p / 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 μ SR data confirm the SDW ground state below 22 K and the much reduced heat capacity is field independent.