8,895 research outputs found
Persistent quantum interfering electron trajectories
The emission of above-ionization-threshold harmonics results from the
recombination of two electron wavepackets moving along a "short" and a "long"
trajectory in the atomic continuum. Attosecond pulse train generation has so
far been attributed to the short trajectory, attempted to be isolated through
targeted trajectory-selective phase matching conditions. Here, we provide
experimental evidence for the contribution of both trajectories to the harmonic
emission, even under phase matching conditions unfavorable for the long
trajectory. This is finger printed in the interference modulation of the
harmonic yield as a function of the driving laser intensity. The effect is also
observable in the sidebands yield resulting from the frequency mixing of the
harmonics and the driving laser field, an effect with consequences in
cross-correlation pulse metrology approaches.Comment: 13 pages, 3 figure
On uniformly rotating fluid drops trapped between two parallel plates
This contribution is about the dynamics of a liquid bridge between two fixed parallel plates. We consider a mathematical model and present some results from the doctoral thesis [10] of the first author. He showed that there is a Poisson bracket and a corresponding Hamiltonian, so that the model equations are in Hamiltonian form. The result generalizes previous results of Lewis et al. on the dynamics of free boundary problems for "free" liquid drops to the case of a drop between two parallel plates, including, especially the effect of capillarity and the angle of contact between the plates and the free fluid surface. Also, we prove the existence of special solutions which represent uniformly rotating fluid ridges, and we present specific stability conditions for these solutions. These results extend work of Concus and Finn [2] and Vogel [18],[19] on static capillarity problems (see also Finn [5]). Using the Hamiltonian structure of the model equations and symmetries of the solutions, the stability conditions can be derived in a systematic way. The ideas that are described will be useful for other situations involving capillarity and free boundary problems as well
Ab-initio shell model with a core
We construct effective 2- and 3-body Hamiltonians for the p-shell by
performing 12\hbar\Omega ab initio no-core shell model (NCSM) calculations for
A=6 and 7 nuclei and explicitly projecting the many-body Hamiltonians onto the
0\hbar\Omega space. We then separate these effective Hamiltonians into 0-, 1-
and 2-body contributions (also 3-body for A=7) and analyze the systematic
behavior of these different parts as a function of the mass number A and size
of the NCSM basis space. The role of effective 3- and higher-body interactions
for A>6 is investigated and discussed
Effective operators from exact many-body renormalization
We construct effective two-body Hamiltonians and E2 operators for the p-shell
by performing ab initio no-core shell model (NCSM) calculations
for A=5 and A=6 nuclei and explicitly projecting the many-body Hamiltonians and
E2 operator onto the space. We then separate the effective E2
operator into one-body and two-body contributions employing the two-body
valence cluster approximation. We analyze the convergence of proton and neutron
valence one-body contributions with increasing model space size and explore the
role of valence two-body contributions. We show that the constructed effective
E2 operator can be parametrized in terms of one-body effective charges giving a
good estimate of the NCSM result for heavier p-shell nuclei.Comment: 9 pages, 8 figure
Chromospheric Variability in SDSS M Dwarfs. II. Short-Timescale H-alpha Variability
[Abridged] We present the first comprehensive study of short-timescale
chromospheric H-alpha variability in M dwarfs using the individual 15 min
spectroscopic exposures for 52,392 objects from the Sloan Digital Sky Survey.
Our sample contains about 10^3-10^4 objects per spectral type bin in the range
M0-M9, with a total of about 206,000 spectra and a typical number of 3
exposures per object (ranging up to a maximum of 30 exposures). Using this
extensive data set we find that about 16% of the sources exhibit H-alpha
emission in at least one exposure, and of those about 45% exhibit H-alpha
emission in all of the available exposures. Within the sample of objects with
H-alpha emission, only 26% are consistent with non-variable emission,
independent of spectral type. The H-alpha variability, quantified in terms of
the ratio of maximum to minimum H-alpha equivalent width (R_EW), and the ratio
of the standard deviation to the mean (sigma_EW/), exhibits a rapid rise
from M0 to M5, followed by a plateau and a possible decline in M9 objects. In
particular, R_EW increases from a median value of about 1.8 for M0-M3 to about
2.5 for M7-M9, and variability with R_EW>10 is only observed in objects later
than M5. For the combined sample we find that the R_EW values follow an
exponential distribution with N(R_EW) exp[-(R_EW-1)/2]; for M5-M9 objects the
characteristic scale is R_EW-1\approx 2.7, indicative of stronger variability.
