Experimental investigation of two-dimensional arrays of ultrasmall Josephson junctions

We have measured Current-Voltage (I-V) characteristics of a large number of two-dimensional arrays of small area Al-AlOx-Al junctions down to temperatures of 40 mK, recording the temperature dependence of the zero bias resistance R0(T). The normal state resistance of the junctions RN varies from 2 kΩ to 270 kΩ and the junction capacitance C from 0.2f F to 2.2f F. The arrays are classified into different types according to the EJ/Ec and the RQ/RN ratios, and mapped onto a parameter diagram. EJ is the Josephson coupling energy, Ec = e2/2C is the charging energy, and RQ = h/4e2 = 6.5 kΩ is the quantum resistance. For low resistance arrays, RQ/RN >> 1 with EJ/Ec > 1, the Josephson current prevails and the arrays are superconducting. When RQ/RN 0) depending on whether RN is greater than RQ or less than RQ respectively.