Ultrathin Niobium in the Si/Nb/Si Trilayers

We study magnetotransport properties of the Si/Nb/Si trilayers, in which the thickness of niobium, d, changes from 1.1 nm to 50 nm, while the thickness of Si is xed at 10 nm. The niobium lms are amorphous for d < 4 nm, while in thicker lms the alligned polycrystalline grains are formed. We observe that the Hall coe cient changes sign into negative in the lms with d < 1.6 nm. We also nd that in the ultrathin lms the magnetic eld induces a transition from the superconducting into a metallic phase with the resistance smaller than the normal-state resistance.