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

TL;DR

This study uses Raman scattering to reveal low energy fluctuations and enhanced out-of-plane coherence in vacancy-ordered NaxCoO2, suggesting a dynamic ground state related to spin polaron condensation.

Contribution

It provides new insights into the low energy excitations and lattice coherence in vacancy-ordered NaxCoO2, highlighting the role of Na vacancy ordering in its electronic properties.

Findings

Low energy fluctuations diverge at low temperatures.

Electronic scattering rate decreases significantly with temperature.

Phonon anomalies indicate increased out-of-plane lattice coherence.

Abstract

We report Raman scattering experiments on the strongly correlated electron system NaxCoO2 with x= 0.71 and ordered Na vacancies. In this doping regime, NaxCoO2 exhibits a large and unusual thermopower and becomes superconducting upon hydration. Our Raman scattering data reveal pronounced low energy fluctuations that diverge in intensity at low temperatures. Related to these fluctuations is a drastic decrease of an electronic scattering rate from 50 to 3 cm-1. This observation is quite different from the behavior of Na disordered samples that have a larger and temperature independent scattering rate. Simultaneously 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.