Abstract
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 << 1 and EJ/Ec < 1, the I-V characteristics show typical Coulomb blockade features. In an intermediate region where RQ/RN similar 0.4, EJ/Ec similar 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 RN is greater than RQ or less than RQ respectively.
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