Heat propagation models for superconducting nanobridges at millikelvin temperatures

Nanoscale superconducting quantum interference devices (nanoSQUIDs) most commonly use Dayem bridges as Josephson elements to reduce the loop size and achieve high spin sensitivity. Except at temperatures close to the critical temperature T-c, the electrical characteristics of these bridges exhibit undesirable thermal hysteresis which complicates device operation. This makes proper thermal analysis an essential design consideration for optimising nanoSQUID performance at ultralow temperatures. However the existing theoretical models for this hysteresis were developed for micron-scale devices operating close to liquid helium temperatures, and are not fully applicable to a new generation of much smaller devices operating at significantly lower temperatures. We have therefore developed a new analytic heat model which enables a more accurate prediction of the thermal behaviour in such circumstances. We demonstrate that this model is in good agreement with experimental results measured down to 100 mK and discuss its validity for different nanoSQUID geometries.