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Register a new userThe Role of hybridization in NaxCoO2 and the Effect of Hydration
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Density functional theory (DFT) within the local density approximation (LDA) is used to understand the electronic properties of Na1/3CoO2 and Na1/3CoO2(H2O)4/3, which was recently found to be superconducting1. Comparing the LDA charge density of CoO2 and the Na doped phases indicates that doping does not simply add electrons to the t2g states. In fact, the electron added in the t2g state is dressed by hole density in the eg state and electron density in the oxygen states via rehybridization. In order to fully understand this phenomenon, a simple extension of the Hubbard Hamiltonian is proposed and solved using the dynamical mean-field theory (DMFT). This simple model confirms that the rehybridization is driven by a competition between the on-site coulomb interaction and the hybridization. In addition, we find that the presence of eg-oxygen hybridization effectively screens the low energy excitations. To address the role that water plays in creating the superconducting state, we compare the LDA band structure of Na1/3CoO2 and its hydrated counterpart. This demonstrates that hydration does cause the electronic structure to become more two-dimensional.
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Crystal structures of a series of La1-xCexIn3 (x = 0.02, 0.2, 0.5, or 0.8) intermetallic compounds have been investigated by both neutron and X-ray diffraction, and their physical properties have been characterized by magnetic susceptibility and specific heat measurements. Our results emphasize atypical atomic displacement parameters (ADP) for the In and the rare-earth sites. Depending on the x value, the In ADP presents either an ellipsoidal elongation (La-rich compounds) or a butterfly-like distortion (Ce-rich compounds). These deformations have been understood by theoretical techniques based on the band theory and are the result of hybridization between conduction electrons and 4f-electrons.
In heavy-fermion compounds, f electrons show both itinerant and localized behaviour depending on the external conditions, and the hybridization between localized f electrons and itinerant conduction bands gives rise to their exotic properties like heavy-fermions, magnetic orders and unconventional superconductivity. Duo to the risk of handling radioactive actinide materials, the direct experimental evidence of the band structure evolution across the localized-itinerant and magnetic transitions for 5f electrons is lacking. Here, by using angle-resolved photoelectron spectroscopy, we revealed the dual nature (localized vs itinerant) and the development of two different kinds of heavy quasi-particle bands of 5f electrons in antiferromagnetic (AFM) USb2. Partially opened energy gaps were observed on one quasi-particle 5f band cross the AFM transition around 203 K, indicating that the magnetic orders in USb2 are of spin density wave (SDW) type similar to Cr. The localized 5f electrons and itinerant conduction bands hybridize to form another heavy quasi-particle band at about 120 K, and then open hybridization gaps at even lower temperature. Our results provide direct spectral demonstration of the localized-itinerant transition, hybridization and SDW transition of 5f electrons for uranium-based materials.
Results of thermo-electric power (S) and electrical resistivity (r) measurements are reported on NaxCoO2 compounds with x = 1.0, 0.7 and 0.6. These are single-phase compounds crystallizing in the hexagonal structure (space group P63/mmc) at room temperature. Thermo-electric power values at 300K (S300K) are, 80mV/K, 39mV/K and 37mV/K for x = 1.0, 0.7 and 0.6 samples, respectively. The samples with x=0.7 and 1.0 are metallic down to 5 K, while the x = 0.6 sample is semiconducting. The value of r300K for x = 1.0 sample is ~0.895 mW-cm and the power factor (S2/r) is = 7.04 x 10-3 W/mK2 which qualifies it as a good thermo-electric material. In x =1.0 sample, S(T) is positive throughout 300-5K temperature range and decreases monotonically to zero as temperature T= 0. In contrast, S(T) of x = 0.7 and 0.6 samples changes sign and shows negative values between 90 K and 16 K before approaching zero as T = 0. Anomalous S(T) behavior of x = 0.6 and 0.7 samples, which are coincidentally the precursor materials to the reported superconductivity in this class of materials, indicates a dramatic change in the electronic structure of these compounds on lowering the Na content.
We have investigated a set of sodium cobaltates (NaxCoO2) samples with various sodium content (0.67 le x le 0.75) using Nuclear Quadrupole Resonance (NQR). The four different stable phases and an intermediate one have been recognized. The NQR spectra of 59Co allowed us to clearly differentiate the pure phase samples which could be easily distinguished from multi-phase samples. Moreover, we have found that keeping samples at room temperature in contact with humid air leads to destruction of the phase purity and loss of sodium content. The high sodium content sample evolves progressively into a mixture of the detected stable phases until it reaches the x=2/3 composition which appears to be the most stable phase in this part of phase diagram.
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 $x<0.75$.
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