46,135 research outputs found
Efimov states in asymmetric systems
The conditions for occurrence of the Efimov effect is briefly described using
hyperspherical coordinates. The strength of the effective hyperradial
potential appearing for two or three large scattering lengths is
computed and discussed as function of two independent mass ratios of the three
constituent particles. The effect is by far most pronounced for asymmetric
systems with three very different masses. One Efimov state may by chance appear
in nuclei. Many states could be present for systems with one electron and two
neutral atoms or molecules. Estimates of the number of states and their sizes
and energies are given.Comment: 7 pages, 3 figure
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
Quantum noise limited and entanglement-assisted magnetometry
We study experimentally the fundamental limits of sensitivity of an atomic
radio-frequency magnetometer. First we apply an optimal sequence of state
preparation, evolution, and the back-action evading measurement to achieve a
nearly projection noise limited sensitivity. We furthermore experimentally
demonstrate that Einstein-Podolsky-Rosen (EPR) entanglement of atoms generated
by a measurement enhances the sensitivity to pulsed magnetic fields. We
demonstrate this quantum limited sensing in a magnetometer utilizing a truly
macroscopic ensemble of 1.5*10^12 atoms which allows us to achieve
sub-femtoTesla/sqrt(Hz) sensitivity.Comment: To appear in Physical Review Letters, April 9 issue (provisionally
Generic Sandpile Models Have Directed Percolation Exponents
We study sandpile models with stochastic toppling rules and having sticky
grains so that with a non-zero probability no toppling occurs, even if the
local height of pile exceeds the threshold value. Dissipation is introduced by
adding a small probability of particle loss at each toppling. Generically, for
models with a preferred direction, the avalanche exponents are those of
critical directed percolation clusters. For undirected models, avalanche
exponents are those of directed percolation clusters in one higher dimension.Comment: 4 pages, 4 figures, minor change
Modeling temporal fluctuations in avalanching systems
We demonstrate how to model the toppling activity in avalanching systems by
stochastic differential equations (SDEs). The theory is developed as a
generalization of the classical mean field approach to sandpile dynamics by
formulating it as a generalization of Itoh's SDE. This equation contains a
fractional Gaussian noise term representing the branching of an avalanche into
small active clusters, and a drift term reflecting the tendency for small
avalanches to grow and large avalanches to be constricted by the finite system
size. If one defines avalanching to take place when the toppling activity
exceeds a certain threshold the stochastic model allows us to compute the
avalanche exponents in the continum limit as functions of the Hurst exponent of
the noise. The results are found to agree well with numerical simulations in
the Bak-Tang-Wiesenfeld and Zhang sandpile models. The stochastic model also
provides a method for computing the probability density functions of the
fluctuations in the toppling activity itself. We show that the sandpiles do not
belong to the class of phenomena giving rise to universal non-Gaussian
probability density functions for the global activity. Moreover, we demonstrate
essential differences between the fluctuations of total kinetic energy in a
two-dimensional turbulence simulation and the toppling activity in sandpiles.Comment: 14 pages, 11 figure
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
Gas gun shock experiments with single-pulse x-ray phase contrast imaging and diffraction at the Advanced Photon Source
The highly transient nature of shock loading and pronounced microstructure
effects on dynamic materials response call for {\it in situ}, temporally and
spatially resolved, x-ray-based diagnostics. Third-generation synchrotron x-ray
sources are advantageous for x-ray phase contrast imaging (PCI) and diffraction
under dynamic loading, due to their high photon energy, high photon fluxes,
high coherency, and high pulse repetition rates. The feasibility of bulk-scale
gas gun shock experiments with dynamic x-ray PCI and diffraction measurements
was investigated at the beamline 32ID-B of the Advanced Photon Source. The
x-ray beam characteristics, experimental setup, x-ray diagnostics, and static
and dynamic test results are described. We demonstrate ultrafast, multiframe,
single-pulse PCI measurements with unprecedented temporal (100 ps) and
spatial (2 m) resolutions for bulk-scale shock experiments, as well
as single-pulse dynamic Laue diffraction. The results not only substantiate the
potential of synchrotron-based experiments for addressing a variety of shock
physics problems, but also allow us to identify the technical challenges
related to image detection, x-ray source, and dynamic loading
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