Role of Doping and Dimensionality in the Superconductivity of NaxCoO2 (original) (raw)
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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.
Revised Superconducting Phase Diagram of Hole-Doped Nax(H3O)zCoO2·yH2O
Physical Review Letters, 2004
We have studied the superconducting phase diagram of Na x CoO 2 yH 2 O 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 H 3 O ions into Na x CoO 2 , together with water, acts as an additional dopant, indicating that Na substoichiometry alone does not control the electronic doping of these materials. We find a superconducting phase diagram where optimal T C is achieved through a Co valence range of 3.24 -3.35, while T C decreases for materials with a higher Co valence. The critical role of dimensionality in achieving superconductivity is highlighted by similarly doped nonsuperconducting anhydrous samples, differing from the superconducting hydrate only in interlayer spacing.
Structure and dynamics of superconducting NaxCoO2 hydrate and its unhydrated analog
Physical Review B, 2003
Neutron scattering has been used to investigate the crystal structure and lattice dynamics of superconducting Na 0.3 CoO 2 •1.4(H/D) 2 O, and the "parent" Na 0.3 CoO 2 material. The structure of Na 0.3 CoO 2 consists of alternate layers of CoO 2 and Na and is the same as the structure at higher Na concentrations. For the superconductor, the water forms two additional layers between the Na and CoO 2 , increasing the c-axis lattice parameter of the hexagonal P6 3 /mmc space group from 11.16 Å to 19.5 Å. The Na ions are found to occupy a different configuration from the parent compound, while the water forms a structure that replicates the structure of ice. Both types of sites are only partially occupied. The CoO 2 layer in these structures is robust, on the other hand, and we find a strong inverse correlation between the CoO 2 layer thickness and the superconducting transition temperature (T C increases with decreasing thickness). The phonon density-ofstates for Na 0.3 CoO 2 exhibits distinct acoustic and optic bands, with a high-energy cutoff of ~100 meV. The lattice dynamical scattering for the superconductor is dominated by the hydrogen modes, with librational and bending modes that are quite similar to ice, supporting the structural model that the water intercalates and forms ice-like layers in the superconductor.
Physical Review Letters, 2004
Superconducting single crystal samples of NaxCoO2 · yH2O have been produced using an electrochemical technique which dispenses with the usual bromine chemical de-intercalation step and permits much more precise control of the Na content. After careful hydration, nearly single-phase crystals have been obtained in which over 90% of the sample's volume corresponds to the superconducting Na0.3CoO2 · 1.3H2O structure. Susceptibility and specific heat measurements confirm that bulk superconductivity has been achieved. The extracted normal state density of states indicates Fermi-liquid behavior with strong mass enhancement and a modest Wilson ratio. Measurements of Hc2 for H c and H ab reveal significant anisotropy. The estimated value of Hc2 for H c yields a coherence length of ∼ 100Å, consistent with an extremely narrow bandwidth.
Specific heat study of the Na0.3CoO2·1.3H2O superconductor: influence of the complex chemistry
Physica C: Superconductivity, 2004
We report results of specific heat measurements on polycrystalline samples of the layered superconductor, Na 0.3 CoO 2 ⋅1.3H 2 O. The electronic contribution to the specific heat, γ, is found to be 12.2 mJ/mol-K 2 . The feature at the superconducting transition is rather sharp, becoming broad and strongly suppressed in an applied magnetic field. The data indicate a residual normal state electronic specific heat at low temperatures, implying that there is a sizable population of normal state electrons in the samples even below T c . Inhomogeneity in the Na content, to which the superconducting state is exquisitely sensitive, appears to be the most likely explanation for these results. These results further indicate that special sample handling is required for an accurate characterization of the superconducting state in this material.
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.
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.
Observation of Bulk Superconductivity in NaxCoO2·yH2O and NaxCoO2·yD2O Powder and Single Crystals
Physical Review Letters, 2003
Poly-and single-crystalline NaxCoO2 has been successfully intercalated with H2O and D2O as confirmed by X-ray diffraction and thermogravimetric analysis. Resistivity, magnetic susceptibility, and specific heat measurements show bulk superconductivity with Tc close to 5 K in both cases. The substitution of deuterium for hydrogen has an effect on Tc of less than 0.2 K. Investigation of the resistivity anisotropy of NaxCoO2·yH2O single crystals shows: (a) almost zero resistivity below Tc, and (b) an abrupt upturn at T * ∼ 52 K in both the ab plane and the c direction. The implications of our results on the possible superconducting mechanism will be discussed.