Nb/Al-AlOx/Nb Overdamped Josephson Junctions Above 4.2 K For Voltage Metrology

The temperature dependence of the quantized voltage steps near the superconducting transition temperature of Nb/Al-AlO(x)/Nb overdamped Josephson junctions irradiated at 75 GHz has been studied. These junctions can switch between the hysteretic and nonhysteretic regime either by varying a fabrication parameter or, as shown in the present paper, by changing the junction operating temperature. Due to the high values of J(C) and V(C) up to 75 kA/cm(2) and 0.5 mV at 4.2 K, respectively, and to the moderate temperature dependence of these parameters, the height of the first rf-induced step was larger than 1 mA at T=6-8 K, corresponding to 0.7-0.9T/T(C) on a junction of 5x5 mu m(2). The flatness at the nanovolt level of this step demonstrates the feasibility of programable or ac voltage standards operating in a cryocooler setup.