Double reentrant superconductor-insulator transition in thin TiN films

We studied the superconductor-insulator transition in very thin films of TiN (and NbN) at very low temperature $(T=35\mathrm{}\mathrm{mK})$ as a function of sheet resistance and magnetic field. The critical temperature dependence versus sheet resistance follows the Finkel’stein theory for homogeneous films. A TiN sample very close to the critical point exhibits a double reentrant behavior below $T=150\mathrm{}\mathrm{mK}$ as the magnetic field increases. At large magnetic field and very low temperature a negative magnetoresistance is observed.