NMR study of the magnetic and metal-insulator transitions in Na0.5CoO2: a nesting scenario

TL;DR

This study uses NMR to investigate magnetic and metal-insulator transitions in Na0.5CoO2, revealing no charge segregation and suggesting a nesting scenario for the observed phenomena.

Contribution

It provides evidence that the transitions are driven by Fermi surface nesting rather than charge ordering, based on NMR data.

Findings

No charge segregation into Co3+ and Co4+ sites.

Magnetic order is only slightly affected by the metal-insulator transition.

Transitions are explained by SDW induced by Fermi surface nesting.

Abstract

Co and Na NMR are used to probe the local susceptibility and charge state of the two Co sites of the Na-ordered orthorhombic Na0.5CoO2. Above T_N=86K, both sites display a similar T-dependence of the spin shift, suggesting that there is no charge segregation into Co3+ and Co4+ sites. Below T_N, the magnetic long range commensurate order found is only slightly affected by the metal-insulator transition (MIT) at T_MIT=51K. Furthermore, the electric field gradient at the Co site does not change at these transitions, indicating the absence of charge ordering. All these observations can be explained by successive SDW induced by two nestings of the Fermi Surface specific to the x=0.5 Na-ordering.