28,487 research outputs found
Ionization of Rydberg atoms in a low frequency field: modelling by maps of transition to chaotic behavior
We investigate a microwave ionization of highly excited atom in a low
frequency field and show that such a process may be studied on the bases of map
for the electron energy change during the period of the electron motion between
two subsequent passages at aphelion. Simple approximate criterion results to
the threshold field for transition to chaotic behavior very close to the
numerical results. We show that transition from adiabatic to chaotic ionization
mechanism takes place when the field frequency to the electron's Kepler
frequency ration approximately equals 0.1.Comment: 5 pages, LaTEX, 37Kb, 2 figures (compressed and uuencoded PostScript)
included in text. To be published in Chaos: The interplay between
stochastics, classics and quanta, Ed. by P. Garbaczewski, et al. Lecture
notes in Physics, 1995
Vanishing of Gravitational Particle Production in the Formation of Cosmic Strings
We consider the gravitationally induced particle production from the quantum
vacuum which is defined by a free, massless and minimally coupled scalar field
during the formation of a gauge cosmic string. Previous discussions of this
topic estimate the power output per unit length along the string to be of the
order of ergs/sec/cm in the s-channel. We find that this production
may be completely suppressed. A similar result is also expected to hold for the
number of produced photons.Comment: 10 pages, Plain LaTex. Minor improvements. To appear in PR
Insect pathogenic fungi in biological control: status and future challenges
In Europe, insect pathogenic fungi have in decades played a significant role in biological control of insects. With respect to the different strategies of biological control and with respects to the different genera of insect pathogenic fungi, the success and potential vary, however. Classical biological control: no strong indication of potential. Inundation and inoculation biological control: success stories with the genera Metarhizium, Beauveria, Isaria/Paecilomyces and Lecanicillium (previously Verticillium). However, the genotypes employed seem to include a narrow spectrum of the many potentially useful genotypes. Conservation biological control: Pandora and Entomophthora have a strong potential, but also Beauveria has a potential to be explored further. The main bottleneck for further exploitation of insect pathogenic fungi in biological control is the limited knowledge of host pathogen interaction at the fungal genotype level
Phonon-induced quadrupolar ordering of the magnetic superconductor TmNiBC
We present synchrotron x-ray diffraction studies revealing that the lattice
of thulium borocarbide is distorted below T_Q = 13.5 K at zero field. T_Q
increases and the amplitude of the displacements is drastically enhanced, by a
factor of 10 at 60 kOe, when a magnetic field is applied along [100]. The
distortion occurs at the same wave vector as the antiferromagnetic ordering
induced by the a-axis field. A model is presented that accounts for the
properties of the quadrupolar phase and explains the peculiar behavior of the
antiferromagnetic ordering previously observed in this compound.Comment: submitted to PR
Self-organized Criticality and Absorbing States: Lessons from the Ising Model
We investigate a suggested path to self-organized criticality. Originally,
this path was devised to "generate criticality" in systems displaying an
absorbing-state phase transition, but closer examination of the mechanism
reveals that it can be used for any continuous phase transition. We used the
Ising model as well as the Manna model to demonstrate how the finite-size
scaling exponents depend on the tuning of driving and dissipation rates with
system size.Our findings limit the explanatory power of the mechanism to
non-universal critical behavior.Comment: 5 pages, 2 figures, REVTeX
Braided Rivers and Superconducting Vortex Avalanches
Magnetic vortices intermittently flow through preferred channels when they
are forced in or out of a superconductor. We study this behavior using a
cellular model, and find that the vortex flow can make braided rivers
strikingly similar to aerial photographs of braided fluvial rivers, such as the
Brahmaputra. By developing an analysis technique suitable for characterizing a
self-affine (multi)fractal, the scaling properties of the braided vortex rivers
in the model are compared with those of braided fluvial rivers. We suggest that
avalanche dynamics leads to braiding in both cases.Comment: 4 pages, 3 figures. To appear in PR
Observation of progressive motion of ac-driven solitons
We report the first experimental observation of phase-locked motion of a
topological soliton at a nonzero average velocity in a periodically modulated
lossy medium, under the action of an ac force with no dc component [the effect
was predicted by G. Filatrella, B.A. Malomed, and R.D. Parmentier, Phys. Lett.
A 198, 43 (1995)]. The velocity is related by a resonant condition to the
driving frequency. The observation is made in terms of the current-voltage,
I(V), characteristics for a fluxon trapped in an annular Josephson junction
placed into dc magnetic field. Large zero-crossing constant-voltage steps,
exactly corresponding to the resonantly locked soliton motion at different
orders of the resonance, are found on the experimental I(V) curves. A measured
dependence of the size of the steps vs. the external magnetic field is in good
agreement with predictions of an analytical model based on the balance equation
for the fluxon's energy. The effect has a potential application as a
low-frequency voltage standard. The work was supported by a grant from the
German-Israeli Foundation.Comment: Physical Review B, in press (Rapid Communication
Shell-Model Effective Operators for Muon Capture in ^{20}Ne
It has been proposed that the discrepancy between the partially-conserved
axial-current prediction and the nuclear shell-model calculations of the ratio
in the muon-capture reactions can be solved in the case of ^{28}Si by
introducing effective transition operators. Recently there has been
experimental interest in measuring the needed angular correlations also in
^{20}Ne. Inspired by this, we have performed a shell-model analysis employing
effective transition operators in the shell-model formalism for the transition
. Comparison of
the calculated capture rates with existing data supports the use of effective
transition operators. Based on our calculations, as soon as the experimental
anisotropy data becomes available, the limits for the ratio can be
extracted.Comment: 9 pages, 3 figures include
Orientational phase transitions in anisotropic rare-earth magnets at low temperatures
Orientational phase transitions are investigated within the Heisenberg model
with single-site anisotropy. The temperature dependence of the cone angle is
calculated within the spin-wave theory. The role of the quantum
renormalizations of anisotropy constants is discussed. A comparison with the
experimental data on the cone-plane orientational transition in holmium is
performed.Comment: 9 pages, LaTeX, 3 figure
Cavity-enhanced room-temperature magnetometry using absorption by nitrogen-vacancy centers in diamond
We demonstrate a cavity-enhanced room-temperature magnetic field sensor based
on nitrogen-vacancy centers in diamond. Magnetic resonance is detected using
absorption of light resonant with the 1042 nm spin-singlet transition. The
diamond is placed in an external optical cavity to enhance the absorption, and
significant absorption is observed even at room temperature. We demonstrate a
magnetic field sensitivity of 2.5 nT/sqrt(Hz), and project a photon
shot-noise-limited sensitivity of 70 pT/sqrt(Hz) for a few mW of infrared
light, and a quantum projection-noise-limited sensitivity of 250 fT/sqrt(Hz)
for the sensing volume of 90 um x 90 um 200 um.Comment: main text 5 pages, supplementary material 3 page
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