Enhancement of superconductivity in thin films of Sn under high pressure

We investigated the pressure effects of superconductivity on thin films of Sn. The elemental superconductor Sn with a body-centered-tetragonal structure, $\ensuremath{\beta}$-Sn, exhibits superconductivity below the superconducting transition temperature (${T}_{\mathrm{c}}=3.72$ K) at ambient pressure. ${T}_{\mathrm{c}}$ of Sn increases with lowering dimension such as in thin-film and nanowire growth, or by high-pressure application. For thin films, ${T}_{\mathrm{c}}$ exhibits a slight increase up to approximately 4 K compared to the bulk value, attributable to the crystalline size and lattice disorder. By applying pressure on a bulk Sn, ${T}_{\mathrm{c}}$ initially decreases from 3.72 K as the pressure increases. Further increasing pressure up to 10 GPa, ${T}_{\mathrm{c}}$ increases to 5.3 K with the structural transformation. However, the combination of these effects on thin films of Sn, namely, thin-film growth and pressure effects, remains underexplored. In this paper, we combined film-growth and pressure-application techniques to further increase ${T}_{\mathrm{c}}$ using a diamond anvil cell with boron-doped diamond electrodes. The drop of electrical resistance suggesting the onset of ${T}_{\mathrm{c}}$ on the thin film reached above 6 K in the $\ensuremath{\gamma}$-Sn phase. Further, the upper critical magnetic field was drastically enhanced. Atomic force microscopy suggests that the refinement of the grain size of the thin film under nonhydrostatic pressure conditions contributes to stabilizing the higher ${T}_{\mathrm{c}}$ of $\ensuremath{\gamma}$-Sn.