In addition, we find that objects with persistent H-alpha emission exhibit
smaller values of R_EW than those with intermittent H-alpha emission. Based on
these results we conclude that H-alpha variability in M dwarfs on timescales of
15 min to 1 hr increases with later spectral type, and that the variability is
larger for intermittent sources.Comment: Submitted to ApJ; 20 pages, 15 figure
Robust formation of morphogen gradients
We discuss the formation of graded morphogen profiles in a cell layer by
nonlinear transport phenomena, important for patterning developing organisms.
We focus on a process termed transcytosis, where morphogen transport results
from binding of ligands to receptors on the cell surface, incorporation into
the cell and subsequent externalization. Starting from a microscopic model, we
derive effective transport equations. We show that, in contrast to morphogen
transport by extracellular diffusion, transcytosis leads to robust ligand
profiles which are insensitive to the rate of ligand production
On the monotone stability approach to BSDEs with jumps: Extensions, concrete criteria and examples
We show a concise extension of the monotone stability approach to backward
stochastic differential equations (BSDEs) that are jointly driven by a Brownian
motion and a random measure for jumps, which could be of infinite activity with
a non-deterministic and time inhomogeneous compensator. The BSDE generator
function can be non convex and needs not to satisfy global Lipschitz conditions
in the jump integrand. We contribute concrete criteria, that are easy to
verify, for results on existence and uniqueness of bounded solutions to BSDEs
with jumps, and on comparison and a-priori -bounds. Several
examples and counter examples are discussed to shed light on the scope and
applicability of different assumptions, and we provide an overview of major
applications in finance and optimal control.Comment: 28 pages. Added DOI
https://link.springer.com/chapter/10.1007%2F978-3-030-22285-7_1 for final
publication, corrected typo (missing gamma) in example 4.1
Yield and Viability of Human Limbal Stem Cells From Fresh and Stored Tissue
Purpose: We compared cell number, putative stem cell markers, and clonogenic ability in fresh uncultured human limbal epithelial cells to that obtained from stored organ-cultured tissue. Methods: Cell suspensions were formed from fresh and organ culture–stored human limbal epithelium. Expression of putative stem cell markers ΔNp63 and TrkA was performed using immunofluorescent staining before culture. Colony-forming efficiency (CFE) assays were performed at first passage. The effects of tissue storage, age, and postmortem/culture times were analyzed in a general linear model. Results: Limbal tissue from 94 donors (34 fresh and 60 stored) was compared. Three times more cells were obtained per eye from fresh (35.34 × 104; SD, 17.39) than stored (11.24 × 104; SD, 11.57; P < 0.01) tissue. A higher proportion of cells from fresh tissue were viable (91.9%; SD, 5.7 vs. 85%; SD, 10.8) P < 0.01. Higher total cell expression of ΔNp63 (20.19 × 104; SD, 15.5 vs. 3.28 104; SD, 4.33) and TrkA (59.24 × 104; SD, 13.21 vs. 7.65 × 104; SD, 1.05) was observed in fresh than stored tissue (P < 0.01). Colony-forming efficiency was higher for fresh (1.42; SD, 0.12) than stored (0.43; SD, 0.15; P < 0.01) cells. For stored tissue only, there was a significant inverse relationship between donor age and total number of cells isolated (R2 = 0.27, P < 0.001). Conclusions: Storage of corneoscleral discs in organ culture medium leads to significant reduction in limbal epithelial cell number, expression of ΔNp63 and TrkA, and viability compared to fresh tissue. There is a smaller basal stem cell population in stored compared to fresh tissue
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