Revised Superconducting Phase Diagram of Hole-Doped Nax(H3O)zCoO2·yH2O (original) (raw)
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Revised superconducting phase diagram of hole doped Na$_{x}$(H$_{3}$O)$_{z}$CoO$_{2}\cdot y$H$_{2}$O
Physical Review Letters, 2004
We have studied the superconducting phase diagram of NaxCoO2 ·yH2O as a function of electronic doping, characterizing our samples both in terms of Na content x and the Co valence state. Our findings are consistent with a recent report that intercalation of H3O + ions into NaxCoO2, together with water, act as an additional dopant indicating that Na sub-stochiometry alone does not control the electronic doping of these materials. We find a superconducting phase diagram where optimal TC is achieved through a Co valence range of 3.24 -3.35, while TC decreases for materials with a higher Co valence. The critical role of dimensionality in achieving superconductivity is highlighted by similarly doped non-superconducting anhydrous samples, differing from the superconducting hydrate only in inter-layer spacing. The increase of the interlayer separation between CoO2 sheets as Co valence is varied into the optimal TC region is further evidence for this criticality.
Crystal structure of the superconducting layered cobaltite Na x CoO 2 . y D 2 O
Acta Crystallographica Section A, 2005
We have used electron diffraction and neutron powder diffraction to elucidate the structural properties of superconducting Na x CoO 2 ·yD 2 O over a wide compositional range. Our measurements show that superconducting samples exhibit a number of supercells ranging from 1 3 a * to 1 15 a * , but the predominant modulation, observed also in the neutron data, is a double hexagonal cell with dimensions 2a × 2a × c. Rietveld analysis reveals that D 2 O is inserted between CoO 2 sheets to form a layered network of NaO 6 triangular prisms. Our model removes the need to invoke a 5 K superconducting point compound and suggests that a solid solution of Na is possible within a relatively constant amount of water y.
Crystal structure of the superconducting layered cobaltate NaxCoO2·yD2O
Journal of Physics-condensed Matter, 2005
We have used electron diffraction and neutron powder diffraction to elucidate the structural properties of superconducting NaxCoO2·yD2O over a wide compositional range. Our measurements show that superconducting samples exhibit a number of supercells ranging from \frac {1}{3}a^{*} to \frac {1}{15}a^{*} , but the predominant modulation, observed also in the neutron data, is a double hexagonal cell with dimensions 2a × 2a × c. Rietveld analysis reveals that D2O is inserted between CoO2 sheets to form a layered network of NaO6 triangular prisms. Our model removes the need to invoke a 5 K superconducting point compound and suggests that a solid solution of Na is possible within a relatively constant amount of water y.
Co59 NMR Study of the Co States in Superconducting and Anhydrous Cobaltates
Physical Review Letters, 2005
59 Co NMR spectra in oriented powders of the superconducting (HSC) Na0.35CoO2,1.3H2O and Na0.35CoO2 compounds reveal a single electronic Co state with identical T independent NMR shift tensor. These phases differ markedly from Na0.7CoO2, in which we resolve 3 types of Co sites. The large T variation of their spin susceptibilities χs and the anisotropy of the orbital susceptibility χ orb allow us to conclude that charge disproportionation occurs, in a non magnetic Co 3+ and two magnetic sites with about 0.3 and 0.7 holes in the t2g multiplet. The data are consistent with those for the single Co site in the anhydrous and HSC phase assuming the expected Co 3.65+ charge.
The role of doping and dimensionality in the superconductivity of NaxCoO2
arXiv (Cornell University), 2004
We report a complete analysis of the formal Co 3+/4+ oxidation state in Na x CoO 2 , in the interval 0.31 ≤ x ≤ 0.67. Iodometric titration and thermoelectric power confirm that a direct relationship between the Na content and the amount of Co 3+ cannot be established in this system. Creation of a significant amount of oxygen vacancies accompanies Na-ion deintercalation, keeping the formal Co valence at 3.45 + for x ≤ 0.45. To the light of new thermoelectric power data which reveals important differences between the hydrated (superconducting) and non-hydrated (non-superconducting) samples, we propose here that water plays an important "chemical" role beyond that of a spacer between the CoO 2 layers.
Role of Doping and Dimensionality in the Superconductivity of NaxCoO2
Chemistry of Materials, 2005
We report a complete analysis of the formal Co 3+/4+ oxidation state in Na x CoO 2 , in the interval 0.31 ≤ x ≤ 0.67. Iodometric titration and thermoelectric power confirm that a direct relationship between the Na content and the amount of Co 3+ cannot be established in this system. Creation of a significant amount of oxygen vacancies accompanies Na-ion deintercalation, keeping the formal Co valence at 3.45 + for x ≤ 0.45. To the light of new thermoelectric power data which reveals important differences between the hydrated (superconducting) and non-hydrated (non-superconducting) samples, we propose here that water plays an important "chemical" role beyond that of a spacer between the CoO 2 layers.
arXiv (Cornell University), 2004
Muon spin relaxation (µSR) measurements on the new layered cobalt oxide superconductor Na0.35CoO2•1.3H2O and its parent, non-superconducting compounds, have revealed unconventional nature of superconductivity through: (1) a small superfluid energy which implies a surprisingly high effective mass of the charge carriers, approximately 100 times the bare electron mass; (2) the superconducting transition temperature Tc scaling with the superfluid energy following the correlations found in high-Tc cuprate and some other two-dimensional superconductors; (3) an anisotropic pairing without broken time-reversal symmetry; and (4) the proximity of a magnetically ordered insulating phase at Na0.5CoO2 below TN = 53 K.
Kinetic Energy Driven Superconductivity in the Electron Doped Cobaltate Na x CoO 2 · y H 2 O
Communications in Theoretical Physics, 2005
Within the charge-spin separation fermion-spin theory, we have shown that the mechanism of superconductivity in the electron doped cobaltate NaxCoO2 • yH2O is ascribed to its kinetic energy. The dressed fermions interact occurring directly through the kinetic energy by exchanging magnetic excitations. This interaction leads to a net attractive force between dressed fermions, then the electron Cooper pairs originating from the dressed fermion pairing state are due to the charge-spin recombination, and their condensation reveals the superconducting ground state. The superconducting transition temperature is identical to the dressed fermion pair transition temperature, and is suppressed to a lower temperature due to the strong magnetic frustration. The optimal superconducting transition temperature occurs in the electron doping concentration δ ≈ 0.29, and then decreases for both underdoped and overdoped regimes, in qualitative agreement with the experimental results.