6,262 research outputs found
Quasi-local contribution to the scalar self-force: Geodesic Motion
We consider a scalar charge travelling in a curved background spacetime. We
calculate the quasi-local contribution to the scalar self-force experienced by
such a particle following a geodesic in a general spacetime. We also show that
if we assume a massless field and a vacuum background spacetime, the expression
for the self-force simplifies significantly. We consider some specific cases
whose gravitational analog are of immediate physical interest for the
calculation of radiation reaction corrected orbits of binary black hole
systems. These systems are expected to be detectable by the LISA space based
gravitational wave observatory. We also investigate how alternate techniques
may be employed in some specific cases and use these as a check on our own
results.Comment: Final Phys. Rev. D version. 24 pages, revtex4. Minor typos correcte
The Refractive Index of Silicon at Gamma Ray Energies
The index of refraction n(E_{\gamma})=1+\delta(E_{\gamma})+i\beta(E_{\gamma})
is split into a real part \delta and an absorptive part \beta. The absorptive
part has the three well-known contributions to the cross section \sigma_{abs}:
the photo effect, the Compton effect and the pair creation, but there is also
the inelastic Delbr\"uck scattering. Second-order elastic scattering cross
sections \sigma_{sca} with Rayleigh scattering (virtual photo effect), virtual
Compton effect and Delbr\"uck scattering (virtual pair creation) can be
calculated by integrals of the Kramers-Kronig dispersion relations from the
cross section \sigma_{abs}. The real elastic scattering amplitudes are
proportional to the refractive indices \delta_{photo}, \delta_{Compton} and
\delta_{pair}. While for X-rays the negative \delta_{photo} dominates, we show
for the first time experimentally and theoretically that the positive
\delta_{pair} dominates for \gamma rays, opening a new era of \gamma optics
applications, i.e. of nuclear photonics.Comment: 4 pages, 3 figure
Chirality of wave functions for three coalescing levels
The coalescence of three levels has particular attractive features. Even
though it may be difficult to realise such event in the laboratory (three
additional real parameters must be adjusted), to take up the challenge seems
worthwhile. In the same way as the chiral behaviour of a usual EP can give a
direction on a line, the state vectors in the vicinity of an EP3 provide an
orientation in the plane. The distinction between left and right handedness
depends on the distribution of the widths of the three levels in the vicinity
of the point of coalescence.Comment: Manuscript has been discussed in June 2007 with the experimental
group under Professor Achim Richter at the TU Darmstadt. It has been
presented at the 6th International Workshop on Pseudo Hermitian Hamiltonians,
London, 16-18 July 2007. An expanded version is being prepared for
publication. 3 Figures, 11 page
Integrated Atom Detector Based on Field Ionization near Carbon Nanotubes
We demonstrate an atom detector based on field ionization and subsequent ion
counting. We make use of field enhancement near tips of carbon nanotubes to
reach extreme electrostatic field values of up to 9x10^9 V/m, which ionize
ground state rubidium atoms. The detector is based on a carpet of multiwall
carbon nanotubes grown on a substrate and used for field ionization, and a
channel electron multiplier used for ion counting. We measure the field
enhancement at the tips of carbon nanotubes by field emission of electrons. We
demonstrate the operation of the field ionization detector by counting atoms
from a thermal beam of a rubidium dispenser source. By measuring the ionization
rate of rubidium as a function of the applied detector voltage we identify the
field ionization distance, which is below a few tens of nanometers in front of
nanotube tips. We deduce from the experimental data that field ionization of
rubidium near nanotube tips takes place on a time scale faster than 10^(-10)s.
This property is particularly interesting for the development of fast atom
detectors suitable for measuring correlations in ultracold quantum gases. We
also describe an application of the detector as partial pressure gauge.Comment: 7 pages, 8 figure
Highly versatile atomic micro traps generated by multifrequency magnetic field modulation
We propose the realization of custom-designed adiabatic potentials for cold
atoms based on multimode radio frequency radiation in combination with static
inhomogeneous magnetic fields. For example, the use of radio frequency combs
gives rise to periodic potentials acting as gratings for cold atoms. In strong
magnetic field gradients the lattice constant can be well below 1 micrometer.
By changing the frequencies of the comb in time the gratings can easily be
propagated in space, which may prove useful for Bragg scattering atomic matter
waves. Furthermore, almost arbitrarily shaped potential are possible such as
disordered potentials on a scale of several 100 nm or lattices with a spatially
varying lattice constant. The potentials can be made state selective and, in
the case of atomic mixtures, also species selective. This opens new
perspectives for generating tailored quantum systems based on ultra cold single
atoms or degenerate atomic and molecular quantum gases.Comment: 12 pages, 6 figure
Investigation of fluorescent properties of plasma exosomes in diagnosis and prognosis of colorectal cancer
Exosomes of blood plasma were studied using multiphoton tomography (two-photon microscopy). Exosomes were isolated in patients with colorectal cancer and in healthy donors. Images of fluorescence of exosomes were obtained at a wavelength of 760 nm and second harmonic generation at a wavelength of 380 nm. As a result of the analysis of the obtained data, qualitative differences were found between samples from patients with colorectal cancer and healthy donors
Temozolomide induces apoptosis and senescence in glioma cells cultured as multicellular spheroids
Temozolomide is an alkylating cytostatic drug that finds increasing application in the treatment of melanoma, anaplastic astrocytoma and glioblastoma multiforme. The compound is a prodrug that decomposes spontaneously, independent of an enzymatic activation step. DNA methylation induces futile mismatch repair cycles and depletion of the DNA repair enzyme O6-methylguanine-DNA methyltransferase should then initiate programmed cell death. We show drug-dependent inhibition of tumour growth in a three-dimensional cell culture model of the glioma cell lines U87MG and GaMG. Migrational behaviour of the glioblastoma cells remained unaltered. However, coincubation of tumour spheroids with primary brain aggregates showed reduced tumour cell invasion into brain tissue in the presence of temozolomide. This was not achieved by slowing cellular migration, as temozolomide-treated cells displayed no reduced motility. By transferase-mediated dUTP nick-end labelling (TUNEL) of apoptotic nuclei, we found that the drug was able to induce apoptosis throughout the tumour cell spheroids. Apoptosis was highest in the core region of the spheroids. Repetitive application of sublethal doses of temozolomide to multicellular spheroids resulted in the development of drug resistance in GaMG cells. We suggest that temozolomide is a strong initiator of apoptosis in glioblastoma tumour cells in a spheroid cell culture system, when cells are already in a stressful environment
Huygens' Principle for the Klein-Gordon equation in the de Sitter spacetime
In this article we prove that the Klein-Gordon equation in the de Sitter
spacetime obeys the Huygens' principle only if the physical mass of the
scalar field and the dimension of the spatial variable are tied by
the equation . Moreover, we define the incomplete Huygens'
principle, which is the Huygens' principle restricted to the vanishing second
initial datum, and then reveal that the massless scalar field in the de Sitter
spacetime obeys the incomplete Huygens' principle and does not obey the
Huygens' principle, for the dimensions , only. Thus, in the de Sitter
spacetime the existence of two different scalar fields (in fact, with m=0 and
), which obey incomplete Huygens' principle, is equivalent to
the condition (in fact, the spatial dimension of the physical world). For
these two values of the mass are the endpoints of the so-called in
quantum field theory the Higuchi bound. The value of the
physical mass allows us also to obtain complete asymptotic expansion of the
solution for the large time. Keywords: Huygens' Principle; Klein-Gordon
Equation; de Sitter spacetime; Higuchi Boun
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