B. J. Taylor

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.

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...

Detection of Far-Field Radio-Frequency Signals by Niobium Superconducting Quantum Interference Device Arrays

The capability of an all-niobium superconducting quantum interference device (SQUID) array to operate as an electrically small antenna capable of detecting radio frequency from distant sources was demonstrated. The measurements were performed in three different arrays, with each array consisting of 2400 identical cells. The intrinsic broadband characteristics of the device were confirmed by the flat frequency response of the ratio of the output and input voltages (the S21 parameter) of the array between 300 kHz and 1 GHz. Setting the device to its optimal operating parameters allowed the detection of signals from local frequency-modulation stations.

Computational Modeling of bi-Superconducting Quantum Interference Devices for High-Temperature Superconducting Prototype Chips

We explore bi-superconducting quantum interference device (bi-SQUID) designs suitable for the fabrication process of high-temperature superconducting step-edge Josephson junctions. The bi-SQUID offers increased linearity and improved signal detection performance over a SQUID. The realization of high-temperature superconducting (HTS) bi-SQUIDs opens up new potential applications given the reduced constraints of the cryogenic package. In this paper, we explore the effects of Josephson junction and temperature variations through modeling a bi-SQUID system of equations for different designs based on step-edge Josephson junctions.

Preparation of novel HTS films and tunnel junctions for advanced C3I sensor applications

Research into the development of advanced RF electronics and devices having high-Temperature Superconducting (HTS) circuitry is being carried out in the Cryogenic Exploitation of RF (CERF) laboratory at SPAWAR Systems Center (SSC) - Pacific. Recently, we have developed a novel annealing process wherein a film of YBa2Cu3Ox is produced having a gradient of oxygen composition along a given direction which we refer to as YBa2Cu3O∇x. Such samples are intended for rapid experimental investigation of the evolution of electronic properties within the compound and in combination with structurally compatible functional oxide materials as integrated sensor devices. We present here an investigation as to the extent to which local oxygen content affects the ion milling process in the formation of Josephson junctions in the HTS compound YBa2Cu3O∇x. We find an abrupt transition in the profile and depth of ion milled trenches at oxygen concentrations at and below the well ordered oxygen level, O6.72. The method described here shows good potential for use in the fabrication of large numbers of uniform Josephson junctions in films of YBa2Cu3Ox, as either a complementary processing tool for grain boundary, step edge, or ion damaged formed JJs, or as a stand alone method for producing nano-bridge JJ’s.

Characterization of large two-dimensional YBa2Cu3O7–δ SQUID arrays

Large two-dimensional SQUID arrays were made using the step-edge Josephson junction process. The performance of the arrays is analyzed with respect to determining the conditions under which the optimal performance is achieved. We find that optimization of the field-voltage transfer function V B is reached at a specific temperature and device current bias point, and arrive at an empirical expression describing the dependence of V B on the critical current and dynamic resistance of the SQUID array and as a function of temperature. The empirical expression for V B of the SQUID arrays is similar to that given by well known theoretical models for a single SQUID.

Voltage Response of Non-Uniform Arrays of Bi-SQUIDs

Multi-loop arrays of Josephson Junctions (JJ) with non-uniform area distributions, which are known as Superconducting Quantum Interference Filters (SQIF), 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-SQUID configuration, in which each individual SQUID loop is made up of three JJs as oppose to using two JJs per loop in standard DC SQUIDs. We show, computationally, that the additional junction offers a viable linearization method for optimizing the voltage response and dynamic range of SQIF arrays. We have realized SQIF arrays based on bi-SQUID cells and present first experimental results.

Flux flushing in superconducting niobium films

Flux trapping in superconducting devices has shown to be detrimental to the consistent operation of superconducting electronics (SCE). Approaches to improve reliability of SCE components have focused on introduction of flux trap regions and highly elaborate degaussing procedures. Nevertheless, a controlled and reproducible method to assure the elimination of trapped flux on SCE devices remains elusive. A substantial body of work on artificial defects utilizing the so called ratchet effect has demonstrate limited control of the magnetic vortices in niobium films. These early attempts to control the spurious vortices distribution have been limited to small geometrical regions having no practical effect on improving SCE devices operational parameters. In this paper, we report simulations and propose an improved method utilizing the ratchet effect that can be extended to physical sizes compatible to existing fabrication techniques of SCE devices.