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Register a new userEnhancement of the effective disorder potential and the thermopower in Na$_x$CoO$_2$ through the electron-phonon coupling
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Added by
Giorgio Sangiovanni
Publication date
2012
fields
Physics
and research's language is
English
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The effects of an electron-phonon ($e$-ph) interaction on the thermoelectric properties of Na$_x$CoO$_2$ are analyzed. By means of dynamical mean field theory calculations we find that the $e$-ph coupling acts in a cooperative way with the disorder, enhancing the effective binary disorder potential strength on the Co sites, which stems from the presence or absence of a neighboring Na atom. Hence, the inclusion of the $e$-ph coupling allows us to assume smaller values of the binary disorder potential strength -- which for Na$_x$CoO$_2$ are in fact also more reasonable. More generally, we can conclude that the interplay between disorder effects and coupling to the lattice can be exploited to engineer more efficient thermoelectric materials.
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Magnetism of layered cobaltites Na$_x$CoO$_2$ with $x$ = 0.6 and 0.9 has been investigated by a positive muon spin rotation and relaxation ($mu^+$SR) spectroscopy together with magnetic susceptibility and specific heat measurements, using single crystal samples in the temperature range between 250 and 1.8 K. Zero-field (ZF-) $mu^+$SR measurements on Na$_{0.9}$CoO$_2$ indicates a transition from a paramagnetic to an incommensurate spin density wave state at 19 K(=$T_{sf SDW}$). The anisotropic ZF-$mu^+$SR spectra suggest that the oscillating moments of the {sf IC-SDW} directs along the c-axis. Since Na$_{0.6}$CoO$_2$ is paramagnetic down to 1.8 K, the magnitude of $T_{sf SDW}$ is found to strongly depend on $x$.This behavior is well explained using the Hubbard model within a mean field approximation on two-dimensional triangle lattice in the CoO$_2$ plane. Also, both the appearance of the {sf IC-SDW} state by the change in $x$ and the magnitude of the electronic specific heat parameter of Na$_{0.6}$CoO$_2$ indicate that Na$_x$CoO$_2$ is unlikely to be a typical strongly correlated electron system.
We propose a minimal model resolving a puzzle of enigmatic correlations observed in sodium-rich Na$_x$CoO$_2$ where one expects a simple, free motion of the dilute $S=1/2$ holes doped into a band insulator NaCoO$_2$. The model also predicts singlet superconductivity at experimentally observed compositions. The model is based on a key property of cobalt oxides -- the spin-state quasidegeneracy of CoO$_6$ octahedral complex -- leading to an unusual physics of, {it e.g.}, LaCoO$_3$. We show that correlated hopping between $t_{2g}$ and $e_g$ states leads to the spin-polaron physics at $xsim 1$, and to an extended s-wave pairing at larger doping when coherent fermionic bands are formed.
Band structure of metallic sodium cobaltate Na$_x$CoO$_2$ ($x$=0.33, 0.48, 0.61 0.72) has been investigated by local density approximation+Hubbard $U$ (LDA+$U$) method and within Gutzwiller approximation for the Co-$t_{2g}$ manifold. Correlation effects being taken into account results in suppression of the $e_g$ hole pockets at the Fermi surface in agreement with recent angle-resolved photo-emission spectroscopy (ARPES) experiments. In the Gutzwiller approximation the bilayer splitting is significantly reduced due to the correlation effects. The formation of high spin (HS) state in Co $d$-shell was shown to be very improbable.
The idea that surface effects may play an important role in suppressing $e_g$ Fermi surface pockets on Na$_x$CoO$_2$ $(0.333 le x le 0.75)$ has been frequently proposed to explain the discrepancy between LDA calculations (performed on the bulk compound) which find $e_g$ hole pockets present and ARPES experiments, which do not observe the hole pockets. Since ARPES is a surface sensitive technique it is important to investigate the effects that surface formation will have on the electronic structure of Na$_{1/3}$CoO$_2$ in order to more accurately compare theory and experiment. We have calculated the band structure and Fermi surface of cleaved Na$_{1/3}$CoO$_2$ and determined that the surface non-trivially affects the fermiology in comparison to the bulk. Additionally, we examine the likelihood of possible hydroxyl cotamination and surface termination. Our results show that a combination of surface formation and contamination effects could resolve the ongoing controversy between ARPES experiments and theory.
We report high-resolution inelastic x-ray measurements of the soft phonon mode in the charge-density-wave compound TiSe$_2$. We observe a complete softening of a transverse optic phonon at the L point, i.e. q = (0.5, 0, 0.5), at T ~ T_{CDW}. Renormalized phonon energies are observed over a large wavevector range $(0.3, 0, 0.5) le mathbf{q} le (0.5, 0, 0.5)$. Detailed ab-initio calculations for the electronic and lattice dynamical properties of TiSe2 are in quantitative agreement with experimental frequencies for the phonon branch involving the soft mode. The observed broad range of renormalized phonon frequencies is directly related to a broad peak in the electronic susceptibility stabilizing the charge-density-wave ordered state. Our analysis demonstrates that a conventional electron-phonon coupling mechanism can explain a structural instability and the charge-density-wave order in TiSe_2 although other mechanisms might further boost the transition temperature.
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