X-ray absorption and optical spectroscopy studies of (Mg1−x_{1-x}Alx_x)B2_2

X-ray absorption spectroscopy and optical reflectance measurements have been carried out to elucidate the evolution of the electronic structure in (Mg$_{1-x}$Al$_{x}$)B$_{2}$ for $\emph{x}$ = 0.0,0.1, 0.2, 0.3, and 0.4. The important role of B 2$\emph{p}$ $\sigma$ hole states to superconductivity has been identified, and the decrease in the hole carrier number is $\emph{quantitatively}$ determined. The rate of the decrease in the hole concentration agree well with the theoretical calculations. On the other hand,while the evolution of the electronic structure is gradual through the doping range, $T_c$ suppression is most significant at $\emph{x}$ = 0.4. These results suggest that the superstructure in (Mg$_{1-x}$Al$_{x}$)B$_{2}$, in addition to the $\sigma$ holes, can affect the lattice dynamics and contributes to the $T_c$ suppression effect. Other possible explanations like the topological change of the $\sigma$ band Fermi surface are also discussed.Comment: 17 pages, 5 figures. Phys. Rev. B, in pres

Unlocking microwatt power: enhanced performance of Fe–V–Al thin films in thermoelectric microgenerators

Microwatt power output was obtained in thermoelectric microgenerators based on cost-effective and non-toxic Fe–V–Al thin films deposited by a DC magnetron co-sputtering process. A maximum electrical power of almost 5 μW at a temperature difference of 134 K was measured. This result leads to a maximum power density of 58.5 ± 6 mW cm−2, which is among the highest values obtained by a microdevice. Contrary to what is observed for other thermoelectric materials like Bi2Te3 or PEDOT composites, the performances of the present devices, assembled with junctions between Fe–V–Al and aluminum electrodes, are weakly impacted by their contact resistance. These results are very encouraging for the development of new architectures based on low-resistance Fe–V–Al thin films to power autonomous sensors in the Internet of Thing domain