Field-Tuned Superconductor-Insulator Transition with and without Current Bias

The magnetic-field-tuned superconductor-insulator transition has been studied in ultrathin beryllium films quench condensed near 20 K. In the zero-current limit, a finite-size scaling analysis yields the scaling exponent product $\nu z=1.35\mathrm{}\pm 0.10$ and a critical sheet resistance, $R_c$, of about ${1.2R}_Q$, with $R_Q{=h/4e}^2$. However, in the presence of dc bias currents that are smaller than the zero-field critical currents, $\nu z$ becomes $0.75\pm 0.10$. This new set of exponents suggests that the field-tuned transitions with and without a dc bias current belong to different universality classes.