Depletion-mode Quantum Dots in Intrinsic Silicon

We report the fabrication and electrical characterization of depletion-mode quantum dots in a two-dimensional hole gas (2DHG) in intrinsic silicon. We use fixed charge in a SiO$_2$/Al$_2$O$_3$ dielectric stack to induce a 2DHG at the Si/SiO$_2$ interface. Fabrication of the gate structures is accomplished with a single layer metallization process. Transport spectroscopy reveals regular Coulomb oscillations with charging energies of 10-15 meV and 3-5 meV for the few- and many-hole regimes, respectively. This depletion-mode design avoids complex multilayer architectures requiring precision alignment, and allows to adopt directly best practices already developed for depletion dots in other material systems. We also demonstrate a method to deactivate fixed charge in the SiO$_2$/Al$_2$O$_3$ dielectric stack using deep ultraviolet light, which may become an important procedure to avoid unwanted 2DHG build-up in Si MOS quantum bits.Comment: Accepted to Applied Physics Letters. 5 pages, 3 figure

Anisotropic Pauli spin blockade in hole quantum dots

We present measurements on gate-defined double quantum dots in Ge-Si core-shell nanowires, which we tune to a regime with visible shell filling in both dots. We observe a Pauli spin blockade and can assign the measured leakage current at low magnetic fields to spin-flip cotunneling, for which we measure a strong anisotropy related to an anisotropic g-factor. At higher magnetic fields we see signatures for leakage current caused by spin-orbit coupling between (1,1)-singlet and (2,0)-triplet states. Taking into account these anisotropic spin-flip mechanisms, we can choose the magnetic field direction with the longest spin lifetime for improved spin-orbit qubits

Quantitative live-cell imaging and computational modelling shed new light on endogenous WNT/CTNNB1 signaling dynamics

WNT/CTNNB1 signaling regulates tissue development and homeostasis in all multicellular animals, but the underlying molecular mechanism remains incompletely understood. Specifically, quantitative insight into endogenous protein behavior is missing. Here, we combine CRISPR/Cas9-mediated genome editing and quantitative live-cell microscopy to measure the dynamics, diffusion characteristics and absolute concentrations of fluorescently tagged, endogenous CTNNB1 in human cells under both physiological and oncogenic conditions. State-of-the-art imaging reveals that a substantial fraction of CTNNB1 resides in slow-diffusing cytoplasmic complexes, irrespective of the activation status of the pathway. This cytoplasmic CTNNB1 complex undergoes a major reduction in size when WNT/CTNNB1 is (hyper)activated. Based on our biophysical measurements, we build a computational model of WNT/CTNNB1 signaling. Our integrated experimental and computational approach reveals that WNT pathway activation regulates the dynamic distribution of free and complexed CTNNB1 across different subcellular compartments through three regulatory nodes: the destruction complex, nucleocytoplasmic shuttling, and nuclear retention

Clinical applications of 7T MRI in the brain

AbstractThis review illustrates current applications and possible future directions of 7Tesla (7T) Magnetic Resonance Imaging (MRI) in the field of brain MRI, in clinical studies as well as clinical practice. With its higher signal-to-noise (SNR) and contrast-to-noise ratio (CNR) compared to lower field strengths, high resolution, contrast-rich images can be obtained of diverse pathologies, like multiple sclerosis (MS), brain tumours, aging-related changes and cerebrovascular diseases. In some of these diseases, additional pathophysiological information can be gained compared to lower field strengths. Because of clear depiction of small anatomical details, and higher lesion conspicuousness, earlier diagnosis and start of treatment of brain diseases may become possible. Furthermore, additional insight into the pathogenesis of brain diseases obtained with 7T MRI could be the basis for new treatment developments. However, imaging at high field comes with several limitations, like inhomogeneous transmit fields, a higher specific absorption rate (SAR) and, currently, extensive contraindications for patient scanning. Future studies will be aimed at assessing the advantages and disadvantages of 7T MRI over lower field strengths in light of clinical applications, specifically the additional diagnostic and prognostic value of 7T MRI

Strong coupling between single-electron tunneling and nano-mechanical motion

Nanoscale resonators that oscillate at high frequencies are useful in many measurement applications. We studied a high-quality mechanical resonator made from a suspended carbon nanotube driven into motion by applying a periodic radio frequency potential using a nearby antenna. Single-electron charge fluctuations created periodic modulations of the mechanical resonance frequency. A quality factor exceeding 10^5 allows the detection of a shift in resonance frequency caused by the addition of a single-electron charge on the nanotube. Additional evidence for the strong coupling of mechanical motion and electron tunneling is provided by an energy transfer to the electrons causing mechanical damping and unusual nonlinear behavior. We also discovered that a direct current through the nanotube spontaneously drives the mechanical resonator, exerting a force that is coherent with the high-frequency resonant mechanical motion.Comment: Main text 12 pages, 4 Figures, Supplement 13 pages, 6 Figure