1,545 research outputs found

### Target effects in negative-continuum assisted dielectronic recombination

The process of recombination of a quasi-free electron into a bound state of
an initially bare nucleus with the simultaneous creation of a
bound-electron--free-positron pair is investigated. This process is called the
negative-continuum assisted dielectronic recombination (NCDR). In a typical
experimental setup, the initial electron is not free but bound in a light
atomic target. In the present work, we study the effects of the atomic target
on the single and double-differential cross sections of the positron production
in the NCDR process. The calculations are performed within the relativistic
framework based on QED theory, with accounting for the electron-electron
interaction to first order in perturbation theory. We demonstrate how the
momentum distribution of the target electrons removes the non-physical
singularity of the differential cross section which occurs for the initially
free and monochromatic electrons

### Post-Wick theorems for symbolic manipulation of second-quantized expressions in atomic many-body perturbation theory

Manipulating expressions in many-body perturbation theory becomes unwieldily
with increasing order of the perturbation theory. Here I derive a set of
theorems for efficient simplification of such expressions. The derived rules
are specifically designed for implementing with symbolic algebra tools. As an
illustration, we count the numbers of Brueckner-Goldstone diagrams in the first
several orders of many-body perturbation theory for matrix elements between two
states of a mono-valent system.Comment: J. Phys. B. (in press); Mathematica packages available from
http://wolfweb.unr.edu/homepage/andrei/WWW-tap/mathematica.htm

### Classical analogy for the deflection of flux avalanches by a metallic layer

Sudden avalanches of magnetic flux bursting into a superconducting sample
undergo deflections of their trajectories when encountering a conductive layer
deposited on top of the superconductor. Remarkably, in some cases flux is
totally excluded from the area covered by the conductive layer. We present a
simple classical model that accounts for this behaviour and considers a
magnetic monopole approaching a semi-infinite conductive plane. This model
suggests that magnetic braking is an important mechanism responsible for
avalanche deflection.Comment: 14 pages, 5 figure

### Quantifying cell-generated forces: Poisson's ratio matters

Quantifying mechanical forces generated by cellular systems has led to key insights into a broad range of biological phenomena from cell adhesion to immune cell activation. Traction force microscopy (TFM), the most widely employed force measurement methodology, fundamentally relies on knowledge of the force-displacement relationship and mechanical properties of the substrate. Together with the elastic modulus, the Poissonâ€™s ratio is a basic material property that to date has largely been overlooked in TFM. Here, we evaluate the sensitivity of TFM to Poissonâ€™s ratio by employing a series of computer simulations and experimental data analysis. We demonstrate how applying the correct Poissonâ€™s ratio is important for accurate force reconstruction and develop a framework for the determination of error levels resulting from the misestimation of the Poissonâ€™s ratio. In addition, we provide experimental estimation of the Poissonâ€™s ratios of elastic substrates commonly applied in TFM. Our work thus highlights the role of Poissonâ€™s ratio underpinning cellular force quantification studied across many biological systems

### Fragmentation and systematics of the Pygmy Dipole Resonance in the stable N=82 isotones

The low-lying electric dipole (E1) strength in the semi-magic nucleus 136Xe
has been measured which finalizes the systematic survey to investigate the
so-called pygmy dipole resonance (PDR) in all stable even N=82 isotones with
the method of nuclear resonance fluorescence using real photons in the entrance
channel. In all cases, a fragmented resonance-like structure of E1 strength is
observed in the energy region 5 MeV to 8 MeV. An analysis of the fragmentation
of the strength reveals that the degree of fragmentation decreases towards the
proton-deficient isotones while the total integrated strength increases
indicating a dependence of the total strength on the neutron-to-proton ratio.
The experimental results are compared to microscopic calculations within the
quasi-particle phonon model (QPM). The calculation includes complex
configurations of up to three phonons and is able to reproduce also the
fragmentation of the E1 strength which allows to draw conclusions on the
damping of the PDR. Calculations and experimental data are in good agreement in
the degree of fragmentation and also in the integrated strength if the
sensitivity limit of the experiments is taken into account

### Direct visualization of magnetic vortex pinning in superconductors

We study the vortex structure in a Pb film deposited on top of a periodic
array of ferromagnetic square microrings by combining two high resolution
imaging techniques: Bitter decoration and scanning Hall probe microscopy
(SHPM). The periodicity and strength of the magnetic pinning potential
generated by the square microrings are controlled by the magnetic history of
the template. When the square rings are in the magnetized dipolar state, known
as the onion state, the strong stray field generated at the domain walls
prevents the decoration of vortices. SHPM images show that the stray field
generated by the dipoles is much stronger than the vortex field in agreement
with the results of simulations. Real space vortex imaging has revealed that,
in the onion state, the corners of the square rings act as effective pinning
centers for vortices.Comment: To be published in Phys. Rev.

### Relativistic polarization analysis of Rayleigh scattering by atomic hydrogen

A relativistic analysis of the polarization properties of light elastically
scattered by atomic hydrogen is performed, based on the Dirac equation and
second order perturbation theory. The relativistic atomic states used for the
calculations are obtained by making use of the finite basis set method and
expressed in terms of $B$ splines and $B$ polynomials. We introduce two
experimental scenarios in which the light is circularly and linearly polarized,
respectively. For each of these scenarios, the polarization-dependent angular
distribution and the degrees of circular and linear polarization of the
scattered light are investigated as a function of scattering angle and photon
energy. Analytical expressions are derived for the polarization-dependent
angular distribution which can be used for scattering by both hydrogenic as
well as many-electron systems. Detailed computations are performed for Rayleigh
scattering by atomic hydrogen within the incident photon energy range 0.5 to 10
keV. Particular attention is paid to the effects that arise from higher
(nondipole) terms in the expansion of the electron-photon interaction.Comment: 8 pages, 5 figure

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