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Register a new userCrystal-to-stripe reordering of sodium ions in NaxCoO2 (x>=0.75)
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The sodium reordering in NaxCoO2 in the vicinity of room temperature is rationalized at high x in terms of phase transitions between square and striped phases. A striking hexagon-of-hexagons diffraction pattern observed for x=0.78 can be reproduced using coexisting square and striped phases that are related by simple shear deformations. All compositions exhibit a partial melting transition to a disordered stripe phase just below room temperature, which alters the topology of the electrical conduction pathways.
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NaxCoO2 has emerged as a material of exceptional scientific interest due to the potential for thermoelectric applications, and because the strong interplay between the magnetic and superconducting properties has led to close comparisons with the physics of the high-Tc cuprates. The density, x, of the sodium in the intercalation layers can be altered electrochemically, directly changing the number of conduction electrons on triangular Co layers. Recent electron diffraction measurements reveal a kaleidoscope of Na+ ion patterns as a function of concentration. Here we use single-crystal neutron diffraction supported by numerical simulations to determine the long-range three-dimensional superstructures of these ions. We show that the sodium ordering and its associated distortion field are governed by pure electrostatics, and the organizational principle is the stabilization of charge droplets that order long range at some simple fractional fillings. Our results provide a good starting point to understand the electronic properties in terms of a Hubbard Hamiltonian taking into account the electrostatic potential from the Na superstructures. The resulting depth of potential wells in the Co layer is greater than the single-particle hopping kinetic energy. As a consequence, holes occupy preferentially the lowest potential regions and, therefore, the Na+ ion patterning plays a decisive role in the transport and magnetic properties.
We report a systematic study of the $c$ lattice parameter in the Na$_{x}$CoO$_{2}$ phases versus Na content $x>0.5$, in which sodium always displays ordered arrangements. This allows us to single out the first phase which exhibits an AF magnetic order at a Neel temperature $T_{N}=$22 K which is found to occur for $xapprox 0.77(1)$. Pure samples of this phase have been studied both as aligned powders and single crystals. They exhibit identical $^{23}$Na NMR spectra in which three sets of Na sites could be fully resolved, and are found to display $T$ dependencies of their NMR shifts which scale with each other. This allows us to establish that the $T$ variation of the shifts is due to the paramagnetism of the Co sites with formal charge state larger than 3$^{+}$. The existence of a sodium site with axial charge symmetry and the intensity ratio between the sets of $^{23}$Na lines permits us to reveal that the 2D structure of the Na order corresponds to 10 Na sites on top of a 13 Co sites unit cell, that is with $x=10/13approx 0.77$. This structure fits with that determined from local density calculations and involves triangles of 3 Na sites located on top of Co sites (so called Na1 sites). The associated ordering of the Na vacancies is quite distinct from that found for $x<0.75$.
The results of DC magnetization measurements under hydrostatic (helium-gas) pressure are reported for an ambient pressure superconductor Na0.35CoO2.1.4D2O and its precursor compound, the gamma-phase Na0.75CoO2 that is known to combine a metallic conductivity with an unusual magnetic state below ~22K. The obtained data allowed us to present for the first time the pressure dependence of the magnetic transition in a metallic sodium cobaltate system. This dependence appears to be positive, with the magnetic transition rapidly shifting towards higher temperatures when an applied pressure increases. We ascribe the observed effect to the pressure-induced enhancement of the out-of-plane antiferromagnetic coupling mediated by localized spins interactions (of either superexchange or RKKY type), the scenario consistent with the A-type antiferromagnetic state suggested by recent neutron-scattering data. As for the pressure effect on the superconductivity in Na0.35CoO2.1.4D2O, our measurements established negative and linear for the entire pressure range from 1 bar to 8.3 kbar pressure dependence of Tc, the behavior quite different from the reported by previous workers strong non-linearity of the Tc (P) dependence. (Dated September 12, 2005) PACS numbers: 74.62.Fj, 74.70.-b, 75.20. En, 75.50 Ee, 75.30 Kz.
Recent studies exposed many remarkable properties of layered cobaltates NaxCoO2. Surprisingly, many-body effects have been found to increase at sodium-rich compositions of NaxCoO2 where one expects a simple, nearly free motion of the dilute S=1/2 holes doped into a band insulator NaCoO2. Here we discuss the origin of enigmatic correlations that turn a doped NaCoO2 into a strongly correlated electronic system. A minimal model including orbital degeneracy is proposed and its predictions are discussed. The model is based on a key property of cobalt oxides - the spin-state quasidegeneracy of CoO6 octahedral complex - which has been known, e.g., in the context of an unusual physics of LaCoO3 compound. Another important ingredient of the model is the 90-degree Co-O-Co bonding in NaxCoO2 which allows nearest-neighbor $t_{2g}-e_g$ hopping. This hopping introduces a dynamical mixture of electronic configurations $t_{2g}^6, S=0$ and $t_{2g}^5e_g^1, S=1$ of neighboring cobalt ions. We show that scattering of charge carriers on spin-state fluctuations suppresses their coherent motion and leads to the spin-polaron physics at $xsim 1$. At larger doping when coherent fermionic bands are formed, the model predicts singlet superconductivity of extended s-wave symmetry. The presence of low-lying spin states of Co$^{3+}$ is essential for the pairing mechanism. Implications of the model for magnetic orderings are also discussed.
We prove the direct link between low temperature magnetism and high temperature sodium ordering in NaxCoO2 using the example of a heretofore unreported magnetic transition at 8 K which involves a weak ferromagnetic moment. The 8 K feature is characterized in detail and its dependence on a diffusive sodium rearrangement around 200 K is demonstrated. Applying muons as local probes this process is shown to result in a reversible phase separation into distinct magnetic phases that can be controlled by specific cooling protocols. Thus the impact of ordered sodium Coulomb potential on the CoO2 physics is evidenced opening new ways to experimentally revisit the NaxCoO2 phase diagram.
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