Self-Heating in SIS Mixers: Experimental Evidence and Theoretical Modeling

This work investigates the relationship between self-heating and the characteristic features observed in the current–voltage characteristics (IVCs) of superconductor–insulator–superconductor (SIS) junctions. Finite-element analysis is employed to evaluate the steady-state temperature distribution around SIS junctions, explicitly accounting for the temperature dependence of the thermal conductivities of the constituent materials. This approach enables flexible estimation of self-heating under various practical conditions, such as different substrate materials, interfacial thermal resistances, and geometric layouts. A heating coefficient is extracted from the simulations and used as an input parameter for IVC modeling. Incorporating self-heating through temperature-dependent gap energy and gap broadening, the simulated IVCs reproduce bending features near the energy gap that agree with measured characteristics. Furthermore, when an unintended weak link, such as a fabrication-induced defect near a via hole, is present near an SIS junction, its critical current can be significantly reduced by junction heating, producing unexpected bends at the linear branch of measured IVCs. Conversely, such bends may serve as indicators that the junction temperature approaches the superconducting transition temperature.