Magnetic phase transition at88KinNa0.5CoO2

Abstract

Na0.5CoO2 exhibits a metal-insulator transition at TMI = 53 K upon cooling. The nature of another transition at TX = 88 K has not been fully clarified yet. We report the results of measurements of the electrical conductivity σ, the magnetic susceptibility χ and 23 Na NMR on a powder sample of Na0.5CoO2, including the mapping of NMR spectra, as well as probing the spin-lattice relaxation rate T −1 1 (T ) and the spin-spin relaxation rate T −1 2 (T ), in the temperature range between 30 K and 305 K. The NMR data reflect the transition at TX very well but provide less evidence for the metal-insulator transition at TMI. The temperature evolution of the shape of the spectra implies the formation of a staggered internal field below TX, not accompained by a rearrangement of the electric charge distribution. Our results thus indicate that in Na0.5CoO2, an unusual type of magnetic ordering in the metallic phase precedes the onset of charge ordering, which finally induces an insulating ground state.

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

“…However, in the proposed magnetic structure, no Na splitting should be observed in contrary to our measurements. Another NMR study [17] submitted just after ours concludes that the 23Na NMR data also supports the absence of charge ordering at the magnetic transition. However, the sensitivity of the Na EFG to Co charge and the accuracy of their data are not sufficient to allow the authors to reach such conclusions independently of our results.…”

Temperature-driven transition from the Wigner crystal to the bond-charge-density wave in the quasi-one-dimensional quarter-filled band

“…Interestingly, these unconventional insulating states in the CTS are often proximate to superconductivity, 27 the mechanism of which has remained perplexing after intensive investigations over several decades. Unconventional behavior at or near 1 4 filling has also been observed in the quasi-two-dimensional organic CTS with higher superconducting critical temperatures, 11,[28][29][30][31] sodium cobaltate, [32][33][34] and oxides of titanium 35 and vanadium. 36,37 In spite of extensive research on 1D instabilities in the CTS, detailed comparisons of theory and experiments remain difficult.…”

Melting of the Na Layers in SolidNa0.8CoO2

“…The recent activities in probing the physical behaviour of the compound series Na x CoO 2 (0.3 < x < 1) revealed a number of exciting features with respect to structural, electronic, magnetic and superconducting properties upon varying x [1,2,3,4,5,6,7,8]. Quite generally these compounds may be regarded as composed of stacks of alternating Na and Co-O layers.…”

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Data of23 Na NMR spectra-and relaxation measurements are interpreted as suggesting that, upon increasing temperature the Na layers in Na0.8CoO2 adopt a 2D liquid state at T = 291 K. The corresponding first order phase transition is preceded by a rapidly increasing mobility and diffusion of Na ions above 200 K. Above 291 K the 23 Na NMR response is similar to that previously observed in superionic conductors with planar Na layers.PACS numbers: 71.27.+a, 75.20.Hr, The recent activities in probing the physical behaviour of the compound series Na x CoO 2 (0.3 < x < 1) revealed a number of exciting features with respect to structural, electronic, magnetic and superconducting properties upon varying x [1,2,3,4,5,6,7,8]. Quite generally these compounds may be regarded as composed of stacks of alternating Na and Co-O layers.

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