Enhanced low-energy fluctuations and increasing out-of-plane coherence in vacancy-ordered NaxCoO2

We report Raman scattering experiments on the strongly correlated electron system NaxCoO2 with x = 0.71 and ordered Na vacancies. This phase exhibits unusual transport and thermodynamic properties. Furthermore, hydrated samples with the same Co valency are superconducting. Our Raman scattering data reveal pronounced low-energy fluctuations that diverge in intensity at low temperatures. There is a related decrease of the electronic scattering rate Gamma from 50 to 3 cm(-1). Simultaneous with the evolution of the scattering rate, phonon anomalies point to an increasing out-of-plane coherence of the lattice with decreasing temperature. These observations may indicate the condensation of spin polarons into an unusual, highly dynamic ground state.