Monotonic decrease of Tcs with thinning of the superconducting MgB2 layer in MgB2/Ni and MgB2/B alternately-layered thin films

We prepared MgB2/Ni and MgB2/B alternately-layered films using electron beam evaporation and coaxial vacuum arc evaporation techniques without any post-annealing. The thickness of each MgB2 layer was designed to be 42, 24 or 15 nm for both MgB2/Ni and MgB2/B alternately-layered films. We confirmed that the layered structure was successfully obtained from the scanning transmission electron microscopic cross-sectional image of the MgB2/Ni alternately-layered film. The critical temperature, Tc, of the alternately-layered film decreased as the MgB2 layer became thinner, for both MgB2/Ni and MgB2/B films. Moreover, the Tc was affected by only the MgB2 layer thickness, and was independent of the inserted layer materials.