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Journal of Applied Physics

Subgap biasing of superconducting tunnel junctions without a magnetic field

K. Segall, J. Moyer, J. J. Mazo

Published Aug 15, 2008• 1 citation
Publisher

Abstract

Superconducting tunnel junctions (STJs) have been successfully used as single-photon detectors but require the use of a magnetic field to operate. A recent paper has proposed the idea to use a circuit of three junctions in place of a single junction in order to achieve the necessary biasing without applying a magnetic field. The nonlinear interaction between the different junctions in the circuit causes the existence of a stable subgap state for one of the junctions, which acts as the detector junction. In this paper, we present the first measurements demonstrating the existence of such a biasing state feasible for STJ detectors. Single junction measurements with an applied magnetic field help determine the functional form of the subgap current versus voltage; then the operating point of a three-junction circuit is measured and fit to theory. The excellent match between theory and experiment demonstrates the existence of the subgap biasing state. The outlook for possible use in detector applications is discussed.

DOI
10.1063/1.2970159

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Superconducting tunnel junctions (STJs) have been successfully used as single-photon detectors but require the use of a magnetic field to operate. A recent paper has proposed the idea to use a circuit of three junctions in place of a single junction in order to achieve the necessary biasing without applying a magnetic field. The nonlinear interaction between the different junctions in the circuit causes the existence of a stable subgap state for one of the junctions, which acts as the detector junction. In this paper, we present the first measurements demonstrating the existence of such a biasing state feasible for STJ detectors. Single junction measurements with an applied magnetic field help determine the functional form of the subgap current versus voltage; then the operating point of a three-junction circuit is measured and fit to theory. The excellent match between theory and experiment demonstrates the existence of the subgap biasing state. The outlook for possible use in detector applications is discussed.
We present numerical simulations of a new biasing circuit for single photon detectors based on superconducting tunnel junctions. A single detector junction is replaced with a circuit of three junctions to achieve biasing of a detector junction at subgap currents without the use of an external magnetic field. The biasing occurs through the nonlinear interaction of the three junctions. We show nonlinear numerical simulation for different values of junction parameters and demonstrate that the biasing state is numerically stable against external fluctuations. The elimination of the external magnetic field potentially increases the capability of these types of photon detectors and eases constraints involved in the fabrication of large detector arrays.
We describe a biasing scheme for single-photon detectors based on superconducting tunnel junctions. It replaces a single detector junction with a circuit of three junctions and achieves biasing of a detector junction at subgap currents without the use of an external magnetic field. This potentially increases the capability of these types of detectors and eases constraints in making large arrays. The biasing occurs through the nonlinear interaction of the three junctions, which we demonstrate through numerical simulation. This nonlinear state is numerically stable against external fluctuations and is compatible with high-fidelity electrical readout of the photon-induced current.
The magnetic field dependence of the Josephson current of a Josephson tunnel junction whose shape was a normal distribution is analyzed for superconducting tunnel junction (STJ) photon detector applications. It was found that the Josephson current of a normal distribution junction is very sensitive to the external magnetic field and follows a function of exp (-B 2), while a conventional rectangular STJ exhibits the well-known Fraunhofer pattern of sin x/x. Numerical calculations show that the Josephson current of the normal distribution junction can be suppressed by an external magnetic field that is one tenth of that needed for conventional rectangular junctions. Calculations of a normal distribution junction approximated with a 1 µm mesh also show that the Josephson current is sufficiently suppressed by a small magnetic field. The effect of the misalignment of the angle between the junction and the magnetic field was evaluated. It was found that for a normal-distribution-shaped junction, misalignment of the magnetic field is allowed within ± 5°, suppressing the Josephson effect by two orders of magnitude more than that of a rectangular junction.
Superconducting Tunnel Junctions (STJs) have traditionally been read-out using low noise JFET based charge sensitive amplifiers. In some applications, low temperature SQUIDs could be used instead. The advantages are that the first amplification stage is also cooled to cryogenic temperatures and can be physically closer to the STJ, the detector can be voltage biased by means of a small fixed resistor and the detected current pulse shapes can be measured more directly. This work will outline the actual SQUID amplifier configuration and explain its virtues as well as its limitations.
We have investigated the effect of a moderately strong magnetic field (generated by superconducting coil, up to 380Gauss, 6 quantum fluxes within a single junction; generated by one piece of permanent magnet; generated by two pieces of permanent magnets respectively) on Nb superconducting tunnel junctions at different temperatures. Both experimental and numerical results show that the superconducting tunnel junction DC Josephson current can be significantly reduced by a magnetic field. A relatively strong magnet field may suppress the gap voltage as well as the sharp nonlinearity at the gap. We also investigated the leakage current and DC Josephson current of the Nb SIS junction at different temperatures.