Low-Tc three-terminal structures for new superconducting devices

Three-terminal superconducting devices which operate on the basis of the non-equilibrium state produced in the intermediate electrode by quasi-particle injection are very promising for their transistor-like properties at T = 4.2 K. Nb-based stacked double-tunnel junction structures with Nb/Al as an intermediate electrode can produce large current amplification at T = 4.2 K when the Al film is in the normal metal state. Experimental results are discussed in the framework of the QUAsiparticle TRApping TRANsistor. The possibility of using a stacked double junction structure to study the proximity effect in thin superconducting bilayers is also discussed. In particular, the stacked devices are particularly useful for determining both the effective interface coupling parameters, the quasi-particle density of states and the Cooper pair potential at both sides of the bilayer. The knowledge of the Nb/Al interface parameters can be very useful for determining non-equilibrium behaviour of proximized structures. Measurements concerning the steady-state and pulsed characterizations of the stacked double Josephson devices are also presented. The possibility of a flip-flop state for these devices has been demonstrated. Experimental results are compared with numerical simulations based on the perturbed sine–Gordon model of the stack in the presence of pulsed electronic pumping.