Magnetic refrigeration for nanoelectronics on a cryogen-free platform

Abstract

Nanostructured samples serve as a playground of solid state physics due to their vast diversity of applications. In addition to various fabrication recipes and measurement methods, the temperature at which these experiments are performed plays a crucial role because thermal excitations can conceal the underlying physics. Thus advancing to lower temperatures in solid state systems might shed light on presently unknown physical phenomena, as e.g. new topological states of matter. We present a novel type of refrigerator using adiabatic nuclear demagnetization with the goal of reaching sub-millikelvin electron temperatures in nanostructured samples. The nuclear stage consists of electronically separated Cu plates, each of which is part of a measurement lead. Before connecting to the nuclear stage, each lead is strongly filtered and then thermalized to the mixing chamber of the dilution refrigerator. This thesis presents measurements on two of these systems: the first operated in a standard, cryostat and the second on a pulse tube refrigerator. Both nuclear stages cool below 300 microkelvin with heat leaks in the order of a few nanowatts per mol of copper. We perform electronic transport measurements on various nanostructured samples. For the wet system, we extract electron temperatures around 5-7 mK after replacing the sample holder material and including an additional filtering stage. These measurements are highly sensitive to noise of the experimental setup and to the electrostatic environment of the devices, e.g. wafer-intrinsic charge noise. In yet another experiment on a high-mobility two-dimensional electron gas, we observe a quantization of the longitudinal resistance Rxx which arises from a density gradient across the wafer. As for the dry system, we attach a home-built magnetic field fluctuation thermometer to the nuclear stage. While calibrated at 4 K, it shows good agreement with various other thermometers down to 5 mK, with the lowest temperature being 700 microkelvin. However, electron temperatures in the samples are around 15 mK, possibly caused by the increased heat leak combined with the weakened thermalization.