Studies of Quantum Materials and Thermal Boundary Resistance in the Microkelvin Temperature Regime

Technical advances in the construction of cryogenic platforms must go hand in hand with the demands of experimental science. This work contains results of two distinct experiments in support of condensed matter research performed on a dry dilution refrigerator equipped with a newly built adiabatic nuclear demagnetisation stage, whose construction and performance characterisation is also presented. The last cooling step carried out by the demagnetisation stage reaching temperatures as low as 0.4 mK stands out, compared to other cryostats, in the time it takes to prepare the starting conditions when compared to the time spent performing experiments. Accompanied by a relatively straightforward in-house construction recipe and modularity of the design, the stage has large industrial potential in ultra-low temperature research. Both experiments performed on this cryostat benefit from the fast turnaround performance of the platform. The first setup is a magnetometer for probing the superconductivity in the heavy Fermion compound YbRh2Si2 . The phase diagram in temperature and magnetic field was studied in great detail, revealing previously unseen phases, and shining light onto the mechanism of suppressing the superconductivity with magnetic field. Heavy Fermions are prototypical compounds hosting unconventional superconductivity. Thus any experimental probe capable of yielding information about the paring mechanism is highly valuable to advancing the understanding of this phenomenon. The second experiment composing this work is a study of thermal boundary resistance between helium and solid matter, known in the field as the Kapitza resistance. The phenomenon is not entirely understood, which is unsatisfactory from theoretical as much as applied perspective. Ultra-low temperature research uses helium in different instances as the subject of interest and as a cooling agent. Heat exchangers enabling effective heat transfer with solid parts are thus crucial parts of the apparatus. The experiment studied the resistance of exfoliated graphite diffusion bonded to a copper foil, investigating if the large surface area of graphite could improve the thermal transfer.