New effects of quasiparticle injection on microwave irradiated Josephson triodes are demonstrated. The device comprises Pb-insulator (Nb2O5)-Nb SIS junction with direct contact of the Nb film to n+-GaAs gate substrate. In this device, the very thin Nb film (300 - 600 Å) as a base electrode of the SIS detector also serves as an acceptor of the quasiparticles injected from GaAs - Nb Sehottky barrier. The emphasis was placed on injections of hot quasiparticles with the energy in excess of 0.5 eV. The zeroth step height of microwave (10 GHz) irradiated Josephson current Iowas efficiently modulated by a sufficiently low density quasiparticle injection current Iinj(1 - 2 A/cm2), though no sign of change was found in the superconductive gap parameter and the Josephson critical current itself. These characteristic features became less pronounced as the energy of injected quasiparticles was lowered below 50 meV. Though the present experiment is in a preliminary stage, a current amplification (dIo/dIinj) of higher than two has been attained on a test specimen.
New effects of quasiparticle injection on microwave irradiated Josephson triodes are demonstrated. The device comprises Pb-insulator (Nb2O5)-Nb SIS junction with direct contact of the Nb film to n+-GaAs gate substrate. In this device, the very thin Nb film (300 - 600 Å) as a base electrode of the SIS detector also serves as an acceptor of the quasiparticles injected from GaAs - Nb Sehottky barrier. The emphasis was placed on injections of hot quasiparticles with the energy in excess of 0.5 eV. The zeroth step height of microwave (10 GHz) irradiated Josephson current Iowas efficiently modulated by a sufficiently low density quasiparticle injection current Iinj(1 - 2 A/cm2), though no sign of change was found in the superconductive gap parameter and the Josephson critical current itself. These characteristic features became less pronounced as the energy of injected quasiparticles was lowered below 50 meV. Though the present experiment is in a preliminary stage, a current amplification (dIo/dIinj) of higher than two has been attained on a test specimen.
We developed submillimeter superconducting oscillators using Josephson effects in the Nb-based superconductor-insulator-superconductor (SIS) parallel array. The radiation emitted from the array is detected using integrated SIS twin junctions. Despite the small junction number (N = 21), the (I-V ) characteristics of the detector clearly exhibit photon-assisted quasiparticle steps when the array is biased upon its Josephson resonances, ranging from 370 to 500 GHz. The array has a moderate current density (Jc≈= 6 kA/cm2) but provides an output power around 0.28 μW, which would be sufficient in achieving sensitive heterodyne reception. The impedance array can be matched easier to microwave circuits than the usual long Josephson junctions. Furthermore, the parallel array approach can be also extended up to about 1.4 THz using Nb-based SIS junctions.
Characteristics of Nb/NbOx/Al injection controlled planar three terminal superconducting devices (injection controlled links) have been studied experimentally. A built-in Nb/AlOx/Nb Josephson junction is used to perform direct tunneling measurement, whose results show that the gap in the Nb strip decreases according as its critical current is suppressed. It is also shown that the static characteristics of the devices with high and low injector resistance can be fitted to a simple heating model and a modified heating model respectively.
We report experimental results on characteristics of SFIFS junctions and multiterminal SFIFSIS and SISFIS devices (where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively). The SFIFS (SFIS) junctions serve as injectors in the SFIFSIS (SISFIS) devices, which have transistorlike properties; for this reason, we call them superconducting-ferromagnetic transistors. We have found the F (Ni) thickness at which the SFIFS current-voltage characteristic becomes linear. For three-terminal SFIFSIS devices, we focused on studying the influence of the injection current through the SFIFS junction on the maximum Josephson current of the SIS acceptor. For four-terminal SISFIS devices, we studied dependence of the transport current through the middle Nb electrode on injection current level through the SFIS junction. In both cases, we found that the output superconducting current (either Josephson or transport) can be efficiently modulated by the quasiparticle injection from SFIFS or SFIS input junction. The experiments indicate that, after optimization of the device parameters, they can be used as input/output isolators and amplifiers for memory, digital, and RF applications.
We report experimental and theoretical results on the current gain in superconductor-ferromagnetic transistors (SFTs) with the SISFIFS structure [where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively]. The Josephson critical current density jcaof the acceptor (SISF) junction in the devices is above 9 kA/cm2, which is higher than that in our previous devices. The critical current gain is defined as |δIca|/|δIi|, where a change |δIi| in the injector (SFIFS junction) current produces a change |δIca| in the acceptor maximum Josephson current. We observed a small-signal gain as high as 7.8 and a large-signal current gain of about 2.2. In addition, we found that the Josephson current of the acceptor junction is sensitive to the state of the ferromagnetic layers. A theoretical model is proposed to describe the nonequilibrium processes in the SFT devices, which agrees with the experimental observations. The calculation shows that, in the present device configuration, the dominant contribution to the gap suppression in the middle Nb electrode is due to the quasiparticle injection; spin injection plays a secondary role. We demonstrate that proper device engineering allows one to efficiently control the maximum Josephson current in the SISF acceptor junction using the quasiparticle injection. We conclude that SFT devices can be used as input/output isolators and amplifiers for memory, digital, and RF applications.
The superconductor-ferromagnetic transistors (SFTs) with the SISFIFS structure (where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively) are promising for various applications in superconducting electronics. Specifically, as current amplifiers, they can be used as line drivers and cell selectors in cryogenic memory. Here we report on results of a study of dependence of current gain on the SFIFS injector and SIS acceptor barrier transparency. We demonstrate that making the acceptor junction much more transparent than the injector junction results in improvement of the current gain; in particular it can exceed unity. Therefore proper device engineering allows one to efficiently control the maximum Josephson current in the SIS acceptor junction using the quasiparticle injection.
Interest in superconducting transistors is due to some potential benefits of these devices, such as the possibility to have large current gain and bandwidth at low temperatures. In this work we present experimental results concerning with a new superconducting 3-terminal device showing true transistor-like properties at T=4.2K. It is a stacked double tunnel junction structure where the intermediate film is a bilayer of superconducting Nb with an Al quasiparticle trap which can be either in the superconducting or in the normal metal state. Current amplification factors of up to 2.0 are observed at a temperature of 4.2 K when the Al is superconducting. When the Al is in the normal state, large current gains of more than 50 are observed. The operation is highly directional, i.e. practically no effect is observed when the roles of the injector and detector junctions are reversed. The results are explained according to the recently proposed QUAsiparticle TRApping TRANnsistor model.