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.

arXiv comment16 pages, 5 figures

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Cited by in corpus (19)

• 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
• NQR and X-ray investigation of the structure of Na${}_{2/3}$CoO${}_2$ compound
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• 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)
• Electronic phase diagram of LixCoO2 revisited with potentiostatically de-intercalated single crystals
• Sodium Trimer Ordering on NaxCoO2 Surface
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• Sodium ion ordering of Na0.77CoO2 under competing multi-vacancy cluster, superlattice and domain formation