| Summary: | The study of the high critical temperature ( T _c ) of hydrogen compounds under high pressure has resulted in a considerable focus on Bardeen–Cooper–Schrieffer superconductors. Nb has the highest T _c among the elemental metals at ambient pressure, so reviewing Nb films again is worthwhile. In this study, we investigated the factors that determine the T _c of Nb films by strain introduction and carrier doping. We deposited Nb films of various thicknesses onto Si substrates and evaluated the T _c variation with thickness. In-plane compressive strain in the (110) plane due to residual stress reduced the T _c . First-principles calculations showed that adjusting the density of states at the Fermi level is key for both strain-induced suppression and doping-induced enhancement of the Nb T _c . The application of hydrostatic pressure compensated for the intrinsic strain of the film and increased its T _c , which could also be enhanced by increasing the hole concentration with an electric double-layer transistor. A liquid electrolyte should be used as a pressure medium for applying hydrostatic pressure to increase the T _c of correlated materials, where this increase results from changes in material structure and carrier concentration.
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