We report electrical resistivity measurements on a high-quality
Ba1βxβKxβFe2βAs2β thin film (x=0.4) under pressure. The
superconducting transition temperature (=39.95 K) of the optimally-doped thin
film shows a dome shape with pressure, reaching a maximal value 40.8 K at 11.8
kbar. The unusually high superconducting transition temperature and its
anomalous pressure dependence are ascribed to a lattice mismatch between the
LaAlO3β substrate and the thin film. The local temperature exponent of the
resistivity (n=dlnΞΟ/dlnT) shows a funnel shape around
the optimal pressure, suggesting that fluctuations associated with the
anomalous normal state are responsible for high-temperature superconductivity.Comment: To appear in Appl. Phys. Let