¹ Department of Applied Physics, University of Twente, 7500 AE Enschede, the Netherlands

² Institute of Nuclear Physics, Moscow State University, 119899 Moscow, Russia

³ Sektion Physik and CeNS, Ludwig-Maximilians-Universität, München, Germany

 

 

   An overview of a theoretical approach to nonequilibrium effects in double-barrier SIS’IS junctions (I is a tunnel barrier, S’ is a thin film with critical temperature lower than that of S) is presented. We demonstrate an application of quasiclassical Keldysh technique to this problem. A number of practically interesting examples are discussed including simplest two-terminal structures and more general multiterminal geometries. Originally, an interest in double-barrier junctions was motivated by the desire to have devices with non-hysteretic current-voltage characteristics, e.g. to apply in large-scale integrated circuits. Double-barrier Josephson junctions are already used in Rapid-Single-Flux-Quantum (RSFQ) logic and digital voltage standards. Critical current densities of the order of 1 kA/cm² have been obtained. Recently, new proposals for application of these junctions as radiation sensors became subject of discussion.

 

   Recent developments in theory will be discussed: (a) microscopic model for the current transport in SIS’IS junctions in stationary case, which quantitatively describes the critical currents in e.g. Nb/Al/Nb junctions, (b) preliminary results of modeling of the nonstationary properties of these devices, which is now in progress. We identify the regime when transport is phase coherent, i.e. both barriers act effectively as a single one. In this case the concept of Multiple Andreev Reflections (MAR) can be applied to calculate the I-V characteristics of the junctions. The resulting I-V curves exhibit the subharmonic structure due to MAR and excess current at high bias. In the low barrier transparency case coherent MAR approach breaks down and the model is extended to treat this non-equilibrium regime. In this case kinetic equations and boundary conditions for the distribution function in the S’ interlayer are formulated taking into account the proximity effect and inelastic relaxation processes. In few limiting cases known results are reproduced, for example the cooling and energy gap stimulation in the interlayer due to effective quasiparticle extraction. Quasiparticle distribution in the subgap energy range becomes strongly nonequilibrium due to quasiparticle trapping and oscillates in time.

 

   Understanding the nonequilibrium processes in SIS’IS structures is important for developing novel particle detectors. First, SIS’IS junctions provides a convenient system to study nonequilibrium quasiparticles in a superconductor. Practically important information can be extracted from I-V characteristics of these structures. Further, multiterminal geometry can be used advantageously to explore nonequilibrium properties of the devices. For example, steep temperature dependence of the critical current around T_c of the S’ interlayer is the key feature of a possible new type of detector, in which critical current can be controlled externally in three- or four-terminal configuration. Important parameters for operation of such a device are quasiparticle trapping, diffusion and tunneling rates. The developed approach can be also applied to modeling the Quasiparticle Trapping Transistor (Quatratran) based on stacked tunnel junctions recently proposed by Booth et al.