Field-induced large resistance peak in superconducting niobium thin films deposited on surface reconstructed SrTiO₃ substrate

It is known that oxygen vacancies in SrTiO₃(STO) can result in surface reconstructions (SR), giving rise to a metallic surface and unusual surface phonon modes. Few earlier works have demonstrated intriguing phenomena of a huge superconducting transition temperature enhancement in a monolayer FeSe on SR-STO substrate, where its intrinsic mechanism remains debatable. In this work, we constructed a phase diagram of SR-STO with surface structures of, where SR-STO samples were prepared via thermal annealing in ultra-high vacuum followed by low energy electron diffraction analysis. Thin Nb films with different thicknesses (t) were deposited on the SR-STO. The detail studies on the magnetotransport and superconducting properties of the Nb(t)/SR-STO films revealed a large positive magnetoresistance, and a pronounced resistance peak occurred near the onset of the resistive superconducting transition in the presence of a large magnetic field. Remarkably, amplitude of the resistance peaks increases with increasing fields, reaching a value of nearly 340 % of the normal state resistance at 12 T. Such resistance peaks were not observed for the control samples of Nb(t)/STO and Nb(t)/SiO₂. Possible mechanisms of the large field-induced resistance peak will be discussed.