1,729 research outputs found
Precision spectroscopy of the 3s-3p fine structure doublet in Mg+
We apply a recently demonstrated method for precision spectroscopy on strong
transitions in trapped ions to measure both fine structure components of the
3s-3p transition in 24-Mg+ and 26-Mg+. We deduce absolute frequency reference
data for transition frequencies, isotope shifts and fine structure splittings
that are in particular useful for comparison with quasar absorption spectra,
which test possible space-time variations of the fine structure constant. The
measurement accuracy improves previous literature values, when existing, by
more than two orders of magnitude
Axon diameters and myelin content modulate microscopic fractional anisotropy at short diffusion times in fixed rat spinal cord
Mapping tissue microstructure accurately and noninvasively is one of the
frontiers of biomedical imaging. Diffusion Magnetic Resonance Imaging (MRI) is
at the forefront of such efforts, as it is capable of reporting on microscopic
structures orders of magnitude smaller than the voxel size by probing
restricted diffusion. Double Diffusion Encoding (DDE) and Double Oscillating
Diffusion Encoding (DODE) in particular, are highly promising for their ability
to report on microscopic fractional anisotropy ({\mu}FA), a measure of the pore
anisotropy in its own eigenframe, irrespective of orientation distribution.
However, the underlying correlates of {\mu}FA have insofar not been studied.
Here, we extract {\mu}FA from DDE and DODE measurements at ultrahigh magnetic
field of 16.4T in the aim to probe fixed rat spinal cord microstructure. We
further endeavor to correlate {\mu}FA with Myelin Water Fraction (MWF) derived
from multiexponential T2 relaxometry, as well as with literature-based
spatially varying axonal diameters. In addition, a simple new method is
presented for extracting unbiased {\mu}FA from three measurements at different
b-values. Our findings reveal strong anticorrelations between {\mu}FA (derived
from DODE) and axon diameter in the distinct spinal cord tracts; a moderate
correlation was also observed between {\mu}FA derived from DODE and MWF. These
findings suggest that axonal membranes strongly modulate {\mu}FA, which - owing
to its robustness towards orientation dispersion effects - reflects axon
diameter much better than its typical FA counterpart. The {\mu}FA exhibited
modulations when measured via oscillating or blocked gradients, suggesting
selective probing of different parallel path lengths and providing insight into
how those modulate {\mu}FA metrics. Our findings thus shed light into the
underlying microstructural correlates of {\mu}FA and are (...
Analytical method for parameterizing the random profile components of nanosurfaces imaged by atomic force microscopy
The functional properties of many technological surfaces in biotechnology,
electronics, and mechanical engineering depend to a large degree on the
individual features of their nanoscale surface texture, which in turn are a
function of the surface manufacturing process. Among these features, the
surface irregularities and self-similarity structures at different spatial
scales, especially in the range of 1 to 100 nm, are of high importance because
they greatly affect the surface interaction forces acting at a nanoscale
distance. An analytical method for parameterizing the surface irregularities
and their correlations in nanosurfaces imaged by atomic force microscopy (AFM)
is proposed. In this method, flicker noise spectroscopy - a statistical physics
approach - is used to develop six nanometrological parameters characterizing
the high-frequency contributions of jump- and spike-like irregularities into
the surface texture. These contributions reflect the stochastic processes of
anomalous diffusion and inertial effects, respectively, in the process of
surface manufacturing. The AFM images of the texture of corrosion-resistant
magnetite coatings formed on low-carbon steel in hot nitrate solutions with
coating growth promoters at different temperatures are analyzed. It is shown
that the parameters characterizing surface spikiness are able to quantify the
effect of process temperature on the corrosion resistance of the coatings. It
is suggested that these parameters can be used for predicting and
characterizing the corrosion-resistant properties of magnetite coatings.Comment: 7 pages, 3 figures, 2 tables; to be published in Analys
Terahertz quantum plasmonics at nanoscales and angstrom scales
Through the manipulation of metallic structures, light-matter interaction can enter into the realm of quantum mechanics. For example, intense terahertz pulses illuminating a metallic nanotip can promote terahertz field-driven electron tunneling to generate enormous electron emission currents in a subpicosecond time scale. By decreasing the dimension of the metallic structures down to the nanoscale and angstrom scale, one can obtain a strong field enhancement of the incoming terahertz field to achieve atomic field strength of the order of V/nm, driving electrons in the metal into tunneling regime by overcoming the potential barrier. Therefore, designing and optimizing the metal structure for high field enhancement are an essential step for studying the quantum phenomena with terahertz light. In this review, we present several types of metallic structures that can enhance the coupling of incoming terahertz pulses with the metals, leading to a strong modification of the potential barriers by the terahertz electric fields. Extreme nonlinear responses are expected, providing opportunities for the terahertz light for the strong light-matter interaction. Starting from a brief review about the terahertz field enhancement on the metallic structures, a few examples including metallic tips, dipole antenna, and metal nanogaps are introduced for boosting the quantum phenomena. The emerging techniques to control the electron tunneling driven by the terahertz pulse have a direct impact on the ultrafast science and on the realization of next-generation quantum devices
Structural Adaptive Smoothing in Diffusion Tensor Imaging: The R Package dti
Diffusion weighted imaging has become and will certainly continue to be an important tool in medical research and diagnostics. Data obtained with diffusion weighted imaging are characterized by a high noise level. Thus, estimation of quantities like anisotropy indices or the main diffusion direction may be significantly compromised by noise in clinical or neuroscience applications. Here, we present a new package dti for R, which provides functions for the analysis of diffusion weighted data within the diffusion tensor model. This includes smoothing by a recently proposed structural adaptive smoothing procedure based on the propagation-separation approach in the context of the widely used diffusion tensor model. We extend the procedure and show, how a correction for Rician bias can be incorporated. We use a heteroscedastic nonlinear regression model to estimate the diffusion tensor. The smoothing procedure naturally adapts to different structures of different size and thus avoids oversmoothing edges and fine structures. We illustrate the usage and capabilities of the package through some examples.
