Harmonic current-phase relation in Nb-Al-based superconductor/insulator/normal conductor/insulator/superconductor-type Josephson junctions between 4.2 K and the critical temperature

For a study of the supercurrent-phase relationship, we have fabricated Nb/A l/AlOx/Al/AlOx/Al/Nb-based Josephson junctions of low critical current dens ity. At 4.2 K, junctions of a nominal area of 10x10 mu m(2) showed a nonhys teretic dc current-voltage characteristic with a critical current of about 10 mu A. Similar junctions were part of washer-type rf superconductor quant um interference devices. These samples were inductively coupled to high-qua lity tank circuits for measurements at 4.2 K or at higher temperatures up t o values close to the critical temperature of niobium. In this range, the c urrent-phase relation, as deduced from impedance measurements of the phase- biased junction, did not deviate significantly from the sinusoidal shape. ( C) 2000 American Institute of Physics. [S0003-6951(00)03635-4].