Thermal relaxation below 1 K by phonons and photons in metallic nanostructures

We present experimental results on thermal energy flow (dissipation) from different metallic nanostructures below 1K, concentrating on the two possible channels: relaxation by phonon or by photon emission. We show how phonon emission can be suppressed either by lowering the dimensionality of the phonon gas (2D membranes), or by introducing impurities into the metal. As an example, we discuss Aluminum doped with Manganese, a material with significant technological importance for ultrasensitive detectors and solid-state coolers. We also discuss relaxation by photon emission in the near field, which becomes siginificant for distances in the sub-micron lengthscale at the sub-Kelvin temperature range. Theoretical understanding for most of the results obtained is still lacking.