High-T$_{c}$ superconductivity in nanostructured Na$_{x}$WO$_{3-y}$: Sol-gel route

Tungsten trioxide, WO$_{3-y}$ infiltrated into various nanoporous matrix structures such as carbon inverse opal, carbon nanotubes paper, or platinum sponge and then intercalated with alkaline ions (Li$^{+}$, Na$^{+})$ exhibits a pronounced diamagnetic onset in ZFC magnetization in a wide range of temperatures, 125-132 K. Resistivity measurements show non zero jump and intensive fluctuations of electrical resistance below observed transition points. The observed magnetic and electrical anomalies in nanostructured tungsten bronzes (Li$_{x}$WO$_{3-y}$, Na$_{x}$WO$_{3-y})$ suggest the possibility of localized non-percolated superconductivity. The direct evidence of polaron formation from temperature dependence of EPR and photoemission spectra and formation of bipolarons in weakly reduced to WO$_{3-y}$, with 3-y typically in the order of 2.95 suggest bipolarons mechanism of a Bose-Einstein condensation of trapped electron pairs in doped WO$_{3-y}$. On the other hand the strong lattice instabilities in 2D systems like layered cuprates and tungsten bronzes place the upper limit on T$_{c}$. Than, the percolative self-organized mechanism on the metal/insulator interface like Na/WO$_{3}$ and NaWO$_{3}$/nanostructured matrix can facilitate the high T$_{c}$ obtained in sodium bronzes infiltrated into inverted carbon opal or carbon nanotube matricies.