Superconductivity in two-dimensional NbSe₂ field effect transistors

We describe investigations of superconductivity in few molecular layer NbSe₂ field effect transistors. While devices fabricated from NbSe₂ flakes less than eight molecular layers thick did not conduct, thicker flakes were superconducting with an onset T_c that was only slightly depressed from the bulk value for 2H-NbSe₂ (7.2 K). The resistance typically showed a small, sharp high temperature transition followed by one or more broader transitions which usually ended in a wide tail to zero resistance at low temperatures. We speculate that these multiple resistive transitions are related to disorder in the layer stacking. The behavior of several flakes has been characterized as a function of temperature, applied field and back-gate voltage. We find that the conductance in the normal state and transition temperature depend weakly on the gate voltage, with both conductivity and T_cdecereasing as the electron concentration is increased. The application of a perpendicular magnetic field allows the evolution of different resistive transitions to be tracked and values of the zero temperature upper critical field, H_c2(0), and coherence length, ξ(0), to be independently estimated. Our results are analyzed in terms of available theories for these phenomena.