872 research outputs found
Numerical model for granular compaction under vertical tapping
A simple numerical model is used to simulate the effect of vertical taps on a
packing of monodisperse hard spheres. Our results are in agreement with an
experimantal work done in Chicago and with other previous models, especially
concerning the dynamics of the compaction, the influence of the excitation
strength on the compaction efficiency, and some ageing effects. The principal
asset of the model is that it allows a local analysis of the packings. Vertical
and transverse density profiles are used as well as size and volume
distributions of the pores. An interesting result concerns the appearance of a
vertical gradient in the density profiles during compaction. Furthermore, the
volume distribution of the pores suggests that the smallest pores, ranging in
size between a tetrahedral and an octahedral site, are not strongly affected by
the tapping process, in contrast to the largest pores which are more sensitive
to the compaction of the packing.Comment: 8 pages, 15 figures (eps), to be published in Phys. Rev. E. Some
corrections have been made, especially in paragraph IV
Mirror Energy Differences at Large Isospin Studied through Direct Two-Nucleon Knockout
The first spectroscopy of excited states in 52Ni (Tz=2) and 51Co (Tz=-3/2)
has been obtained using the highly selective two-neutron knockout reaction.
Mirror energy differences between isobaric analogue states in these nuclei and
their mirror partners are interpreted in terms of isospin nonconserving
effects. A comparison between large scale shell-model calculations and data
provides the most compelling evidence to date that both electromagnetic and an
additional isospin nonconserving interactions for J=2 couplings, of unknown
origin, are required to obtain good agreement.Comment: Accepted for publication in Physical Review Letter
Random Packings of Frictionless Particles
We study random packings of frictionless particles at T=0.
The packing fraction where the pressure becomes nonzero is the same as the
jamming threshold, where the static shear modulus becomes nonzero. The
distribution of threshold packing fractions narrows and its peak approaches
random close-packing as the system size increases. For packing fractions within
the peak, there is no self-averaging, leading to exponential decay of the
interparticle force distribution.Comment: 4 pages, 3 figure
Recent direct reaction experimental studies with radioactive tin beams
Direct reaction techniques are powerful tools to study the single-particle
nature of nuclei. Performing direct reactions on short-lived nuclei requires
radioactive ion beams produced either via fragmentation or the Isotope
Separation OnLine (ISOL) method. Some of the most interesting regions to study
with direct reactions are close to the magic numbers where changes in shell
structure can be tracked. These changes can impact the final abundances of
explosive nucleosynthesis. The structure of the chain of tin isotopes is
strongly influenced by the Z=50 proton shell closure, as well as the neutron
shell closures lying in the neutron-rich, N=82, and neutron-deficient, N=50,
regions. Here we present two examples of direct reactions on exotic tin
isotopes. The first uses a one-neutron transfer reaction and a low-energy
reaccelerated ISOL beam to study states in 131Sn from across the N=82 shell
closure. The second example utilizes a one-neutron knockout reaction on
fragmentation beams of neutron-deficient 106,108Sn. In both cases, measurements
of gamma rays in coincidence with charged particles proved to be invaluable.Comment: 11 pages, 5 figures, Zakopane Conference on Nuclear Physics "Extremes
of the Nuclear Landscape", Zakopane, Poland, August 31 - September 7, 201
Nonlinear anomalous diffusion equation and fractal dimension: Exact generalized gaussian solution
In this work we incorporate, in a unified way, two anomalous behaviors, the
power law and stretched exponential ones, by considering the radial dependence
of the -dimensional nonlinear diffusion equation where , ,
, and are real parameters and is a time-dependent
source. This equation unifies the O'Shaugnessy-Procaccia anomalous diffusion
equation on fractals () and the spherical anomalous diffusion for
porous media (). An exact spherical symmetric solution of this
nonlinear Fokker-Planck equation is obtained, leading to a large class of
anomalous behaviors. Stationary solutions for this Fokker-Planck-like equation
are also discussed by introducing an effective potential.Comment: Latex, 6 pages. To appear in Phys. Rev.
