353 research outputs found
Gate Adjustable Coherent Three and Four Level Mixing in a Vertical Quantum Dot Molecule
We study level mixing in the single particle energy spectrum of one of the
constituent quantum dots in a vertical double quantum dot by performing
magneto-resonant-tunneling spectroscopy. The device used in this study differs
from previous vertical double quantum dot devices in that the single side gate
is now split into four separate gates. Because of the presence of natural
perturbations caused by anharmonicity and anistrophy, applying different
combinations of voltages to these gates allows us to alter the effective
potential landscape of the two dots and hence influence the level mixing. We
present here preliminary results from one three level crossing and one four
level crossings high up in the energy spectrum of one of the probed quantum
dots, and demonstrate that we are able to change significantly the energy
dispersions with magnetic field in the vicinity of the crossing regions.Comment: 5 pages, 4 figures. MSS-14 conference proceedings submitted to
Physica
Single charge sensing and transport in double quantum dots fabricated from commercially grown Si/SiGe heterostructures
We perform quantum Hall measurements on three types of commercially available
modulation doped Si/SiGe heterostructures to determine their suitability for
depletion gate defined quantum dot devices. By adjusting the growth parameters,
we are able to achieve electron gases with charge densities 1-3 X 10^{11}/cm^2
and mobilities in excess of 100,000 cm^2/Vs. Double quantum dot devices
fabricated on these heterostructures show clear evidence of single charge
transitions as measured in dc transport and charge sensing and exhibit electron
temperatures of 100 mK in the single quantum dot regime.Comment: Related papers at http://pettagroup.princeton.ed
Investigation of Mobility Limiting Mechanisms in Undoped Si/SiGe Heterostructures
We perform detailed magnetotransport studies on two-dimensional electron
gases (2DEGs) formed in undoped Si/SiGe heterostructures in order to identify
the electron mobility limiting mechanisms in this increasingly important
materials system. By analyzing data from 26 wafers with different
heterostructure growth profiles we observe a strong correlation between the
background oxygen concentration in the Si quantum well and the maximum
mobility. The highest quality wafer supports a 2DEG with a mobility of 160,000
cm^2/Vs at a density 2.17 x 10^11/cm^2 and exhibits a metal-to-insulator
transition at a critical density 0.46 x 10^11/cm^2. We extract a valley
splitting of approximately 150 microeV at a magnetic field of 1.8 T. These
results provide evidence that undoped Si/SiGe heterostructures are suitable for
the fabrication of few-electron quantum dots.Comment: Related papers at http://pettagroup.princeton.ed
Charge Relaxation in a Single Electron Si/SiGe Double Quantum Dot
We measure the interdot charge relaxation time T_1 of a single electron
trapped in an accumulation mode Si/SiGe double quantum dot. The energy level
structure of the charge qubit is determined using photon assisted tunneling,
which reveals the presence of a low lying excited state. We systematically
measure T_1 as a function of detuning and interdot tunnel coupling and show
that it is tunable over four orders of magnitude, with a maximum of 45
microseconds for our device configuration. Measured relaxation times are
consistent with a phonon mediated energy relaxation process and indicate that
low lying excited states may have important implications in the development of
silicon spin qubits.Comment: Related papers at http://pettagroup.princeton.ed
Serum Concentrations of Myostatin and Myostatin-Interacting Proteins do not differ between young and Scarcopenic elderly men
Peer reviewedPostprin
Efficient multi-class fetal brain segmentation in high resolution MRI reconstructions with noisy labels
Segmentation of the developing fetal brain is an important step in
quantitative analyses. However, manual segmentation is a very time-consuming
task which is prone to error and must be completed by highly specialized
indi-viduals. Super-resolution reconstruction of fetal MRI has become standard
for processing such data as it improves image quality and resolution. However,
dif-ferent pipelines result in slightly different outputs, further complicating
the gen-eralization of segmentation methods aiming to segment super-resolution
data. Therefore, we propose using transfer learning with noisy multi-class
labels to automatically segment high resolution fetal brain MRIs using a single
set of seg-mentations created with one reconstruction method and tested for
generalizability across other reconstruction methods. Our results show that the
network can auto-matically segment fetal brain reconstructions into 7 different
tissue types, regard-less of reconstruction method used. Transfer learning
offers some advantages when compared to training without pre-initialized
weights, but the network trained on clean labels had more accurate
segmentations overall. No additional manual segmentations were required.