Nanomechanical sensing using spins in diamond
Nanomechanical sensors and quantum nanosensors are two rapidly developing
technologies that have diverse interdisciplinary applications in biological and
chemical analysis and microscopy. For example, nanomechanical sensors based
upon nanoelectromechanical systems (NEMS) have demonstrated chip-scale mass
spectrometry capable of detecting single macromolecules, such as proteins.
Quantum nanosensors based upon electron spins of negatively-charged
nitrogen-vacancy (NV) centers in diamond have demonstrated diverse modes of
nanometrology, including single molecule magnetic resonance spectroscopy. Here,
we report the first step towards combining these two complementary technologies
in the form of diamond nanomechanical structures containing NV centers. We
establish the principles for nanomechanical sensing using such
nano-spin-mechanical sensors (NSMS) and assess their potential for mass
spectrometry and force microscopy. We predict that NSMS are able to provide
unprecedented AC force images of cellular biomechanics and to, not only detect
the mass of a single macromolecule, but also image its distribution. When
combined with the other nanometrology modes of the NV center, NSMS potentially
offer unparalleled analytical power at the nanoscale.Comment: Errors in the stress susceptibility parameters present in the
original arXiv version have been correcte
On stable reconstructions from nonuniform Fourier measurements
We consider the problem of recovering a compactly-supported function from a
finite collection of pointwise samples of its Fourier transform taking
nonuniformly. First, we show that under suitable conditions on the sampling
frequencies - specifically, their density and bandwidth - it is possible to
recover any such function in a stable and accurate manner in any given
finite-dimensional subspace; in particular, one which is well suited for
approximating . In practice, this is carried out using so-called nonuniform
generalized sampling (NUGS). Second, we consider approximation spaces in one
dimension consisting of compactly supported wavelets. We prove that a linear
scaling of the dimension of the space with the sampling bandwidth is both
necessary and sufficient for stable and accurate recovery. Thus wavelets are up
to constant factors optimal spaces for reconstruction
The Kondo Resonance in Electron Spectroscopy
The Kondo resonance is the spectral manifestation of the Kondo properties of
the impurity Anderson model, and also plays a central role in the dynamical
mean-field theory (DMFT) for correlated electron lattice systems. This article
presents an overview of electron spectroscopy studies of the resonance for the
4f electrons of cerium compounds, and for the 3d electrons of V_2O_3, including
beginning efforts at using angle resolved photoemission to determine the
k-dependence of the resonance. The overview includes the comparison and
analysis of spectroscopy data with theoretical spectra as calculated for the
impurity model and as obtained by DMFT, and the Kondo volume collapse
calculation of the cerium alpha-gamma phase transition boundary, with its
spectroscopic underpinnings.Comment: 32 pages, 11 figures, 151 references; paper for special issue of J.
Phys. Soc. Jpn. on "Kondo Effect--40 Years after the Discovery
Adaptive smoothing of multi-shell diffusion-weighted magnetic resonance data by msPOAS
In this article we present a noise reduction method (msPOAS) for multi-shell diffusion-weighted magnetic resonance data. To our knowledge, this is the first smoothing method which allows simultaneous smoothing of all q-shells. It is applied directly to the diffusion weighted data and consequently allows subsequent analysis by any model. Due to its adaptivity, the procedure avoids blurring of the inherent structures and preserves discontinuities. MsPOAS extends the recently developed position-orientation adaptive smoothing (POAS) procedure to multi-shell experiments. At the same time it considerably simplifies and accelerates the calculations. The behavior of the algorithm msPOAS is evaluated on diffusion-weighted data measured on a single shell and on multiple shells
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