64,737 research outputs found
Fragmentation of spherical radioactive heavy nuclei as a novel probe of transient effects in fission
Peripheral collisions with radioactive heavy-ion beams at relativistic
energies are discussed as an innovative approach for probing the transient
regime experienced by fissile systems evolving towards quasi-equilibrium. A
dedicated experiment using the advanced technical installations of GSI,
Darmstadt, permitted to realize ideal conditions for the investigation of
relaxation effects in the meta-stable well. Combined with a highly sensitive
experimental signature, it provides a measure of the transient effects with
respect to the flux over the fission barrier. Within a two-step reaction
process, 45 proton-rich unstable spherical isotopes produced by
projectile-fragmentation of a stable 238U beam have been used as secondary
projectiles. The fragmentation of the radioactive projectiles on lead results
in nearly spherical compound nuclei which span a wide range in excitation
energy and fissility. The decay of these excited systems by fission is studied
with a dedicated set-up which permits the detection of both fission products in
coincidence and the determination of their atomic numbers with high resolution.
The width of the fission-fragment nuclear charge distribution is shown to be
specifically sensitive to pre-saddle transient effects and is used to establish
a clock for the passage of the saddle point. The comparison of the experimental
results with model calculations points to a fission delay of (3.3+/-0.7).10-21s
for initially spherical compound nuclei, independent of excitation energy and
fissility. This value suggests a nuclear dissipation strength at small
deformation of (4.5+/-0.5).1021s-1. The very specific combination of the
physics and technical equipment exploited in this work sheds light on previous
controversial conclusions.Comment: 38 pages, 15 figure
Dynamical Masses in Modified Gravity
Differences in masses inferred from dynamics, such as velocity dispersions or
X-rays, and those inferred from lensing are a generic prediction of modified
gravity theories. Viable models however must include some non-linear mechanism
to restore General Relativity (GR) in dense environments, which is necessary to
pass Solar System constraints on precisely these deviations. In this paper, we
study the dynamics within virialized structures in the context of two modified
gravity models, f(R) gravity and DGP. The non-linear mechanisms to restore GR,
which f(R) and DGP implement in very different ways, have a strong impact on
the dynamics in bound objects; they leave distinctive signatures in the
dynamical mass-lensing mass relation as a function of mass and radius. We
present measurements from N-body simulations of f(R) and DGP, as well as
semi-analytical models which match the simulation results to surprising
accuracy in both cases. The semi-analytical models are useful for making the
connection to observations. Our results confirm that the environment- and
scale-dependence of the modified gravity effects have to be taken into account
when confronting gravity theories with observations of dynamics in galaxies and
clusters.Comment: 18 pages, 16 figures; submitted to PRD; v2: typos corrected,
references added, minor additions (Sec. IID
Transport properties of a superconducting single-electron transistor coupled to a nanomechanical oscillator
We investigate a superconducting single-electron transistor capacitively
coupled to a nanomechanical oscillator and focus on the double Josephson
quasiparticle resonance. The existence of two coherent Cooper pair tunneling
events is shown to lead to pronounced backaction effects. Measuring the current
and the shot noise provides a direct way of gaining information on the state of
the oscillator. In addition to an analytical discussion of the linear-response
regime, we discuss and compare results of higher-order approximation schemes
and a fully numerical solution. We find that cooling of the mechanical
resonator is possible, and that there are driven and bistable oscillator states
at low couplings. Finally, we also discuss the frequency dependence of the
charge noise and the current noise of the superconducting single electron
transistor.Comment: 19 pages, 11 figures, published in PR
Nanoscale tunnel field effect transistor based on a complex oxide lateral heterostructure
We demonstrate a tunnel field effect transistor based on a lateral
heterostructure patterned from an electron gas.
