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 νz0ex0ex=0ex0ex1.35±0.10 and a critical sheet resistance, Rc, of about 1.2RQ, with RQ0ex0ex=0ex0exh/4e². However, in the presence of dc bias currents that are smaller than the zero-field critical currents, νz becomes 0.75±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.