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Japanese Journal of Applied Physics

Quasiparticle Characteristics and Noise Properties of Superconductor-Normal Metal-Superconductor Quantum Well Devices

Akio Takada, Toshiaki Ikuta, A. Tachibana, T. Fukumoto, M. Hátle, Katsuyoshi Hamasaki

Published Feb 1, 1995• 1 citation
Publisher

Abstract

All-thin film superconducting Nb-Nb nanoconstriction-NbN quantum well devices with improved low-frequency noise properties have been successfully fabricated. The quasiparticle current characteristics of this device are well explained by the Andreev reflection current. The low-frequency noise parameter η in Hooge's empirical theory decreases quickly with increasing device quality factor Q eff=R D(1 mV)/R N(10 mV), i.e., with decreasing effective strength of the barrier potential at the superconductor (Nb, or NbM-nanoconstriction interface.

DOI
10.1143/jjap.34.1401

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All-thin film superconducting Nb-Nb nanoconstriction-NbN quantum well devices with improved low-frequency noise properties have been successfully fabricated. The quasiparticle current characteristics of this device are well explained by the Andreev reflection current. The low-frequency noise parameter η in Hooge's empirical theory decreases quickly with increasing device quality factor Q eff=R D(1 mV)/R N(10 mV), i.e., with decreasing effective strength of the barrier potential at the superconductor (Nb, or NbM-nanoconstriction interface.
All-thin film superconducting Nb-Nb nanoconstriction-NbN quantum well devices with improved low-frequency noise properties have been successfully fabricated. The quasiparticle current characteristics of this device are well explained by the Andreev reflection current. The low-frequency noise parameter η in Hooge's empirical theory decreases quickly with increasing device quality factor Q eff = R D (1 mV)/R N (10 mV), i.e., with decreasing effective strength of the barrier potential at the superconductor (Nb, or NbM-nanoconstriction interface
We have fabricated dc-superconducting quantum interference devices (SQUIDs) with all-thin-film point contacts (Nb–Nb nanoconstriction-NbN). The size of the nanoconstriction may be smaller than the superconducting coherence length of bulk Nb. The quasiparticle characteristic of the all-thin-film point contact can be explained using the Blonder, Tinkham and Klapwijk (BTK) quasiparticle current which is applicable to the superconducting quantum point contact. The flux noise of SQUID with 10 pH operated in a flux locked loop is 8 µΦ0/ Hz1/2 at 1 Hz, and 4 µΦ0/ Hz1/2 at frequencies above 10 Hz is limited by the preamplifier.
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.
High quality NbN-NbOX-Pb Josephson devices have been fabricated by r.f. diode sputtering. The critical temperatures of the NbN films were as high as 16.8 K. The junction geometry was defined by photolithography and the oxide barrier was obtained by thermal oxidation or by a plasma oxidation technique. Very low leakage currents have been obtained, corresponding to values of the parameter Vm = 80 mV (Vm = IO× 2 mV/I (2mV)). The junction microwave quality factor Q and other a.c. properties have been measured in the frequency range 20 - 300 GHz by looking at the devices self-resonant modes. Q values up to 120 at 4.2 K have been found in the low frequency range, indicating potential performances of these devices for microwave applications such as Josephson voltage standards. Theoretical analysis of the results shows that the main contribution to the losses is related to the NbN films surface impedance.
Abstract Decoherence in quantum bits (qubits) is a major challenge for realizing scalable quantum computing. One of the primary causes of decoherence in qubits and quantum circuits based on superconducting Josephson junctions is the critical current fluctuation. Many efforts have been devoted to suppressing the critical current fluctuation in Josephson junctions. Nonetheless, the efforts have been hindered by the defect-induced trapping states in oxide-based tunnel barriers and the interfaces with superconductors in the traditional Josephson junctions. Motivated by this, along with the recent demonstration of 2D insulator h -BN with exceptional crystallinity and low defect density, we fabricated a vertical NbSe 2 / h -BN/Nb Josephson junction consisting of a bottom NbSe 2 superconductor thin layer and a top Nb superconductor spaced by an atomically thin h -BN layer. We further characterized the superconducting current and voltage ( I – V ) relationships and Fraunhofer pattern of the NbSe 2 / h -BN/Nb junction. Notably, we demonstrated the critical current noise (1/ f noise power) in the h -BN-based Josephson device is at least a factor of four lower than that of the previously studied aluminum oxide-based Josephson junctions. Our work offers a strong promise of h -BN as a novel tunnel barrier for high-quality Josephson junctions and qubit applications.
We have developed a low-noise 850 GHz superconductor–insulator–superconductor quasiparticle mixer with NbTiN thin-film microstrip tuning circuits and hybrid Nb/AlN/NbTiN tunnel junctions. The mixer uses a quasioptical configuration with a planar twin-slot antenna feeding a two-junction tuning circuit. At 798 GHz, we measured an uncorrected double-sideband receiver noise temperature of TRX=260 K at 4.2 K bath temperature. This mixer outperforms current Nb SIS mixers by a factor of nearly 2 near 800 GHz. The high-gap frequency and low loss at 800 GHz make NbTiN an attractive material with which to fabricate tuning circuits for SIS mixers. NbTiN mixers can potentially operate up to the gap frequency, 2Δ/h∼1.2 THz.