A new design for a multi-terminal superconducting transistor-like device exploiting tunnelling injection of quasiparticles is proposed, in which parasitic back-action of the acceptor junction (S1IS2) bias current on the injector junction (S2FIS3) current–voltage characteristic (CVC) is dramatically reduced as compared with that for the formerly reported quiteron (here S, I, and F denote a superconductor, an insulator and a ferromagnetic material). Improvement of the isolation is achieved owing to the short electron mean free path and short coherence length in a thin F layer that screens the superconducting energy gap in the perturbed S2 layer preventing its manifestation in the CVC of the injector.
A new design for a multi-terminal superconducting transistor-like device exploiting tunnelling injection of quasiparticles is proposed, in which parasitic back-action of the acceptor junction (S1IS2) bias current on the injector junction (S2FIS3) current–voltage characteristic (CVC) is dramatically reduced as compared with that for the formerly reported quiteron (here S, I, and F denote a superconductor, an insulator and a ferromagnetic material). Improvement of the isolation is achieved owing to the short electron mean free path and short coherence length in a thin F layer that screens the superconducting energy gap in the perturbed S2 layer preventing its manifestation in the CVC of the injector.
We have fabricated and characterized double-barrier multi-terminal superconducting transistor-like devices with the Nb/Al/AlOx/Al/Nb/Ni/Al/AlOx/Al/Nb (S1IS2FIS3) structure, where S, I, and F refer to superconducting, insulating, and ferromagnetic layers, respectively. A thin ferromagnetic Ni layer screens the superconductivity of the middle Nb layer on the injector-barrier side, so that the Nb/Ni bilayer manifests itself as a superconductor in the current-voltage characteristic (I-V curve) of the acceptor S1IS2junction, but as a normal metal in the I-V curve of the injector S2FIS3junction. It is shown that this property allows for considerable improvement of the input-output isolation of the quasiparticle-injection devices as compared with that for the formerly reported quiteron.
We report experimental results on characteristics of SFIFS junctions and multi-terminal SFIFSIS devices (where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively). The SFIFS junctions serve as injectors in the SFIFSIS devices which have transistor-like properties; for this reason we call them Superconducting-Ferromagnetic Transistors (SFTs). We have found the F (Ni) thickness at which the SFIFS current-voltage characteristic (CVC) becomes linear. Furthermore, we investigated the DC and AC characteristics of SFTs of two types: ordinary devices with a single acceptor (SIS) junction, and devices with a double acceptor. In the first case, we focused on studying the influence of the injection current through the SFIFS junction on the maximum Josephson current of the SIS acceptor. For devices of the second type, we studied voltage amplification properties when the operating point was chosen in the sub-gap region of the acceptor CVC. By applying an AC signal (in the kHz range) while biasing the injector (SFIFS) junction with a constant DC current, we observed a voltage gain above 25 on the double acceptor. In the reverse transmission experiment, we applied DC current and an AC modulation to the acceptor junction and, within the accuracy of the experiment, observed no response on the injector junction, which implies an excellent input-output isolation in our SFIFSIS devices. 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.
A multiterminal superconducting device with the S1IS2FIS3 structure (where S, I, and F denote a superconductor, an insulator, and a ferromagnetic material) is fabricated and characterized. Introducing a thin ferromagnetic layer into the middle electrode dramatically reduces parasitic back action of the acceptor junction (S1IS2) bias current on the injector junction (S2FIS3) current-voltage characteristic as compared with that for the formerly reported quiteron, a device exploiting similar operation principle.
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 results on the dc and ac characterization of multiterminal SFIFSIS devices (where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively), which display transistor-like properties. We investigated two types of such superconducting-ferromagnetic transistors (SFTs): ordinary devices with a single acceptor (SIS) junction, and devices with a double acceptor. The devices with the single SIS acceptor were investigated and demonstrated a modulation of the maximum Josephson current as a function of the SFIFS current injection level. For devices of the second type, by applying an ac signal (in the kilohertz range) with a constant dc bias current to the injector (SFIFS) junction, we observed a voltage gain of about 25 on the double acceptor with the operating point chosen in the subgap region of the acceptor current-voltage characteristic. We also observed an excellent input-output isolation in our SFIFSIS devices. 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 on the measurement results of the superconducting-ferromagnetic transistors (SFTs) made at Northwestern University and Hypres, Inc. [IEEE Trans. Appl. Supercond. vol. 24, 1800506 (2014); vol. 25, 1800705 (2015)]. SFT is a multi-terminal device with the SIS'FIFS structure (where S, I, and F denote a superconductor, an insulator, and a ferromagnetic material, respectively) exploiting intense quasiparticle injection in order to modify the non-linear I-V curve of a superconducting tunnel junction. Potentially, SFT is capable of providing voltage, current and power amplification while having good input/output isolation. We characterized the devices at frequencies up to 5 MHz at 4 K. Our setup did not allow for accurate measurement of the voltage gain of low-impedance SFT devices because of contribution of resistance of the bias-T connected in series with the SFT. Nevertheless we observed a voltage gain above unity for some measurement configurations. It is very interesting that we confirmed that the isolation between the input and output of the device is quite good. We suggest that further improvement of the SFT device parameters is possible in optimized devices, so that the device potentially may serve as a preamplifier for readout of output signals of cryogenic detectors and be useful as an element of other superconductor-based circuits.