Externally Shunted Josephson Junctions: Generalized Weak Links

Using externally shunted thin-film Sn-I-Sn Josephson junctions, we have experimentally verified the Stewart-McCumber predictions for the current-voltage (I-V) characteristics of a generalized weak link treated as a lumped circuit consisting of an ideal Josephson junction with critical current ic, conductance G, and capacitance C. In addition, we have shown experimentally and theoretically that the predictions for the amount of hysteresis in the I-V characteristic as a function of the dimensionless circuit parameter βC=(2e) (icCG²) can be generalized to include shunt inductance L by replacing the capacitance with an effective capacitance C^'=C-G²L. An externally shunted Josephson junction acts as a generalized weak link in that it can reproduce the behavior of other types of weak links. In fact, if the temperature of a single shunted junction is varied between 1.12 and 3.7 K, or the applied magnetic field is varied between 0 and 1 Oe, βC can be varied over two orders of magnitude while the junction is in the cryostat. Since βC determines the qualitative nature of the I-V characteristic, this variation of βC simulates widely different types of weak-link devices. Furthermore, the accessible range of βC for the single shunted junction can be changed while it is out of the cryostat by altering its external shunt conductance. Thus, externally shunted Josephson junctions are theoretically well-understood models of, and simulators for, the behavior of many weak-link junctions; they have the desirable property of adjustability both before and after cryostat installation. This versatility should find many significant applications.