Georgy V. Prokopenko

DC and RF Measurements of Serial Bi-SQUID Arrays

SQUID arrays are promising candidates for low-profile antennas and low-noise amplifier applications. We present the integrated circuit designs and results of dc and radio frequency measurements of wideband serial arrays based on the integration of linear bi-SQUID cells forming a superconducting quantum interference filter (bi-SQUID SQIF). Various configurations of serial array designs are described. The measured linearity, power gain, and noise temperature are analyzed and compared. The experimental results are matched to results of mathematical modeling. A serial bi-SQUID SQIF arrays are integrated into a coplanar waveguide, and symmetrically grounded to corresponding sides of the coplanar waveguide. The radio frequency output comes out from the central common line, which is also used for dc biasing, and forms a symmetrical balanced output. The signal and dc flux biasing line is designed as coplanar lines passed in parallel over each bi-SQUID cell in a bidirectional fashion concentrating magnetic flux inside of each cell. Serial bi-SQUID SQIF arrays are fabricated on 5 mm × 5 mm chips using a standard HYPRES niobium 4.5 kA/cm2 fabrication process.

High quality Nb-based tunnel junctions for high frequency and digital applications

A number of new fabrication techniques are developed and optimized in order to fit the requirements of contemporary superconducting electronics. To achieve ultimate performance of integrated submm receivers with operational frequency of 1 THz, tunnel junctions with AlN tunnel barrier having a R/sub n/S value as low as 1 /spl Omega//spl mu/m/sup 2/ have been developed. High quality characteristics of Nb/AlN/Nb tunnel junctions with R/sub j//R/sub n/=16 and R/sub n/S=10 /spl Omega//spl mu/m/sup 2/ have been demonstrated. Electron Beam Lithography (EBL) in combination with Chemical Mechanical Polishing (CMP) has been incorporated to produce Nb/AlN/Nb junctions with 0.03 /spl mu/m/sup 2/ area. A new approach to obtain overdamped Nb/AlO/sub x//Nb tunnel junctions has been proposed and realized. The dependencies of the main parameters of novel junctions on the current density and circuit geometry have been studied. These junctions may have a good potential in Josephson junction arrays and Single-Flux-Quantum applications (RSFQ).

Development of 2-D Bi-SQUID Arrays With High Linearity

We develop a two-dimensional (2-D) superconducting quantum interference filter (SQIF) array based on the recently introduced high-linearity tri-junction bi-SQUIDs (superconducting quantum interference device). Our bi-SQUID SQIF array design is based on a tight integration of individual bi-SQUID cells sharing inductances with adjacent cells. We provide extensive computer simulations, analysis, and experimental measurements, in which we explore the phase dynamics and linearity of the array voltage response. The nonuniformity in inductances of the bi-SQUIDs produces a pronounced zero-field single antipeak in the voltage response. The antipeak linearity and size can be optimized by varying the critical current of the additional junction of each bi-SQUID. The layout implementation of the tight 2-D array integration leads to a distinct geometrical diamond shape formed by the merged dual bi-SQUID cells. Different-sized 2-D arrays are fabricated using the standard HYPRES niobium 4.5 kA/cm2 fabrication process. The measured linearity, power gain, and noise properties will be analyzed for different array sizes and the results will be compared with circuit simulations. We will discuss a design approach for the electrically small magnetic field antenna and low-noise amplifiers with high bandwidth based on these 2-D bi-SQUID SQIF arrays. The results from this work will be used to design chips densely and completely covered in bi-SQUIDs that have optimized parameters such as linearity and power gain.

Voltage response of non-uniform arrays of bi-superconductive quantum interference devices

Multi-loop arrays of Josephson junctions (JJs) with non-uniform area distributions, which are known as superconducting quantum interference filters (SQIFs), are the most highly sensitive sensors of changes in applied magnetic field as well as the absolute magnitude of magnetic fields. The non-uniformity of the loop sizes allows the array to produce a unique collective voltage response that has a pronounced single peak with a large voltage swing around zero magnetic field. To obtain high linear dynamic range, which is critical for a wide variety of applications, the linearity of the slope of the anti-peak response must be improved. We propose a novel scheme for enhancing linearity—a new configuration combining the SQIF array concept with the recently introduced bi-superconductive quantum interference device (SQUID) configuration, in which each individual SQUID loop is made up of three JJs as opposed to using two JJs per loop in standard dc SQUIDs. We show, computationally, that the additional junction offe...

