Subgap biasing of superconducting tunnel junctions without a magnetic field

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.

Breather decay into a vortex/antivortex pair in a Josephson ladder

“…In Figure 4 we can also see clearly that the breather junction goes unstable at almost the same voltage each time: around 1.4 mV or about half of the gap voltage (2.8 mV). Previous experiments on the subgap currents of similar junctions [24] showed a large increase in the subgap current at this voltage, so this is not surprising. At these points, the whole ladder switches into the whirling mode.…”