384 research outputs found
In-vitro resistance of cloned human glioma cells to natural killer activity of allogeneic peripheral lymphocytes.
Cells from an established culture of a human astrocytoma were incubated with normal allogeneic peripheral lymphocytes (PBL) in order to study the natural killer (NK) sensitivity of the in vitro propagated cell line. A proportion of cells in culture formed halos, into which lymphocytes did not penetrate. These cells were successfully cloned and showed a decreased susceptibility to NK cytolysis compared with the parent line. Both cell lines could be transplanted into athymic nude mice. The cloned NK-resistant cells underwent a frequent spontaneous regression in nu/nu mice, despite the fact that when used as targets for nu/nu NK cells in vitro they were only moderately susceptible. Phase-contrast microscopy of the mass-cultured cells co-cultivated with lymphocytes suggested that their morphology and ability to form inpenetrable translucent halos might influence their susceptibility to NK lysis. Experiments performed on this assumption revealed that quiescent and halo forming tumour cells were not the primary targets for NK lysis. Cells in mass culture, although tumorigenic, were thus heterogeneous in respect of susceptibility to NK attack. These findings might be relevant to the mechanism of immune escape and tumour heterogeneity in respect of spontaneous cell-mediated lysis
One-dimensional quantum chaos: Explicitly solvable cases
We present quantum graphs with remarkably regular spectral characteristics.
We call them {\it regular quantum graphs}. Although regular quantum graphs are
strongly chaotic in the classical limit, their quantum spectra are explicitly
solvable in terms of periodic orbits. We present analytical solutions for the
spectrum of regular quantum graphs in the form of explicit and exact periodic
orbit expansions for each individual energy level.Comment: 9 pages and 4 figure
Diagnostic criterion for crystallized beams
Small ion crystals in a Paul trap are stable even in the absence of laser
cooling. Based on this theoretically and experimentally well-established fact
we propose the following diagnostic criterion for establishing the presence of
a crystallized beam: Absence of heating following the shut-down of all cooling
devices. The validity of the criterion is checked with the help of detailed
numerical simulations.Comment: REVTeX, 11 pages, 4 figures; submitted to PR
Quantum Fractal Fluctuations
We numerically analyse quantum survival probability fluctuations in an open,
classically chaotic system. In a quasi-classical regime, and in the presence of
classical mixed phase space, such fluctuations are believed to exhibit a
fractal pattern, on the grounds of semiclassical arguments. In contrast, we
work in a classical regime of complete chaoticity, and in a deep quantum regime
of strong localization. We provide evidence that fluctuations are still
fractal, due to the slow, purely quantum algebraic decay in time produced by
dynamical localization. Such findings considerably enlarge the scope of the
existing theory.Comment: revtex, 4 pages, 5 figure
Correlations and pair emission in the escape dynamics of ions from one-dimensional traps
We explore the non-equilibrium escape dynamics of long-range interacting ions
in one-dimensional traps. The phase space of the few ion setup and its impact
on the escape properties are studied. As a main result we show that an
instantaneous reduction of the trap's potential depth leads to the synchronized
emission of a sequence of ion pairs if the initial configurations are close to
the crystalline ionic configuration. The corresponding time-intervals of the
consecutive pair emission as well as the number of emitted pairs can be tuned
by changing the final trap depth. Correlations between the escape times and
kinetic energies of the ions are observed and analyzed.Comment: 17 pages, 9 figure
Veränderung der subchondralen Mineralisierung nach Rekonstruktion des vorderen Kreuzbandes beim Schaf
Failure of Effective Potential Approach: Nucleus-Electron Entanglement in the He-Ion
Entanglement may be considered a resource for quantum-information processing,
as the origin of robust and universal equilibrium behaviour, but also as a
limit to the validity of an effective potential approach, in which the
influence of certain interacting subsystems is treated as a potential. Here we
show that a closed three particle (two protons, one electron) model of a He-ion
featuring realistic size, interactions and energy scales of electron and
nucleus, respectively, exhibits different types of dynamics depending on the
initial state: For some cases the traditional approach, in which the nucleus
only appears as the center of a Coulomb potential, is valid, in others this
approach fails due to entanglement arising on a short time-scale. Eventually
the system can even show signatures of thermodynamical behaviour, i.e. the
electron may relax to a maximum local entropy state which is, to some extent,
independent of the details of the initial state.Comment: Submitted to Europhysics Letter
Excitation of Small Quantum Systems by High-Frequency Fields
The excitation by a high frequency field of multi--level quantum systems with
a slowly varying density of states is investigated. A general approach to study
such systems is presented. The Floquet eigenstates are characterized on several
energy scales. On a small scale, sharp universal quasi--resonances are found,
whose shape is independent of the field parameters and the details of the
system. On a larger scale an effective tight--binding equation is constructed
for the amplitudes of these quasi--resonances. This equation is non--universal;
two classes of examples are discussed in detail.Comment: 4 pages, revtex, no figure
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