At low temperatures, the thermal wavelength of acoustic phonons in a metallic thin film on a substrate can widely exceed the film thickness. It is thus generally believed that a mesoscopic device operating at low temperature does not carry an individ
ual phonon population. In this work, we provide direct experimental evidence for the thermal decoupling of phonons in a mesoscopic quantum device from its substrate phonon heat bath at a sub-Kelvin temperature. A simple heat balance model assuming an independent phonon bath following the usual electron-phonon and Kapitza coupling laws can account for all experimental observations.
A novel method to fabricate large-area superconducting hybrid tunnel junctions with a suspended central normal metal part is presented. The samples are fabricated by combining photo-lithography and chemical etch of a superconductor - insulator - norm
al metal multilayer. The process involves few fabrication steps, is reliable and produces extremely high-quality tunnel junctions. Under an appropriate voltage bias, a significant electronic cooling is demonstrated.
