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تسجيل مستخدم جديدThe structure of intercalated water in superconducting Na$_{0.35}$CoO$_{2}cdot$1.37D$_{2}$O: Implications for the superconducting phase diagram
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D. N. Argyriou
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We have used electron and neutron powder diffraction to elucidate the structural properties of superconducting NaD. Our measurements show that our superconducting sample exhbits a number of supercells ranging from ${1/3}a^{*}$ to ${1/15}a^{*}$, but the most predominant one, observed also in the neutron data, is a double hexagonal cell with dimensions dhx. Rietveld analysis reveals that deutspace is inserted between CoO$_{2}$ sheets as to form a layered network of NaO$_{6}$ triangular prisms. Our model removes the need to invoke a 5K superconducting point compound and suggests that a solid solution of Na is possible within a constant amount of water $y$.
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We report the in-plane resistivity and magnetic susceptibility of the layered cobalt oxide Na$_{0.35}$CoO$_{2}{cdot}1.3$H$_{2}$O single crystal. The temperature dependence of the resistivity shows metallic behavior from room temperature to the superc
onducting transition temperature $T_{c}$ of 4.5 K. Sharp resistive transition, zero resistivity and almost perfect superconducting volume fraction below $T_{c}$ indicate the good quality and the bulk superconductivity of the single crystal. The upper critical field $H_{c2}$ and the coherence length $xi$ are obtained from the resistive transitions in magnetic field parallel to the c-axis and the $ab$-plane. The anisotropy of $xi$, $xi_{ab} / xi_{c} =$ 12 nm/1.3 nm $simeq$ 9.2, suggests that this material is considered to be an anisotropic three dimensional superconductor. In the field parallel to the $ab$-plane, $H_{c2}$ seems to be suppressed to the value of Pauli paramagnetic limit. It may indicate the spin singlet superconductivity in the cobalt oxide.
We have studied the superconducting phase diagram of NaHspace 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 oxpspace ions into Na$_{x}$CoO$_{2}$, 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 Tcspace is achieved through a Co valence range of 3.24 - 3.35, while Tcspace 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 CoO$_{2}$ sheets as Co valence is varied into the optimal Tcspace region is further evidence for this criticality.
A weak magnetic order was found in a non-superconducting bilayered-hydrate Na$_{x}$CoO$_{2}cdot y$H$_{2}$O sample by a Co Nuclear Quadrupole Resonance (NQR) measurement. The nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ shows a
prominent peak at 5.5 K, below which a Co-NQR peak splits due to an internal field at the Co site. From analyses of the Co NQR spectrum at 1.5 K, the internal field is evaluated to be $sim$ 300 Oe and is in the $ab$-plane. The magnitude of the internal field suggests that the ordered moment is as small as $sim 0.015$ $mu_B$ using the hyperfine coupling constant reported previously. It is shown that the NQR frequency $ u_Q$ correlates with magnetic fluctuations from measurements of NQR spectra and $1/T_1T$ in various samples. The higher-$ u_Q$ sample has the stronger magnetic fluctuations. A possible phase diagram in Na$_{x}$CoO$_{2}cdot y$H$_{2}$O is depicted using $T_c$ and $ u_Q$, in which the crystal distortion along the c-axis of the tilted CoO$_2$ octahedron is considered to be a physical parameter. Superconductivity with the highest $T_c$ is seemingly observed in the vicinity of the magnetic phase, suggesting strongly that the magnetic fluctuations play an important role for the occurrence of the superconductivity.
In order to investigate the role of the water molecules in Na$_{0.35}$CoO$_{2}cdot$1.3H$_{2}$O, we synthesized superconducting Na$_{0.35}$CoO$_{2}cdot$1.3H$_{2}$O and nonsuperconducting Na$_{0.35}$CoO$_{2}cdot$0.7H$_{2}$O, and measured their normal-s
tate magnetic susceptibilities. The susceptibility of Na$_{0.35}$CoO$_{2}cdot$1.3H$_{2}$O has an enhancement below ~150 K probably caused by ferromagnetic fluctuation, whereas no such enhancement was observed in Na$_{0.35}$CoO$_{2}cdot$0.7H$_{2}$O. The water molecules in Na$_{0.35}$CoO$_{2}cdot$1.3H$_{2}$O may work to shield random coulomb potential of the Na ions with smoother potential at the CoO$_{2}$ layer. This effect may account for the appearance of superconductivity in Na$_{0.35}$CoO$_{2}cdot$1.3H$_{2}$O.
High-energy (h$ u$ = 5.95 keV) synchrotron Photoemission spectroscopy (PES) is used to study bulk electronic structure of Na$_{0.35}$CoO$_{2}$.1.3H$_{2}$O, the layered superconductor. In contrast to 3-dimensional doped Co oxides, Co $it{2p}$ core lev
el spectra show well-separated Co$^{3+}$ and Co$^{4+}$ ions. Cluster calculations suggest low spin Co$^{3+}$ and Co$^{4+}$ character, and a moderate on-site Coulomb correlation energy U$_{dd}sim$3-5.5 eV. Photon dependent valence band PES identifies Co $it{3d}$ and O $it{2p}$ derived states, in near agreement with band structure calculations.
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