No Arabic abstract
Thermoelectric properties of the layered cobalt oxide system LixCoO2 were investigated in a wide range of Li composition, 0.98 >= x >= 0.35. Single-phase bulk samples of LixCoO2 were successfully obtained through electrochemical deintercalation of Li from the pristine LiCoO2 phase. While LixCoO2 with x >= 0.94 is semiconductive, the highly Li-deficient phase (0.75 >= x >= 0.35) exhibits metallic conductivity. The magnitude of Seebeck coefficient at 293 K (S293K) significantly depends on the Li content (x). The S293K value is as large as +70 ~ +100 uV/K for x >= 0.94, and it rapidly decreases from +90 uV/K to +10 uV/K as x is lowered within a Li composition range of 0.75 >= x >= 0.50. This behavior is in sharp contrast to the results of x <= 0.40 for which the S293K value is small and independent of x (+10 uV/K), indicating that a discontinuous change in the thermoelectric characteristics takes place at x = 0.40 ~ 0.50. The unusually large Seebeck coefficient and metallic conductivity are found to coexist in a narrow range of Li composition at about x = 0.75. The coexistence, which leads to an enhanced thermoelectric power factor, may be attributed to unusual electronic structure of the two-dimensional CoO2 block.
We investigate the low temperature magnetic field dependence of the resistivity in the thermoelectric misfit cobalt oxide [Bi1.7Ca2O4]0.59CoO2 from 60 K down to 3 K. The scaling of the negative magnetoresistance demonstrates a spin dependent transport mechanism due to a strong Hunds coupling. The inferred microscopic description implies dual electronic states which explain the coexistence between localized and itinerant electrons both contributing to the thermopower. By shedding a new light on the electronic states which lead to a high thermopower, this result likely provides a new potential way to optimize the thermoelectric properties.
We report infrared spectroscopic properties of the strongly correlated layered cobalt oxide [BiBa$_{0.66}$K$_{0.36}$O$_2$]CoO$_2$. These measurements performed on single crystals allow us to determine the optical conductivity as a function of temperature. In addition to a large temperature dependent transfer of spectral weight, an unconventional low energy mode is found. We show that both its frequency and damping scale as the temperature itself. In fact, a basic analysis demonstrates that this mode fully scales onto a function of $omega$/T up to room temperature. This behavior suggests low energy excitations of non-Fermi liquid type originating from quantum criticality.
We show that important anomalous features of the normal-state thermoelectric power S of high-Tc materials can be understood as being caused by doping dependent short-range antiferromagnetic correlations. The theory is based on the fluctuation-exchange approximation applied to Hubbard model in the framework of the Kubo formalism. Firstly, the characteristic maximum of S as function of temperature can be explained by the anomalous momentum dependence of the single-particle scattering rate. Secondly, we discuss the role of the actual Fermi surface shape for the occurrence of a sign change of S as a function of temperature and doping.
Misfit-layered (ML) cobalt oxides of the general formula of [MmA2Om+2]qCoO2 have been proven to be efficient thermoelectric materials as the structure is capable in accommodating the two seemingly contradictory characteristics of high electrical conductivity and large thermo-electric power. They are also potential hosts for other oxymoron-like functions. The known phases all contain one or two square-planar MO (M = Co, Bi, Pb, Tl, etc.) layers sandwiched together with AO (A = Ca, Sr, Ba, etc.) planes of square symmetry and CoO2 layers of hexagonal symmetry. Here we report realization of the simplest (m = 0) ML phase forming in the Sr-Co-O system with the cation ratio, Sr/Co = 1. Atomic-resolution TEM imaging confirms for the new phase the parent three-layer crystal structure, SrO-SrO-CoO2, which is compatible with the formula of [Sr2O2]qCoO2. Electron diffraction reveals that the phase is rather commensurate, i.e. the misfit parameter q is 0.5. Nevertheless, in terms of the transport-property characteristics the new ML parent is comparable to its earlier-established and more complex derivatives.
We report details of single crystal growth of stoichiometric bismuthide PtBi$_2$ whose structure consists of alternate stacking of Pt layer sandwiched by Bi bilayer along the $c$-axis. The compound crystallizes in space group P-3 with a hexagonal unit cell of $a$=$b$=6.553$AA$, $c$=6.165$AA$. The magnetization data show opposite sign for fields parallel and perpendicular to the Pt layers, respectively. The $T$-dependent resistivity is typical of a metal and the magnetic response shows clear two types of charge carriers and the validity of the semi-classical Kohlers rule. Its physical properties was discussed in comparison with recently proposed topological superconductor $beta$-PdBi$_2$.