EXPERIMENTAL INVESTIGATION OF 2-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 R(N) varies from 2 kOMEGA to 270 kOMEGA and the junction capacitance C from 0.2fF to 2.2fF. The arrays are classified into different types according to the E(J)/E(C) and the R(Q)/R(N) ratios, and mapped onto a parameter diagram. E(J) is the Josephson coupling energy, E(C) = e2/2C is the charging energy, and R(Q) = h/4e2 = 6.5 kOMEGA is the quantum resistance. For low resistance arrays, R(Q)/R(N) much greater than 1 with E(J)/E(C) > 1, the Josephson current prevails and the arrays are superconducting. When R(Q)/R(N) much less than 1 and E(J)/E(C) < 1, the I-V characteristics show typical Coulomb blockade features. In an intermediate region where R(Q)/R(N) approximately 0.4, E(J)/E(C) approximately 0.2, we find a "back-bending" I-V curve with negative differential resistance. The behavior of R0(T) also falls into three categories which can be correlated with the type of I-V characteristic. At the lowest temperature the arrays appear to be either insulating (dR0/dT < 0) or superconducting (dR0/dT > 0) depending on whether R(N) is greater than R(Q) or less than R(Q) respectively.