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LA SUPERCONDUCTIVIDAD Y SUS APLICACIONES (PARTE 2)

Gustavo Sarmiento Muñoz, Santiago Sanz, Ainhoa Pujana, Aitor Kortajarena, Raúl Rodríguez-Sánchez, Susana Apiñaniz

Published May 1, 2013• 0 citations
OpenAlex

Abstract

Applications to electronics are mainly based on Josephson junctions special properties. As it was pointed before, a single Josephson junction is a perfect voltage to frequency converter (AC Josephson effect). One example is the application into RF amplifiers where a radiofrequency signal generates a magnetic flux in a circuit coupled to, for example, DC-SQUID. Josephson effect allows converting the flux into the SQUID to an amplified voltage signal, proportional to the number of 0. That is the base of the microwave applications. The SQUID inductance together with the capacitance of the junction, allows also using the device as resonator.

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Applications to electronics are mainly based on Josephson junctions special properties. As it was pointed before, a single Josephson junction is a perfect voltage to frequency converter (AC Josephson effect). One example is the application into RF amplifiers where a radiofrequency signal generates a magnetic flux in a circuit coupled to, for example, DC-SQUID. Josephson effect allows converting the flux into the SQUID to an amplified voltage signal, proportional to the number of 0. That is the base of the microwave applications. The SQUID inductance together with the capacitance of the junction, allows also using the device as resonator.
Superconductive electronics is an extensive field covering both low- and high-temperature superconductors (HTS), and their applications in quantum interference devices, power transmission, microwave filters and logic circuits. The microscopic origins of superconductivity are explained by Bardeen–Cooper–Schieffer (BCS) theory whereas a macroscopic treatment is provided by the Ginzburg–Landau theory. The London equations can explain the well-known Meissner effect of exclusion of magnetic induction by superconductors. A widely used device in superconductive electronics is the Josephson junction (JJ). The DC and AC Josephson effects as well as the inverse AC Josephson effects are described. The application of DC and AC superconducting quantum interference devices (SQUIDs) in magnetometry are discussed. Rapid single flux quantum (RSFQ) logic, touted as a competitive beyond-CMOS technology in nanoelectronics, is briefly surveyed.
The unique ability of a Josephson junction to control the flow of magnetic flux quanta leads to a perfect relationship between frequency and voltage. Over the last 30 years, metrology laboratories have used this effect to greatly improve the accuracy of dc voltage standards. More recent research is focused on combining the ideas of digital signal processing with quantum voltage pulses to achieve similar gains in ac voltage metrology. The integrated circuits that implement these ideas are the only complex superconducting electronic devices that have found wide commercial application.
The goal of this studies is to use dc Squids as low noise radiofrequency amplifier (Tn ≈1K) in the frequency range 1-150 Mhz and to integrate them in NMR probe. DC Squids have been built using planar technology and with two types of superconducting materials : low Tc(Nb) and High Tc(YBa2Cu3O7). Contrary to the traditional set up used in magnetometry (flux locked loop) the dc Squid is here fluxed biased near Φz=(2n+1)Φo/4 and no flux locked loop is connected. Small variations of flux (Φs<<Φo) introduced from signal source via the input coil are amplified following the transfer characteristic VΦ=dV/dΦ. The low Tc devices are set up to now the best, concerning the reproducibility and the qualities of the Josephson Junctions obtained with a three layer process Nb/Al-Al2O3/Nb. Such junctions have been realized following the so called SNOP process, without anodization. The main characteristic obtained at 4.2K are RJ/RN=10, 2Δ≈2.8 meV and a Josephson current density J≈103 A/cm2. The high Tc devices have been built with bi-epitaxial YBa2Cu3O7 junctions and with a 30x30µm2 area loop. I(V) and V(Φ) characteristics curves obtained are promizing for applications as RF detection in the temperature range : 4-77K. Modelization studies are currently in progress for the optimization of those devices (low Tc and high Tc) as RF amplifiers (minimization of parasitic capacitance between the input coil and the Squid loop) in order to increase their upper frequency limit.\n* This work is partially supported by Ultimatech (CNRS), by the "région Rhône-Alpes" and E.E.C. "HTSC-GBJ" programs n°7100.
The dynamics and noise of a dc SQUID (the Superconducting Quantum Interference Device), with the McCumber parameter /spl beta//sub c/=2/spl pi/R/sup 2/l/sub c/C//spl Phi//sub 0/ close to the unity (where I/sub c/, R, C is the critical current, the shunt resistance and the capacitance of the Josephson junctions comprising the SQUID, respectively, /spl Phi//sub 0/=2.07 10/sup -15/ Wb is the magnetic flux quantum) integrated with a planar spiral input coil, have been experimentally studied. The length of the spiral input coil was chosen so as to match its /spl lambda//4 microwave resonance frequency to the plasma resonance frequency of the SQUID. The input coil resonance is found to enhance the overall quality factor Q of the Josephson oscillations in the SQUID and, as a result, to increase the dynamic resistance R/sub d/ and the gradient of the flux-to-voltage characteristics, /spl delta/V//spl delta//spl Phi/, without hysteresis. A dc SQUID with a loop inductance L=30.5 pH, /spl beta//sub c/=0.72, and a 6 turn input coil demonstrated a non-distorted quasi-sinusoidal flux-to-voltage transfer function with an exceptionally large modulation depth of 140 /spl mu/V peak-to-peak. The spectral density of the magnetic flux noise was as low as 3.5/spl times/10/sup -7/ /spl Phi//sub 0//Hz/sup 1/2/ in the double stage configuration, measured at a temperature 4.2 K using direct read out electronics. In combination with an intermediary transformer, the current resolution of the SQUID is as low as 1.25 pA/Hz/sup 1/2/ with an input coil inductance of 100 nH.
A series of dc SQUID radiofrequency amplifiers in the frequency range 0.1-1.1 GHz with noise temperature 0.4-2 K has been designed, fabricated and experimentally studied. For resonant input tank circuit an analytical model of noise temperature has been developed. A new effect of the output noise increasing or decreasing with changing the sign of voltage-to-flux transfer coeff 1 cient has been observed experimentally and described analytically and numerically. Application of SQUID amplifiers as IF amplifiers in SIS, Josephson, hot-electron heterodyne detectors as well as in cellular base stations has been discussed.
Superconducting quantum sensors use flux quantization, the Josephson effect and low-loss microwave circuits to measure magnetic flux, microwave signals and photon flux close to fundamental limits. In this review we follow the field from the Bardeen–Cooper–Schrieffer theory and the discovery of Josephson tunnelling to SQUID magnetometers, microwave kinetic inductance detectors and quantum-limited parametric amplifiers. We explain how dc and rf SQUIDs, superconducting resonators, and Josephson and kinetic-inductance amplifiers work, and we describe one measurement chain that covers them all, with separate excitation, response, amplification, control and readout paths and a fluxlocked-loop variant for SQUIDs. Flux and field noise, noise-equivalent power, added noise, bandwidth and dynamic range are defined and compared using numbers taken directly from the primary literature, together with the conditions under which they were measured. We then survey applications in biomagnetism, astronomical imaging, qubit readout and dark-matter searches, and close with the open problems of scalability, environmental noise and fair benchmarking.