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In recent years, rapid single flux quantum (RSFQ) circuits, which are promising for the development of high-speed digital electronics, have attracted widespread interest. Niobium nitride (NbN) films have also attracted significant attention for high-frequency applications in superconducting electronics because of their superior material parameters, including their high critical temperature Tc and large gap voltage Vg. To enable the development of all-NbN SFQ circuits, we have developed a fabrication process based on NbN junctions and inductors made of NbN strip lines. It is important to evaluate the inductance because it can affect the performance of an RSFQ circuit and is essential for the RSFQ design. We measured the inductance properties of both epitaxial and polycrystalline NbN strip lines using a superconducting quantum interference device modulation technique at 4.2 K and obtained the width dependence on the inductance of the NbN films. The penetration depth was then calculated from these measurements. The NbN inductance values were calculated numerically with respect to the penetration depth and were then compared with the measured values from the experiments. It was demonstrated that the differences between these values were within 5%. The results reported here will be useful for the design and fabrication of all-NbN RSFQ circuits. [ABSTRACT FROM AUTHOR]

Superconducting quantum interference filter (SQIF) is an array of superconducting quantum interference devices (SQUIDs) with different loop sizes. Such a device is capable of providing a much higher transfer factor and much higher magnetic field sensitivity with respect to those of single SQUID. Series niobium nitride (NbN) SQIF containing 400 SQUIDs with randomly varied loop areas and epitaxial NbN/AlN/NbN Josephson junctions have been developed, fabricated, and measured at helium temperature. The device shows a nonperiodic voltage response with a single sharp peak at zero magnetic field. The measured magnetic flux to voltage trans fer factor reaches 37.8 mV/Ф0 at 4.2 K. [ABSTRACT FROM AUTHOR]

Abstract: SQUID bootstrap circuit (SBC) is a SQUID direct readout operating in the voltage bias mode. It consists of two branches connected in parallel: SQUID-inductance La branch and inductance Lb-resistance Rb branch. Its bias voltage is connected to the input terminal of preamplifier. In the SBC the voltage noise from the preamplifier creates the current noise which can be modified by the parameters. Based on the equivalent circuit for SBC and its network equations we studied its characteristics and the effect of parameters on current noise due to biased voltage noise by numerical simulation. Results suggest that SBC current noise can be effectively reduced by modifying parameters, in optimal condition the current noise can be totally suppressed. The effect of resistance Rb on the excess noise is discussed. [Copyright &y& Elsevier]

The SQUID Bootstrap Circuit (SBC) for direct-coupled readout of SQUID signals in voltage bias mode was recently demonstrated. In addition to the conventional dc SQUID, the SBC incorporates a shunt resistor Rs, and two coils coupled to the SQUID via mutual inductances M1 and M2. In this paper, basic equations of SBC are formulated based on its equivalent circuit model. The expression of equivalent flux noise from the preamplifier is also given. The effect of the three adjustable parameters (M1, M2 and Rs) on the characteristics of SBC and the preamplifier noise suppression are numerically simulated. The SBC combines current and voltage feedbacks in one circuit, allowing for an effective suppression of the preamplifier voltage noise through increased flux-current transfer coefficient and dynamic resistance. In contrast to other direct-coupled schemes, it offers not only a good noise performance, but also tolerance to a wide range of adjustable parameters. [ABSTRACT FROM AUTHOR]

We recently presented a direct readout technique for the dc Superconducting QUantum Interference Device (SQUID) without flux modulation (FM), operated in voltage bias mode, and named it the SQUID Bootstrap Circuit (SBC). The SBC combines additional voltage and current feedbacks to minimize the room-temperature preamplifier noise. The main point of this paper is to compare the flux noise performance of the SBC readout with that of the FM scheme using a sine wave modulation signal. Several liquid-helium-cooled SQUID magnetometers with different layouts and loop inductances were characterized using these two readout schemes. Measured noise was comparable to or even lower than that measured by FM electronics. Furthermore, the SBC noise performance was evaluated as function of resistance which, when properly adjusted, permits us to nearly fulfill the critical noise suppression condition. We believe SBC to be a promising candidate for multi-channel SQUID systems. [ABSTRACT FROM AUTHOR]

Presents as single-channel high-Tc dc-superconducting quantum interference device magnetometer and gradiometer developed to record the magnetic field component perpendicular to the human chest generated by the heartbeat. Magnetocardiogram measurements carried out inside a magnetically shielded room; Sequential adjustment of a non-magnetic patient table.