Frontiers in refrigeration and cooling: how to obtain and sustain ultralow temperatures beyond natureʹs ambience

The investigation of matter at very low temperatures close to absolute zero is a powerful and important subject as it provides the unique possibility for studying physical properties, e.g. phase transitions, in an environment where most of the unavoidable disturbances present at higher temperatures are almost completely frozen out. In particular, the improvement of the standard cooling techniques for mK- and μK temperatures, i.e. 3He–4He dilution refrigeration and adiabatic nuclear refrigeration, respectively, opens up a temperature range for the study of liquid and solid matter several orders of magnitude below the minimum temperature existing in nature (background temperature of the universe, ∼2.7 K). In this paper, some important contributions obtained at the ultralow temperature facility at the University of Bayreuth will be discussed: (1) the achievement of a minimum electronic temperature of Tel=1.5 μK by adiabatic nuclear refrigeration of platinum, (2) the progress in improving the thermal coupling between different materials at low temperatures, and (3) recent developments in low temperature thermometry.