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We have developed low-noise direct current-superconducting quantum interference devices (DC-SQUIDs) based on Nb/Al-AlO x /Nb Josephson junctions on thermally oxidized silicon substrate. The SQUID fabrication was realized with a simple 5-mask-level process, in which one single insulting layer (SiO 2 ) was used not only to insulate different metal layers, but also to protect the junction shunt resistor from oxidation or corrosion. In our SQUID design, a washer-type loop with an inductance of 140 pH and a coupled flux transformer with a multi-turn spiral input coil connecting to a pickup coil of 8.5 × 8.5 mm 2 were used, thus leading to a SQUID magnetometer with magnetic field sensitivity of 0.7 nT/Φ 0 . We measured the voltage-flux characteristics and flux noise in superconducting niobium shielding at 4.2 K. The measured white flux noise in a flux-locked loop was 5 µΦ 0 /√Hz, corresponding to magnetic filed noise of 3.5 fT/√Hz. [ABSTRACT FROM AUTHOR]

We investigated the formation and dissolution of natural oxide layers on 4-nm ultrathin NbTiN films at room temperatures and after heat treatment. By x-ray reflection (XRR) at a low angle and adopting a tri-oxidized-layer model to fit the XRR data, the thickness of each oxidized layer and the actual NbTiN layer below was obtained. This was confirmed by scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy. The x-ray photoelectron spectroscopy of the NbTiN film indicated the presence of the oxidized layers Nb 2 O 5 , TiO 2 , and NbO on the surface. Besides, the analysis of the films gave consistent results for the oxidized layer with saturation values of approximately 1.3 ± 0.1 nm. We found that the thickness of oxidized layers remains unchanged under heat treatment of 200 °C. Our approach can help us further understand the surface properties of ultrathin NbTiN films for improving the performance of NbTiN-based devices. [ABSTRACT FROM AUTHOR]

• A new figure of merit of Josephson comparator is proposed. • The design method of improving the quality factor of Josephson comparator circuit parameters is verified by experiments. • The reference value of circuit parameters quality factor of a comparator consisting of critically damped Josephson junctions is obtained by fitting experimental data. The Josephson comparator, composed of two Josephson junctions in series, is the decision-making pair of the single flux quantum (SFQ) logic and also performs critical roles in superconducting analog-to-digital converter (ADC) as sampling elements. Its current resolution and speed determine the performance of these circuits. Balancing the inherent trade-off between the resolution and speed is an issue in optimizing a Josephson comparator. In this research, we propose the Normalized Speed Resolution Ratio (NSRR) to analyze the comprehensive performance of the Josephson comparator under various critical currents (I c) and critical current densities (j c). Based on numerical simulations and results, we suggested an NSRR-based optimization criterion for the manufacturing process and design parameters of a Josephson comparator. Furthermore, a fitted value for the NSRR is determined by fitting our experimental data and the results of other research organizations. This reference value may be used to assess the complete performance of the Josephson comparator under different processes. [ABSTRACT FROM AUTHOR]

• Accurate extraction of capacitance and parasitic inductance of Josephson junction. • Extraction of a set of model parameters for post simulation of SFQ circuits. • The simulation accuracy of the RCLSJ model for the SFQ circuit is verified. Current-voltage (I-V) characteristics of shunted Nb/Al-AlO x /Nb Josephson junctions (JJs) can be precisely described in the resistive capacitive inductive shunted junction (RCLSJ) model derived from resistive capacitive shunted junction (RCSJ) model with additional parasitic inductance. However, the methods for extraction of current values in the RCLSJ model are not fully developed, which causes unsatisfactor accuracy of the simulations of superconducting integrated circuits. In this work, we propose an innovative method for extraction of parameters that is capable to solve inaccuracy problem. The enhancement is achieved by including the fluctuation of device parameters due to fabrication process in the analysis. A Josephson comparator is designed in the RCLSJ model to test the performance of the proposed parameter extraction method. Obtained results show that our method ensures 2 to 3 times higher simulation accuracy at frequencies above 60 GHz as compared to the traditional method. [ABSTRACT FROM AUTHOR]

• We have developed a high-frequency analysis method to remove the intrinsic resonances of dc-SQUID amplifiers from the operating region of interest, so that the distortions in the IVΦ characteristics are eliminated and ensure stable operations for the dc-SQUID amplifier without excess noise. • Based on the proposed method, we designed and fabricated a dc-SQUID amplifier. The measured results verified the reliability of the proposed method and demonstrated smooth IVΦ characteristics in the operating region of interest and a white noise level of 1.7 μΦ 0 /√Hz, corresponding to a current noise level of 15 pA/√Hz. • Based on the simulated and measured results, we concluded that these resonance frequencies of the dc-SQUID amplifier mainly originates from the lossless embedded section without the JJ and the whole geometric size including the JJ. • Based on the simulated and measured results, we deduced that these intrinsic resonance frequencies from dc-SQUID amplifier may produce crossovers at the corresponding voltages in the I-V characteristics. Further measurements and simulations will be carried out in future work. We performed high-frequency simulation to optimize the geometric size of dc-SQUID amplifiers and to remove their intrinsic resonance characteristics from the operating range. The aim is to eliminate distortions in the IVΦ characteristics that ensures low noise and stable operation over a large range of bias current. With this method, we designed and fabricated a dc-SQUID amplifier with two series-connected octagonal loops to achieve a robust operating performance. The measured results not only show smooth IVΦ characteristics without any distortion, but also demonstrate that the swing voltage is much larger than the corresponding voltage realized without this method at the same bias current. In addition, the white flux noise level as low as 1.7 μΦ 0 /√Hz was achieved at 0.1 K corresponding to a low current noise level of 15 pA/√Hz. [ABSTRACT FROM AUTHOR]

Abstract: The / La0.88Ca0.12MnO3 (YBCO/LCMOi) and / La0.33Ca0.67MnO3 (YBCO/LCMOj) bilayers are fabricated with the method of pulsed laser deposition, and the magnetic dependent proximity effects are investigated. The magnetic fields were applied parallel (in-plane) and perpendicular (out-of-plane) to the film plane. Magnetic properties curves in different temperature range and magnetic field along two crystal orientation show a complex behavior due to the interplay between Meissner currents in YBCO layer and the magnetic fields present in LCMOi (or LCMOj) layer. Ac susceptibility measurements show that the suppression of superconductivity in LCMOi and LCMOj bilayers. The obtained results are discussed in terms of the effect of hole charge transfer from YBCO to LCMOi (or LCMOj) on the proximity between superconductivity and ferromagnetism. [Copyright &y& Elsevier]