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

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

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

Impulse Control Disorders and Effort‐Based Decision‐Making in Parkinson's Disease Patients with Subthalamic Nucleus Deep Brain Stimulation

Background: Impulse control disorders (ICD) are common side effects of dopaminergic treatment in Parkinson's disease (PD). Whereas some studies show a reduction in ICD after subthalamic nucleus deep brain stimulation (STN‐DBS), others report worsening of ICD or impulsivity. Objective: The aim was to study ICD in the context of STN‐DBS using an objective measure of decision‐making. Methods: Ten PD patients performed an effort‐based decision‐making task alongside neuropsychiatric and cognitive evaluation before and 4 months after STN‐DBS. Further, 33 PD patients underwent the same experimental procedures just once after an average 40 months of chronic STN‐DBS. Participants were examined preoperatively in the medication on state and postoperatively in the medication on/stimulation ON state. Mixed linear models were used to assess the impact of ICD and STN‐DBS on acceptance rate and decision time in the task while controlling for motor symptom burden, cognitive measures, and dopaminergic medication. Results: Results revealed an increased willingness to exert high levels of effort in return for reward in patients with ICD, but acceptance rate was not modulated by chronic STN‐DBS. Further, ICD, cognitive processing speed, and STN‐DBS were all identified as positive predictors for faster decision speed. ICD scores showed a tendency to improve 4 months after STN‐DBS, without an increase in apathy scores. Conclusions: Chronic STN‐DBS and ICD facilitate effort‐based decision‐making by speeding up judgment. Furthermore, ICD enhances the willingness to exert high levels of effort for reward. Both STN‐DBS and dopaminergic medication impact motivated behavior and should be titrated carefully to balance neuropsychiatric symptoms

Oral abstracts of the 21st International AIDS Conference 18-22 July 2016, Durban, South Africa

The rate at which HIV-1 infected individuals progress to AIDS is highly variable and impacted by T cell immunity. CD8 T cell inhibitory molecules are up-regulated in HIV-1 infection and associate with immune dysfunction. We evaluated participants (n=122) recruited to the SPARTAC randomised clinical trial to determine whether CD8 T cell exhaustion markers PD-1, Lag-3 and Tim-3 were associated with immune activation and disease progression.Expression of PD-1, Tim-3, Lag-3 and CD38 on CD8 T cells from the closest pre-therapy time-point to seroconversion was measured by flow cytometry, and correlated with surrogate markers of HIV-1 disease (HIV-1 plasma viral load (pVL) and CD4 T cell count) and the trial endpoint (time to CD4 count <350 cells/μl or initiation of antiretroviral therapy). To explore the functional significance of these markers, co-expression of Eomes, T-bet and CD39 was assessed.Expression of PD-1 on CD8 and CD38 CD8 T cells correlated with pVL and CD4 count at baseline, and predicted time to the trial endpoint. Lag-3 expression was associated with pVL but not CD4 count. For all exhaustion markers, expression of CD38 on CD8 T cells increased the strength of associations. In Cox models, progression to the trial endpoint was most marked for PD-1/CD38 co-expressing cells, with evidence for a stronger effect within 12 weeks from confirmed diagnosis of PHI. The effect of PD-1 and Lag-3 expression on CD8 T cells retained statistical significance in Cox proportional hazards models including antiretroviral therapy and CD4 count, but not pVL as co-variants.Expression of ‘exhaustion’ or ‘immune checkpoint’ markers in early HIV-1 infection is associated with clinical progression and is impacted by immune activation and the duration of infection. New markers to identify exhausted T cells and novel interventions to reverse exhaustion may inform the development of novel immunotherapeutic approaches

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