Hot-electron effects in the two-dimensional superconductor-insulator transition

The parallel magnetic field tuned two-dimensional superconductor-insulator transition has been investigated in ultrathin films of amorphous Bi. The resistance is found to be independent of temperature on both sides of the transition below approximately 120 mK. Several observations suggest that this regime is not intrinsically "metallic" but results from the failure of the films' electrons to cool. The onset of this temperature-independent regime can be moved to higher temperatures by either increasing the measuring current or the level of electromagnetic noise. Temperature scaling is successful above 120 mK. Electric field scaling can be mapped onto temperature scaling by relating the electric fields to elevated electron temperatures. These results cast doubt in the case of ultrathin films on the existence of an intrinsic metallic regime and on the independent determination of the correlation length and dynamical critical exponents obtained by combining the results of electric field and temperature scaling.