Patterning of sodium ions and the control of electrons in sodium cobaltate

Abstract

Sodium cobaltate (Na_xCoO₂) has emerged as a material of exceptional scientific interest due to the potential for thermoelectric applications^1,2, and because the strong interplay between the magnetic and superconducting properties has led to close comparisons with the physics of the superconducting copper oxides³. The density x of the sodium in the intercalation layers can be altered electrochemically, directly changing the number of conduction electrons on the triangular Co layers⁴. Recent electron diffraction measurements reveal a kaleidoscope of Na⁺ ion patterns as a function of concentration⁵. Here we use single-crystal neutron diffraction supported by numerical simulations to determine the long-range three-dimensional superstructures of these ions. We show that the sodium ordering and its associated distortion field are governed by pure electrostatics, and that the organizational principle is the stabilization of charge droplets that order long range at some simple fractional fillings. Our results provide a good starting point to understand the electronic properties in terms of a Hubbard hamiltonian⁶ that takes into account the electrostatic potential from the Na superstructures. The resulting depth of potential wells in the Co layer is greater than the single-particle hopping kinetic energy and as a consequence, holes preferentially occupy the lowest potential regions. Thus we conclude that the Na⁺ ion patterning has a decisive role in the transport and magnetic properties.