Susan Berggren

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.

Modeling the effects of fabrication spreads and noise on series coupled arrays of bi-SQUIDs

We explore the effects of fabrication spreads and noise in a series array of DC bi-SQUIDs. The investigation is performed through numerical simulations of phase equations derived from circuit diagrams. It has been previously determined that the series coupled bi-SQUID arrays dynamic range increases with the number of bi-SQUIDs in the array and that non-uniform bi-SQUID loops sizes produces a zero-field single anti-peak in the voltage response. Results show the effects of industry standard fabrication spreads to the anti-peak feature are minimal, while the effects to the voltage response of arrays of identical bi-SQUIDs can be drastic. The findings presented here will be used to support the design of an electrically small magnetic field antenna and low-noise amplifier with high bandwidth.

Effects of Spread in Critical Currents for Series- and Parallel-Coupled Arrays of SQUIDs and Bi-SQUIDs

High-temperature-superconductor (HTS) superconducting quantum interference devices (SQUIDs) have a much greater variance in the values of the critical currents than their low-temperature-superconductor counterparts. To explore the effects of this increased variance, we add a noise term into the critical current of each junction. We perform numerical simulations of previously derived phase equations from series- and parallel-coupled arrays of SQUIDs, and series-and parallel-coupled arrays of bi-SQUIDs. These results support ongoing work to design an electrically small magnetic field antenna and low-noise amplifier with high bandwidth using HTS SQUIDs.

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.

Superconducting Quantum Interference Devices Arranged in Pyramid Shaped Arrays

We explore SQUID array designs that could be beneficial in a robust 3-D structure involving several substrates and find a design for which there may be some advantages in linearity and dynamic range. Traditional substrates are square in shape, for a 3-D structure three squares would need to be attached together to create a structure with three sides of a cube. We explored how we would arrange an array of SQUIDs on this structure if the substrates were cut into triangles. This would result in a three-sided pyramid (or tetrahedron) shape. The arrays have a feed line at the top entering a single SQUID (or small array) and the number of SQUIDs (small arrays) increased in number each row and a decreasing critical current. We simulated the effects of varying the critical currents and SQUID loop sizes on the average response output and determined that for arrays of 21 10 × 10 bi-SQUIDs have a more linear anti-peak and larger voltage swing than a comparable size 35 × 60 array.

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.

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.