Photonic negative differential thermal conductance enabled by NIS junctions

Due to their sensitivity to temperature variations, normal metal–insulator-superconductor (NIS) junctions are utilized in various thermal devices. This study illustrates that two NISIN reservoirs can achieve a measurable negative differential thermal conductance (NDTC). This phenomenon is enabled by photon-mediated heat exchange, which is profoundly affected by the temperature-dependent impedance matching between the reservoirs. Under suitable configurations, the heat current is suppressed for increasingly large temperature gradients, resulting in NDTC. We also propose experimental configurations that allow for the unambiguous discrimination of this effect. We employ superconducting aluminum in conjunction with either silver or epitaxial InAs to facilitate the experimental observation of NDTC at low temperatures over significant sub-Kelvin ranges. This advances the development of devices that exploit NDTC to enhance the regulation of heat and temperature in cryogenic environments, such as thermal switches, transistors, and amplifiers.