Specific heat study of the Na0.3CoO2·1.3H2O superconductor: influence of the complex chemistry (original) (raw)
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Physica C: Superconductivity, 2004
The heat capacity of superconducting Na x CoO 2 ·yH 2 O was measured and the data are discussed based on two different models: The BCS theory and a model including the effects of line nodes in the superconducting gap function. The electronic heat capacity is separated from the lattice contribution in a thermodynamically consistent way maintaining the entropy balance of superconducting and normal states at the critical temperature. It is shown that for a fully gapped superconductor the data can only be explained by a reduced (≈ 50 %) superconducting volume fraction. The data are compatible with 100 % superconductivity in the case where line nodes are present in the superconducting gap function.
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.
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.
Specific Heat and Superconductivity in (La1-xCax)2CuO4
Japanese Journal of Applied Physics, 1987
Specific heat of (La0.925Ca0.075)2CuO4 was measured by the thermal relaxation method both under zero magnetic field and under H=5.7T. The discontinuity in the specific heat C was clearly observed at the superconducting transition temperature T c (∼18 K). The electronic specific heat coefficient γ value, estimated from the discontinuity ΔC, was about 20 mJ/(mol·K2).
Magnetism, Specific Heat, and Surface Topography of Oxide Superconductors
MRS Proceedings, 1987
Samples of the composition AfBa 2 Cu 3 07_x with N = Y, Eu, Er, and Gd have been prepared by the standard solid state reaction method. The magnetic susceptibility has been investigated above room temperature up to 1870 K in vacuum. Specific heat measurements have been performed with a differential scanning calorimeter (DSC) working at low temperatures in a differential mode using superconducting and non-superconducting samples. The surface topography of the high-Tc superconductors has been investigated at room temperature by scanning tunneling microscopy (STM).
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 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.
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.