Valence and Na content dependences of superconductivity in NaxCoO2.yH2O

Various samples of sodium cobalt oxyhydrate with relatively large amounts of Na$^{+}$ ions were synthesized by a modified soft-chemical process in which a NaOH aqueous solution was added in the final step of the procedure. From these samples, a superconducting phase diagram was determined for a section of a cobalt valence of $\sim$+3.48, which was compared with a previously obtained one of $\sim$+3.40. The superconductivity was significantly affected by the isovalent exchanger of Na$^{+}$ and H$_{3}$O$^{+}$, rather than by variation of Co valence, suggesting the presence of multiple kinds of Fermi surface. Furthermore, the high-field magnetic susceptibility measurements for one sample up to 30 T indicated an upper critical field much higher than the Pauli limit supporting the validity of the spin-triplet pairing mechanism.Comment: 4 figures and 1 tabl

Planar-type silicon thermoelectric generator with phononic nanostructures for 100 {\mu}W energy harvesting

Energy harvesting is essential for the internet-of-things networks where a tremendous number of sensors require power. Thermoelectric generators (TEGs), especially those based on silicon (Si), are a promising source of clean and sustainable energy for these sensors. However, the reported performance of planar-type Si TEGs never exceeded power factors of 0.1 ${\mu} Wcm^{-2} K^{-2}$ due to the poor thermoelectric performance of Si and the suboptimal design of the devices. Here, we report a planar-type Si TEG with a power factor of 1.3 ${\mu} Wcm^{-2} K^{-2}$ around room temperature. The increase in thermoelectric performance of Si by nanostructuring based on the phonon-glass electron-crystal concept and optimized three-dimensional heat-guiding structures resulted in a significant power factor. In-field testing demonstrated that our Si TEG functions as a 100-${\mu}W$-class harvester. This result is an essential step toward energy harvesting with a low-environmental load and cost-effective material with high throughput, a necessary condition for energy-autonomous sensor nodes for the trillion sensors universe

Pressure-induced anomalous valence crossover in cubic YbCu5-based compounds

A pressure-induced anomalous valence crossover without structural phase transition is observed in archetypal cubic YbCu5 based heavy Fermion systems. The Yb valence is found to decrease with increasing pressure, indicating a pressure-induced crossover from a localized 4f (13) state to the valence fluctuation regime, which is not expected for Yb systems with conventional c-f hybridization. This result further highlights the remarkable singularity of the valence behavior in compressed YbCu5-based compounds. The intermetallics Yb2Pd2Sn, which shows two quantum critical points (QCP) under pressure and has been proposed as a potential candidate for a reentrant Yb(2+) state at high pressure, was also studied for comparison. In this compound, the Yb valence monotonically increases with pressure, disproving a scenario of a reentrant non-magnetic Yb(2+) state at the second QCP

High-resolution photoelectron spectroscopy study of Kondo metals : SmSn3 and Sm0.9La0.1Sn3

We performed a high-resolution photoelectron spectroscopy study on the Kondo metals SmSn3 and Sm0.9La0.1Sn3. The experimental results are compared with calculations of density of state performed within the local density approximation plus the dynamical mean-field theory. The theory is found to reproduce the experimental valence-band spectra well. In both SmSn3 and Sm0.9La0.1Sn3 the bulk Sm valence is nearly trivalent, with a small fraction of divalent component. Resonant photoelectron spectroscopy indicates a decrease in the Kondo effect in the diluted system Sm0.9La0.1Sn3

Electronic structure of Kondo lattice compounds YbNi3X9 (X = Al, Ga) studied by hard x-ray spectroscopy

We have performed hard x-ray photoemission spectroscopy (HAXPES) for Yb-based Kondo lattice compounds; an antiferromagnetic heavy-fermion system YbNi3Al9 and a valence fluctuation system YbNi3Ga9. The Yb 3d5/2 spectra of YbNi3Ga9 showed both Yb2+ and Yb3+-derived structures indicating strong valence fluctuation, and the intensity of Yb2+ (Yb3+) structures gradually increased (decreased) on cooling. The Yb 3d5/2 spectra of YbNi3Al9 mostly consisted of Yb3+-derived structures and showed little temperature dependence. The Yb valences of YbNi3Ga9 and YbNi3Al9 at 22 K were evaluated to be 2.43 and 2.97, respectively. Based on the results of the Ni 2p and valence-band HAXPES spectra together with soft x-ray valence-band spectra, we described that the difference of physical properties of YbNi3X9 (X= Al, Ga) is derived from the differences of the 4f-hole level relative to the Fermi level (EF) and Ni 3d density of states at EF. The HAXPES results on the Yb valences were consistent with those obtained by x-ray absorption spectroscopy using the partial fluorescence yield mode and resonant x-ray emission spectroscopy at the Yb L3 edge

Microstructure analysis and thermoelectric properties of iron doped CuGaTe2

Chalcopyrite related compounds have attracted much attention in recent years due to their promising thermoelectric properties. In this research we report Fe doping in chalcopyrite-type CuGaTe2 and its influence on structural and thermal transport properties. We synthesized polycrystalline samples with composition CuGa1-xFexTe2 with x = 0.0 to 0.05 by spark plasma sintering method. For structural analysis powder X-ray diffraction and electron probe micro analysis were employed. Solubility of Fe in CuGaTe2 was found to be very small, and other phases like FeTe2 and CuTe were identified. Thermal conductivity showed a significant decrease with the addition of Fe up to x = 0.02, which started to increase for x ≥ 0.03. On the other hand, the addition of Fe caused slight increase in the power factor from 1.3 mW/K2m for x = 0.0 to 1.6 mW/K2m for x = 0.02 at T = 770 K. As a result, ZT peak value of 0.92 is recorded for x = 0.02 at 870 K, which corresponds to an enhancement of 60% from that of non-doped CuGaTe2. This work demonstrates that thermoelectric properties of composite materials can be greatly improved by controlling its microstructure. Keywords: Thermoelectric, CuGaTe2, Microstructure, Seebeck coefficient, Thermal conductivity, Composite materia

The low and high temperature thermoelectric properties of Yb3Si5

Silicides have been of great interest for thermoelectric applications due to their abundant elements as well as thermal and chemical stability. In this paper, we examined the thermoelectric properties of Yb _3 Si _5 polycrystalline samples in a wide temperature range from 10 to 800 K. The temperature dependence of the Seebeck coefficient was successfully analyzed by assuming a narrow 4f quasi-particle band, indicating the intermediate valence state of Yb ^2+ -Yb ^3+ is responsible for the high power factor. A very large maximum power factor of ∼ 4.70 mWm ^−1 K ^−2 was observed at 72 K and room temperature value ∼ 1.56 mWm ^−1 K ^−2 for Yb _3 Si _5 . These results shows that Yb-Si compounds have large potential to be used as low temperature TE applications in the future. We also studied the Co-doping effect in Yb _3 Si _5 , namely, Yb _3 Co _x Si _5− _x where x  = 0, 0.1, 0.15, 0.20 and investigated their thermoelectric properties. While powder X-ray diffraction analysis confirmed all main peaks indexed to Yb _3 Si _5 phase, SEM and EDX analyses revealed that Co is precipitated as metal particles, forming a composite material with Yb _3 Si _5 phase. Thermoelectric properties of the Co-doped samples are also reported