Dynamic characteristics of S-band DC SQUID amplifier

A low-noise RF amplifier based on a dc SQUID (SQA) has been tested in the frequency range 3.0-4.6 GHz in the open-loop configuration. The following parameters have been measured for the single-stage balanced type SQA at 4.0 GHz: gain (12/spl plusmn/1) dB, 3 dB bandwidth of 500 MHz and noise temperature (1.0/spl plusmn/0.25) K. For the nonbalanced type SQA at 4.0 GHz gain was (15/spl plusmn/1) dB, 3 dB bandwidth 200 MHz and noise temperature (0.5/spl plusmn/0.25) K. The improved performance is obtained due to the increased characteristic voltage (/spl ap/420 /spl mu/V) of the small-area (down to 0.7-0.9 /spl mu/m/sup 2/) high-quality Nb-AlO/sub x/-Nb SIS junctions. The saturation power (normalized to 1 GHz) referred to the input at 1 dB gain compression is estimated as /spl ap/55 K*GHz at a bias voltage of 60 /spl mu/V. The reasons for saturation of the SQA are discussed.

Integrated rf amplifier based on dc SQUID

Integrated radio-frequency amplifiers comprising a 4-loop dc SQUID, seriesly connected input coil turns, a resonant capacitor parallel to the input coil, series capacitors at the input and output ports and bias resistors have been designed, fabricated and experimentally studied. A multiloop dc SQUID with parallel loops and seriesly connected single-turn input coils placed inside each loop and integration with the input resonant matching circuit elements and with elements of dc bias circuit allows one to increase signal frequency and reduce the influence of external noise. The amplifiers with three different capacitors have resonant frequencies 560, 656, 758 MHz and bandwidth about 50 MHz. The noise temperature of such amplifiers below 1.5 K has been measured using cold attenuator and room-temperature noise sources. The layout comprising three pairs of such amplifiers placed on the same 15/spl times/24 mm substrate was designed to increase the bandwidth over the bandwidth of the individual amplifiers.<<ETX>>

A dc SQUID based low-noise 4 GHz amplifier

The dc SQUID based RF amplifier (SQA) looks very attractive as an IF amplifier for integration with a SIS mixer and a flux-flow oscillator (FFO) in a fully superconducting submillimeter wave receiver suitable for space applications. Important advantages of the SQA are its low noise, extremely low power consumption, and complete compatibility with the fabrication process currently used for SIS mixers. Single stage amplifiers with a novel signal coupling circuit have been developed and tested in the frequency range 3.6-4.1 GHz. The 1 /spl mu/m/sup 2/ area Nb-AlO/sub x/-Nb junctions shunted by Ti resistors are grouped in a double washer dc SQUID. Two samples with slightly different designs showed the following gain, noise temperature and 3 dB bandwidth: 10.0/spl plusmn/1 dB, 5.0/spl plusmn/1.5 K, 3.86-3.90 GHz, and 6.8/spl plusmn/1 dB, 22/spl plusmn/7 K, 3.89-4.05 GHz, respectively.

DC SQUID RF amplifiers

The noise and signal parameters of several types of RF amplifiers based on different SQUIDs with integrated and hybrid input coils were studied. A new type of multiloop DC SQUID with an integrated input coil and extremely low stray capacitances was designed. The inductance of a four-loop SQUID was 100 pH, the input coil inductance 1.3 nH, and mutual inductance 300 pH. The tuned integrated four-loop amplifier at 420 MHz had a noise temperature lower 0.5 K and a power gain of nearly 20 dB in a 60-MHz bandwidth. For the noise calibration of such amplifiers, SIS junctions were used as a shot noise source, or a cooled attenuator and a room temperature semiconductor noise source were used. >

Quantum Sensitivity: Superconducting Quantum Interference Filter-Based Microwave Receivers

Several critical applications of superconducting electronics that have been successfully commercialized are the Josephson voltage standard, superconducting quantum interference device (SQUID) magnetometers, superconductor-insulator-superconductor (SIS) mixers, and analog filters. More recently, digital and mixed-signal circuits based on rapid single flux quantum (RSFQ) logic have also made an impression on the high-speed/high-frequency electronics application markets.

Superconducting-Ferromagnetic Transistor

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.