Influence of Temperature Gradients on Tunnel Junction Thermometry below 1 K: Cooling and Electron–Phonon Coupling

Abstract

We have studied thermal gradients in thin Cu and AlMn wires, both experimentally and theoretically. In the experiments, the wires were Joule heated non-uniformly at sub-Kelvin temperatures, and the resulting temperature gradients were measured using normal metal–insulator–superconducting tunnel junctions. The data clearly shows that even in reasonably well-conducting thin wires with a short (~10 μm) non-heated portion, significant temperature differences can form. In most cases, the measurements agree well with a model which includes electron–phonon interaction and electronic thermal conductivity by the Wiedemann–Franz law.