Microkelvin Nanoelectronics

Of all parameters, determining the behaviour of a physical system in the laboratory, temperature is one of the most important, if not themost important. The study of solid matter at cryogenic temperatures revealed unexpected phenomena like superconductivity and was closely related to the verification of new and revolutionary concepts in solidstate physics in the last century. The nowadays pursued development of quantum effect devices is closely related to technologies of creating ultralow temperatures on the microand nanometerscale. Achievable electron temperatures in miniaturized electronic devices are currently limited to the millikelvin temperature regime, caused by the technical limits of 3He/4He dilution refrigeration in combination with hot-electron effects, due to a strongly weakening electron-phonon coupling strength in miniaturized electronic conductors with lowering the temperature. New physical and technological concepts, like the use of topological materials for quantuminformation processing, created an interest in reaching lower electron temperatures in nanoelectronic devices than currently possible. For this purpose, a bridge to classical ultralow temperature research, where microkelvin cooling of bulk solids is achieved for several decades, has to be built. This work is dedicated to the goal of enabling cooling of nanoelectronic devices to the yet unreached microkelvin regime. For enabling microkelvin refrigeration of nanodevices, methods for chipscale nuclear magnetic cooling are developed, in order to cool electrons on a chip to microkelvin temperatures by direct spin-spin thermalization with nuclear spins, bypassing the weak electron-phonon interaction. A decisive key for reaching a nuclear cooling power in miniaturized volumes, which is sufficient to refrigerate a nanoelectronic circuit to microkelvin temperatures, is the utilization of nonequidistant nuclear level splitting by nuclear quadrupole interaction in the nuclear refrigerant. The metal indium is proposed and utilized as nuclear refrigerant for this purpose, since it combines a strong quadrupolar interaction with a strong hyperfine interaction. The ultilization of indium for magnetic cooling on the micro- and nanoscale is studied by integrating electrochemically deposited indiumfilms onto a Coulomb blockade thermometer. Coulomb blockade thermometry is based on thermally activated single charge transport by tunneling between metallic islands and comprises an ideal and scalable solution for nuclear magnetic cooling and electronic thermometry on a combined, miniaturized platform. By combining indiummicrorefrigerators with an off-chip nuclear magnetic cooling stage, tailor made to couple the chip to an ultracold environment, cooling of a nanoelectronic device to microkelvin temperatures is demonstrated for the first time. With the cooling schemes for miniaturized electronic devices developed in this work, the foundation for quantum nanoelectronics at microkelvin temperatures is laid, opening the door to an experimentally unknown territory in physics.