Presentation Information
[ED8-04]Frequency-dependent damping of Josephson junctions in dc-SQUID sensors and Zappe interferometer switches
*Joern Beyer1 (1. Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany (Germany))
Keywords:
dc-SQUID sensors,Zappe interferometer switches,Josephson junction damping
Dc Superconducting Quantum Interference Device (dc-SQUID) sensors based on low-temperature superconductors, such as Nb or Al, predominately employ superconductor-insulator-superconductor Josephson junctions (JJs). An established model to describe the JJ dynamics is the resistively- and capacitively-shunted junction (RSCJ) model. In this model, the Stewart-McCumber (damping) parameter, βc = (2π/Φ0) I0 R2 CJJ with I0 - JJ critical current, R - JJ normal resistance, CJJ - capacitance associated with the JJ and Φ0 - the flux quantum, determine the degree of damping of the Josephson oscillations in the dc-SQUID circuit. The JJ normal resistance is most often realized as external resistor element wired in parallel to each of the two JJs, integrated into the SQUID circuit, and its value R is chosen such that βc < 1 for a chosen critical current and given capacitance of the JJs. This way non-hysteretic transport characteristics of the dc-SQUID sensor are obtained.It has been proposed to use R-C [1] or R-L [2] series connections as replacements of purely resistive JJ shunts. For the so-called “unshunted” dc-SQUID [1] it was shown, by means of numerical simulations, that improved noise performance compared to a conventional dc-SQUID may be obtained. Stable operation, though, required voltage biasing the un-SQUID. Numerical simulations of R-L-shunting of the JJs improve the linearity of dc-SQUID voltage-flux characteristics [2].
This presentation discusses, based on simulations using the Superconducting Circuit Simulator JoSIM [3], the use of R-L-C series/parallel circuits as JJ shunts for dc-SQUID sensors and for Zappe-style interferometers. The latter are 3-JJ interferometers used as flux-actuated superconducting-to-normal conducting switches for SQUID time division or code division multiplexers. In this work two shunt circuit configurations have been considered: a) R in parallel to R-C in series (R || RC) and b) R in parallel to R-L in series (R || RL). Replacing the resistance R of a purely resistive JJ shunt by the absolute value of the impedance of the above shunt circuits in the expression of the damping parameter, βc becomes dependent on frequency f. We consider values for R, C and L so that βc ( f ) < 1 in the range that corresponds to the Josephson frequencies in the circuit under consideration. Current-voltage and voltage-flux characteristics, transport parameters (δV/δΦ, Rdyn) as well as noise values obtained for dc-SQUIDs with realistic device parameters (SQUID inductance LSQ = 100pH, I0 = 5uA .. 10uA, CJJ = 40fF) show that stable current-biased sensor operation as well as improved linearity and noise can be obtained by the above shunt circuits compared to dc-SQUIDs with the same device parameters, but purely resistive JJ shunts. In Zappe-style flux-actuated switches, the proposed JJ shunt circuits result in increased maximum critical current of a switch and a wider range of the control flux for which the minimum critical current is obtained. Practical implementations of the shunt circuit configurations in integrated dc-SQUID sensor and Zappe-style switch circuits will be discussed.
[1] M. Kiviranta, The unshunted SQUID revisited, Supercond. Sci. Technol. 24 (2011) 065003
[2] S.T.P Boyd and Tijmen de Haan Development of SQUID Array Amplifiers for the LiteBIRD CMB Satellite, https://arxiv.org/pdf/2501.05592
[3] https://github.com/Superconductor-Electronics/JoSIM
Presentation Materials
https://iss-archives.jp/iss2025.jp/slides/ED8-04.pdf
This presentation discusses, based on simulations using the Superconducting Circuit Simulator JoSIM [3], the use of R-L-C series/parallel circuits as JJ shunts for dc-SQUID sensors and for Zappe-style interferometers. The latter are 3-JJ interferometers used as flux-actuated superconducting-to-normal conducting switches for SQUID time division or code division multiplexers. In this work two shunt circuit configurations have been considered: a) R in parallel to R-C in series (R || RC) and b) R in parallel to R-L in series (R || RL). Replacing the resistance R of a purely resistive JJ shunt by the absolute value of the impedance of the above shunt circuits in the expression of the damping parameter, βc becomes dependent on frequency f. We consider values for R, C and L so that βc ( f ) < 1 in the range that corresponds to the Josephson frequencies in the circuit under consideration. Current-voltage and voltage-flux characteristics, transport parameters (δV/δΦ, Rdyn) as well as noise values obtained for dc-SQUIDs with realistic device parameters (SQUID inductance LSQ = 100pH, I0 = 5uA .. 10uA, CJJ = 40fF) show that stable current-biased sensor operation as well as improved linearity and noise can be obtained by the above shunt circuits compared to dc-SQUIDs with the same device parameters, but purely resistive JJ shunts. In Zappe-style flux-actuated switches, the proposed JJ shunt circuits result in increased maximum critical current of a switch and a wider range of the control flux for which the minimum critical current is obtained. Practical implementations of the shunt circuit configurations in integrated dc-SQUID sensor and Zappe-style switch circuits will be discussed.
[1] M. Kiviranta, The unshunted SQUID revisited, Supercond. Sci. Technol. 24 (2011) 065003
[2] S.T.P Boyd and Tijmen de Haan Development of SQUID Array Amplifiers for the LiteBIRD CMB Satellite, https://arxiv.org/pdf/2501.05592
[3] https://github.com/Superconductor-Electronics/JoSIM
Presentation Materials
https://iss-archives.jp/iss2025.jp/slides/ED8-04.pdf