NMR Study of the Magnetic and Metal-Insulator Transitions inNa0.5CoO2: A Nesting Scenario

Abstract

Co and Na NMR are used to probe the local susceptibility and charge state of the two Co sites of the Naordered orthorhombic Na 0.5 CoO 2 . Above . 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 two successive SDW induced by nestings of the Fermi Surface specific to the x=0.5 Na-ordering.

Ionic Hubbard model on a triangular lattice forNa0.5CoO2,Rb0.5Powell1, Merino2, McKenzie3 2009Phys. Rev. B9020View full textAdd to dashboardCite

“…25 However, the authors noted that the degree of charge disproportionation is rather small. More recent 59 Co and 23 Na NMR measurements find no signature of differences in the charge state of the cobalt ions at the two distinct sites 26 suggesting that all the Co atoms are in the charge state Co ͑3.5Ϯ ͒+ , where Ͻ 0.2 gives an upper bound on the extent of charge ordering. 26 This is in contrast to the case of x = 0.7 or 0.75 for which NMR clearly detecting a charge disproportionation.…”

“…More recent 59 Co and 23 Na NMR measurements find no signature of differences in the charge state of the cobalt ions at the two distinct sites 26 suggesting that all the Co atoms are in the charge state Co ͑3.5Ϯ ͒+ , where Ͻ 0.2 gives an upper bound on the extent of charge ordering. 26 This is in contrast to the case of x = 0.7 or 0.75 for which NMR clearly detecting a charge disproportionation. 27,28 High resolution neutron crystallography detects small differences in Co-O bond lengths and an analysis based on bond valence sums is consistent with a charge order of Ӎ 0.06.…”

“…4,26 This suggests an antiferromagnetic exchange between magnetic ions of order a few hundred Kelvin.…”

“…26 The change in the resistivity from a bad metal to an activated behavior at 51 K has little effect on the magnetic state. Elastic neutron scattering also detects longrange antiferromagnetic order below 88 K, with a magnetic moment of 0.26͑2͒ B per magnetic cobalt ion.…”

Quasiparticle dispersion near the Fermi surface in NaxCoO2

“…The crystallographic structure of Na x CoO 2 consists in electronically active layers of edge-shared CoO 6 octahedra alternating with layers of Na. The physics of these compounds is believed to be essentially two dimensional because of the weak coupling between CoO 2 layers, and Na atoms act as a charge reservoir whose order can dramatically affect the CoO 2 planes like in Na 0.5 CoO 2 [3]. The knowledge of the band structure and the Fermi surface (FS) of these systems is crucial to understand their physical properties.…”