Electric field effect on superconductivity in atomically thin flakes of NbSe2

We studied the electric field effect on superconductivity in atomically thin flakes of NbSe2 prepared by mechanical exfoliation. We found that these NbSe2 flakes are superconducting down to a thickness of a single unit cell consisting of two molecular layers of NbSe2 with an onset superconducting transition temperature (Tc) up to 2.5 K. We demonstrated that the Tc of the thinnest flakes can be modulated by an applied gate voltage with a Tc shift as large as 200 mK, which is larger than expected from a simple consideration based on Bardeen-Cooper-Schrieffer theory. We discuss the possible reasons for the observed enhanced electric field modulation of Tc.