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

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.

16 pages, 5 figures

References in corpus (5)

• U(1) Gauge Theory of the Hubbard Model : Spin Liquid States and Possible Application to k-(BEDT-TTF)_2 Cu_2 (CN)_3
• Na Induced Correlations in Na${}_x$CoO${}_2$
• Searching for Stable Na-ordered Phases in Single Crystal Samples of gamma-NaxCoO2
• Local moment, itinerancy and deviation from Fermi liquid behavior in Na${}_x$CoO${}_2$ for $0.71\le x\le 0.84$
• Sodium Trimer Ordering on NaxCoO2 Surface

Cited by in corpus (9)

• Electronic phase diagram of the layered cobalt oxide system, LixCoO2 (0.0 <= x <= 1.0)
• Aperiodic topological order in the domain configurations of functional materials
• Na ordering imprints a metallic kagome lattice onto the Co planes of Na2/3CoO2
• Impact of lithium composition on the thermoelectric properties of the layered cobalt oxide system LixCoO2
• Crystal-to-stripe reordering of sodium ions in NaxCoO2 (x>=0.75)
• Sodium Trimer Ordering on NaxCoO2 Surface
• Mott and Wigner-Mott transitions in doped correlated electron systems: effects of superlattice potential and inter-site correlation
• Evidence for Oxygen Holes due to d-p Rehybridization in Thermoelectric Sr_{1-x}Rh_{2}O_{4}
• Transition from a phase-segregated state to single-phase incommensurate sodium ordering in Na_xCoO_2 with x \approx 0.53