Method for Cooling Nanostructures to Microkelvin Temperatures

We propose a new scheme aimed at cooling nanostructures to microkelvin temperatures, based on the well established technique of adiabatic nuclear demagnetization: we attach each device measurement lead to an individual nuclear refrigerator, allowing efficient thermal contact to a microkelvin bath. On a prototype consisting of a parallel network of nuclear refrigerators, temperatures of $\sim 1\,$mK simultaneously on ten measurement leads have been reached upon demagnetization, thus completing the first steps toward ultracold nanostructures.Comment: 4 pages, 3 (color) figure

GaAs Quantum Dot Thermometry Using Direct Transport and Charge Sensing

We present measurements of the electron temperature using gate defined quantum dots formed in a GaAs 2D electron gas in both direct transport and charge sensing mode. Decent agreement with the refrigerator temperature was observed over a broad range of temperatures down to 10 mK. Upon cooling nuclear demagnetization stages integrated into the sample wires below 1 mK, the device electron temperature saturates, remaining close to 10 mK. The extreme sensitivity of the thermometer to its environment as well as electronic noise complicates temperature measurements but could potentially provide further insight into the device characteristics. We discuss thermal coupling mechanisms, address possible reasons for the temperature saturation and delineate the prospects of further reducing the device electron temperature.Comment: 8 pages, 3 (color) figure

Metallic Coulomb Blockade Thermometry down to 10 mK and below

We present an improved nuclear refrigerator reaching 0.3 mK, aimed at microkelvin nanoelectronic experiments, and use it to investigate metallic Coulomb blockade thermometers (CBTs) with various resistances R. The high-R devices cool to slightly lower T, consistent with better isolation from the noise environment, and exhibit electron-phonon cooling ~ T^5 and a residual heat-leak of 40 aW. In contrast, the low-R CBTs display cooling with a clearly weaker T-dependence, deviating from the electronphonon mechanism. The CBTs agree excellently with the refrigerator temperature above 20 mK and reach a minimum-T of 7.5 +/- 0.2 mK.Comment: 3 pages, 3 (color) figure

GaAs Quantum Dot Thermometry Using Direct Transport and Charge Sensing

We present measurements of the electron temperature using gate-defined quantum dots formed in a GaAs 2D electron gas in both direct transport and charge sensing mode. Decent agreement with the refrigerator temperature was observed over a broad range of temperatures down to 10mK. Upon cooling nuclear demagnetization stages integrated into the sample wires below 1mK, the device electron temperature saturates, remaining close to 10mK. The extreme sensitivity of the thermometer to its environment as well as electronic noise complicates temperature measurements but could potentially provide further insight into the device characteristics. We discuss thermal coupling mechanisms, address possible reasons for the temperature saturation and delineate the prospects of further reducing the device electron temperature

Magnetic refrigeration for nanoelectronics on a cryogen-free platform

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, "wet" cryostat and the second on a "dry" 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

GaAs Quantum Dot Thermometry Using Direct Transport and Charge Sensing

We present measurements of the electron temperature using gate defined quantum dots formed in a GaAs 2D electron gas in both direct transport and charge sensing mode. Decent agreement with the refrigerator temperature was observed over a broad range of temperatures down to 10 mK. Upon cooling nuclear demagnetization stages integrated into the sample wires below 1 mK, the device electron temperature saturates, remaining close to 10 mK. The extreme sensitivity of the thermometer to its environment as well as electronic noise complicates temperature measurements but could potentially provide further insight into the device characteristics. We discuss thermal coupling mechanisms, address possible reasons for the temperature saturation and delineate the prospects of further reducing the device electron temperature