Therefore, the proposed network has the potential to eliminate the need for
manual segmentations needed in quantitative analyses of the fetal brain
independent of reconstruction method used, offering an unbiased way to quantify
normal and pathological neurodevelopment.Comment: Accepted for publication at PIPPI MICCAI 202
Eurasian Arctic greening reveals teleconnections and the potential for novel ecosystems
Arctic warming has been linked to observed increases in tundra shrub cover and growth in recent decades on the basis of significant relationships between deciduous shrub growth/biomass and temperature. These vegetation trends have been linked to Arctic sea ice decline and thus to the sea ice/albedo feedback known as Arctic amplification. However, the interactions between climate, sea ice and tundra vegetation remain poorly understood. Here we reveal a 50- year growth response over a >100,000 km2 area to a rise in summer temperature for alder (Alnus) and willow (Salix), the most abundant shrub genera respectively at and north of the continental treeline. We demonstrate that whereas plant productivity is related to sea ice in late spring, the growing season peak responds to persistent synoptic-scale air masses over West Siberia associated with Fennoscandian weather systems through the Rossby wave train. Substrate is important for biomass accumulation, yet a strong correlation between growth and temperature encompasses all observed soil types. Vegetation is especially responsive to temperature in early summer. These results have significant implications for modelling present and future Low Arctic vegetation responses to climate change, and emphasize the potential for structurally novel ecosystems to emerge fromwithin the tundra zone.Vertaisarviointia edeltävä käsikirjoitu
Melanoma cells break down LPA to establish local gradients that drive chemotactic dispersal.
The high mortality of melanoma is caused by rapid spread of cancer cells, which occurs unusually early in tumour evolution. Unlike most solid tumours, thickness rather than cytological markers or differentiation is the best guide to metastatic potential. Multiple stimuli that drive melanoma cell migration have been described, but it is not clear which are responsible for invasion, nor if chemotactic gradients exist in real tumours. In a chamber-based assay for melanoma dispersal, we find that cells migrate efficiently away from one another, even in initially homogeneous medium. This dispersal is driven by positive chemotaxis rather than chemorepulsion or contact inhibition. The principal chemoattractant, unexpectedly active across all tumour stages, is the lipid agonist lysophosphatidic acid (LPA) acting through the LPA receptor LPAR1. LPA induces chemotaxis of remarkable accuracy, and is both necessary and sufficient for chemotaxis and invasion in 2-D and 3-D assays. Growth factors, often described as tumour attractants, cause negligible chemotaxis themselves, but potentiate chemotaxis to LPA. Cells rapidly break down LPA present at substantial levels in culture medium and normal skin to generate outward-facing gradients. We measure LPA gradients across the margins of melanomas in vivo, confirming the physiological importance of our results. We conclude that LPA chemotaxis provides a strong drive for melanoma cells to invade outwards. Cells create their own gradients by acting as a sink, breaking down locally present LPA, and thus forming a gradient that is low in the tumour and high in the surrounding areas. The key step is not acquisition of sensitivity to the chemoattractant, but rather the tumour growing to break down enough LPA to form a gradient. Thus the stimulus that drives cell dispersal is not the presence of LPA itself, but the self-generated, outward-directed gradient
Can forest management based on natural disturbances maintain ecological resilience?
Given the increasingly global stresses on forests, many ecologists argue that managers must maintain ecological resilience: the capacity of ecosystems to absorb disturbances without undergoing fundamental change. In this review we ask: Can the emerging paradigm of natural-disturbance-based management (NDBM) maintain ecological resilience in managed forests? Applying resilience theory requires careful articulation of the ecosystem state under consideration, the disturbances and stresses that affect the persistence of possible alternative states, and the spatial and temporal scales of management relevance. Implementing NDBM while maintaining resilience means recognizing that (i) biodiversity is important for long-term ecosystem persistence, (ii) natural disturbances play a critical role as a generator of structural and compositional heterogeneity at multiple scales, and (iii) traditional management tends to produce forests more homogeneous than those disturbed naturally and increases the likelihood of unexpected catastrophic change by constraining variation of key environmental processes. NDBM may maintain resilience if silvicultural strategies retain the structures and processes that perpetuate desired states while reducing those that enhance resilience of undesirable states. Such strategies require an understanding of harvesting impacts on slow ecosystem processes, such as seed-bank or nutrient dynamics, which in the long term can lead to ecological surprises by altering the forest's capacity to reorganize after disturbance
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