Charge is injected by tunneling from the /
contacts and the current through a narrow channel of insulating
is controlled via an electrostatic side gate. Drain-source
I/V-curves have been measured at low and elevated temperatures. The transistor
shows strong electric-field and temperature-dependent behaviour with a steep
sub-threshold slope %of up to as small as and a
transconductance as high as . A fully
consistent transport model for the drain-source tunneling reproduces the
measured steep sub-threshold slope.Comment: 20 pages, 6 figures, Supplementary material: 4 pages, 2 figure
Non-perturbative Interband Response of InSb Driven Off-resonantly by Few-cycle Electromagnetic Transients
Intense multi-THz pulses are used to study the coherent nonlinear response of
bulk InSb by means of field-resolved four-wave mixing spectroscopy. At
amplitudes above 5 MV/cm the signals show a clear temporal substructure which
is unexpected in perturbative nonlinear optics. Simulations based on a
two-level quantum system demonstrate that in spite of the strongly off-resonant
character of the excitation the high-field pulses drive the interband
resonances into a non-perturbative regime of Rabi flopping.Comment: 4 pages, 4 figure
The Odyssey Approach for Optimizing Federated SPARQL Queries
Answering queries over a federation of SPARQL endpoints requires combining
data from more than one data source. Optimizing queries in such scenarios is
particularly challenging not only because of (i) the large variety of possible
query execution plans that correctly answer the query but also because (ii)
there is only limited access to statistics about schema and instance data of
remote sources. To overcome these challenges, most federated query engines rely
on heuristics to reduce the space of possible query execution plans or on
dynamic programming strategies to produce optimal plans. Nevertheless, these
plans may still exhibit a high number of intermediate results or high execution
times because of heuristics and inaccurate cost estimations. In this paper, we
present Odyssey, an approach that uses statistics that allow for a more
accurate cost estimation for federated queries and therefore enables Odyssey to
produce better query execution plans. Our experimental results show that
Odyssey produces query execution plans that are better in terms of data
transfer and execution time than state-of-the-art optimizers. Our experiments
using the FedBench benchmark show execution time gains of at least 25 times on
average.Comment: 16 pages, 10 figure
Raman Response of Magnetic Excitations in Cuprate Ladders and Planes
An unified picture for the Raman response of magnetic excitations in cuprate
spin-ladder compounds is obtained by comparing calculated two-triplon Raman
line-shapes with those of the prototypical compounds SrCu2O3 (Sr123),
Sr14Cu24O41 (Sr14), and La6Ca8Cu24O41 (La6Ca8). The theoretical model for the
two-leg ladder contains Heisenberg exchange couplings J_parallel and J_perp
plus an additional four-spin interaction J_cyc. Within this model Sr123 and
Sr14 can be described by x:=J_parallel/J_perp=1.5, x_cyc:=J_cyc/J_perp=0.2,
J_perp^Sr123=1130 cm^-1 and J_perp^Sr14=1080 cm^-1. The couplings found for
La6Ca8 are x=1.2, x_cyc=0.2, and J_perp^La6Ca8=1130 cm^-1. The unexpected sharp
two-triplon peak in the ladder materials compared to the undoped
two-dimensional cuprates can be traced back to the anisotropy of the magnetic
exchange in rung and leg direction. With the results obtained for the isotropic
ladder we calculate the Raman line-shape of a two-dimensional square lattice
using a toy model consisting of a vertical and a horizontal ladder. A direct
comparison of these results with Raman experiments for the two-dimensional
cuprates R2CuO4 (R=La,Nd), Sr2CuO2Cl2, and YBa2Cu3O(6+delta) yields a good
agreement for the dominating two-triplon peak. We conclude that short range
quantum fluctuations are dominating the magnetic Raman response in both,
ladders and planes. We discuss possible scenarios responsible for the
high-energy spectral weight of the Raman line-shape, i.e. phonons, the
triple-resonance and multi-particle contributions.Comment: 10 pages, 6 figure
Vanishing conductivity of quantum solitons in polyacetylene
Quantum solitons or polarons are supposed to play a crucial role in the
electric conductivity of polyacetylene, in the intermediate doping regime. We
present an exact fully quantized calculation of the quantum soliton
conductivity in polyacetylene and show that it vanishes exactly. This is
obtained by applying a general method of soliton quantization, based on
order-disorder duality, to a Z(2)-symmetric complex extension of the TLM
dimerization effective field theory. We show that, in this theory,
polyacetylene solitons are sine-Gordon solitons in the phase of the complex
field.Comment: To appear in J. Phys. A: Math. Theor., 15 page
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