Sodium vacancy ordering and the co-existence of localized spins and itinerant charges in NaxCoO2

TL;DR

This paper investigates the sodium vacancy ordering in NaxCoO2, revealing stable Na concentrations linked to superlattice structures, and discusses the coexistence of localized spins with itinerant charges, providing insights into its complex electronic behavior.

Contribution

It introduces a detailed model connecting Na ordering with localized spins and itinerant electrons, advancing understanding of NaxCoO2's electronic phases.

Findings

Stable Na concentrations associated with superlattice ordering.

Coexistence of localized spins and itinerant charges.

Small Fermi energy of 87 K at x=0.84.

Abstract

The sodium cobaltate family (NaxCoO2) is unique among transition metal oxides because the Co sits on a triangular lattice and its valence can be tuned over a wide range by varying the Na concentration x. Up to now detailed modeling of the rich phenomenology (which ranges from unconventional superconductivity to enhanced thermopower) has been hampered by the difficulty of controlling pure phases. We discovered that certain Na concentrations are specially stable and are associated with superlattice ordering of the Na clusters. This leads naturally to a picture of co-existence of localized spins and itinerant charge carriers. For x = 0.84 we found a remarkably small Fermi energy of 87 K. Our picture brings coherence to a variety of measurements ranging from NMR to optical to thermal transport. Our results also allow us to take the first step towards modeling the mysterious ``Curie-Weiss'' metal state at x = 0.71. We suggest the local moments may form a quantum spin liquid state and we propose experimental test of our hypothesis.