Global Equation of State of two-dimensional hard sphere systems
Hard sphere systems in two dimensions are examined for arbitrary density.
Simulation results are compared to the theoretical predictions for both the low
and the high density limit, where the system is either disordered or ordered,
respectively. The pressure in the system increases with the density, except for
an intermediate range of volume fractions , where a
disorder-order phase transition occurs. The proposed {\em global equation of
state} (which describes the pressure {\em for all densities}) is applied to the
situation of an extremely dense hard sphere gas in a gravitational field and
shows reasonable agreement with both experimental and numerical data.Comment: 4 pages, 2 figure
Triplet energy differences and the low lying structure of Ga 62
Background: Triplet energy differences (TED) can be studied to yield information on isospin-non-conserving interactions in nuclei.
Purpose: The systematic behavior of triplet energy differences (TED) of T=1, J\u3c0=2+ states is examined. The A=62 isobar is identified as having a TED value that deviates significantly from an otherwise very consistent trend. This deviation can be attributed to the tentative assignments of the pertinent states in Ga62 and Ge62.
Methods: An in-beam \u3b3-ray spectroscopy experiment was performed to identify excited states in Ga62 using Gamma-Ray Energy Tracking In-Beam Nuclear Array with the S800 spectrometer at NSCL using a two-nucleon knockout approach. Cross-section calculations for the knockout process and shell-model calculations have been performed to interpret the population and decay properties observed.
Results: Using the systematics as a guide, a candidate for the transition from the T=1, 2+ state is identified. However, previous work has identified similar states with different J\u3c0 assignments. Cross-section calculations indicate that the relevant T=1, 2+ state should be one of the states directly populated in this reaction.
Conclusions: As spins and parities were not measurable, it is concluded that an unambiguous identification of the first T=1, 2+ state is required to reconcile our understanding of TED systematics
Nodal dynamics, not degree distributions, determine the structural controllability of complex networks
Structural controllability has been proposed as an analytical framework for
making predictions regarding the control of complex networks across myriad
disciplines in the physical and life sciences (Liu et al.,
Nature:473(7346):167-173, 2011). Although the integration of control theory and
network analysis is important, we argue that the application of the structural
controllability framework to most if not all real-world networks leads to the
conclusion that a single control input, applied to the power dominating set
(PDS), is all that is needed for structural controllability. This result is
consistent with the well-known fact that controllability and its dual
observability are generic properties of systems. We argue that more important
than issues of structural controllability are the questions of whether a system
is almost uncontrollable, whether it is almost unobservable, and whether it
possesses almost pole-zero cancellations.Comment: 1 Figures, 6 page
Single-particle structure at N=29 : The structure of Ar 47 and first spectroscopy of S 45
Comprehensive spectroscopy of the N=29 nucleus Ar47 is presented, based on two complementary direct reaction mechanisms: one-neutron pickup onto Ar46 projectiles and one-proton removal from the 1- ground state of K48. The results are compared with shell-model calculations that use the state-of-the-art SDPF-U and SDPF-MU effective interactions. Also, from the Be9(Cl46,S45+γ)X one-proton-removal reaction, we report the first γ-ray transitions observed from S45. By using comparisons with shell-model calculations, and from the observed intensities and energy sums, we propose a first tentative level scheme for S45
Theoretical interpretation of the experimental electronic structure of lens shaped, self-assembled InAs/GaAs quantum dots
We adopt an atomistic pseudopotential description of the electronic structure
of self-assembled, lens shaped InAs quantum dots within the ``linear
combination of bulk bands'' method. We present a detailed comparison with
experiment, including quantites such as the single particle electron and hole
energy level spacings, the excitonic band gap, the electron-electron, hole-hole
and electron hole Coulomb energies and the optical polarization anisotropy. We
find a generally good agreement, which is improved even further for a dot
composition where some Ga has diffused into the dots.Comment: 16 pages, 5 figures. Submitted to Physical Review
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