Superconductor-Insulator Transition in a Two-Dimensional Array of Resistively Shunted Small Josephson Junctions

We have fabricated two-dimensional (2D) small-Josephson-junction arrays of which each Al-AlOₓ-Al junction is shunted by a Cr resistor. The arrays with large junction resistance and large charging energy show a transition from insulating to superconducting behavior when the shunt resistance is lowered below a critical value, which is close to 2RQ ( RQ≡h/4e²0ex0ex=0ex0ex6.45kΩ). The measured phase diagram is consistent with theories of quantum-fluctuation-driven and dissipation-driven phase transitions in the 2D Josephson-junction array with Ohmic shunt resistors.