24,444 research outputs found
Quantum Entanglement in -Spherium
There are very few systems of interacting particles (with continuous
variables) for which the entanglement of the concomitant eigenfunctions can be
computed in an exact, analytical way. Here we present analytical calculations
of the amount of entanglement exhibited by -states of \emph{spherium}. This
is a system of two particles (electrons) interacting via a Coulomb potential
and confined to a -sphere (that is, to the surface of a -dimensional
ball). We investigate the dependence of entanglement on the radius of the
system, on the spatial dimensionality , and on energy. We find that
entanglement increases monotonically with , decreases with , and also
tends to increase with the energy of the eigenstates. These trends are
discussed and compared with those observed in other two-electron atomic-like
models where entanglement has been investigated.Comment: 14 pages, 6 figures. J. Phys. A (2015). Accepte
Oscillatory dynamics of a superconductor vortex lattice in high amplitude ac magnetic fields
In this work we study by ac susceptibility measurements the evolution of the
solid vortex lattice mobility under oscillating forces. Previous work had
already shown that in YBCO single crystals, below the melting transition, a
temporarily symmetric magnetic ac field (e.g. sinusoidal, square, triangular)
can heal the vortex lattice (VL) and increase its mobility, but a temporarily
asymmetric one (e.g. sawtooth) of the same amplitude can tear the lattice into
a more pinned disordered state. In this work we present evidence that the
mobility of the VL is reduced for large vortex displacements, in agreement with
predictions of recent simulations. We show that with large symmetric
oscillating fields both an initially ordered or an initially disordered VL
configuration evolve towards a less mobile lattice, supporting the scenario of
plastic flow.Comment: 5 pages, 4 figures. To appear in Phys. Rev.
Fluctuation-induced traffic congestion in heterogeneous networks
In studies of complex heterogeneous networks, particularly of the Internet,
significant attention was paid to analyzing network failures caused by hardware
faults or overload, where the network reaction was modeled as rerouting of
traffic away from failed or congested elements. Here we model another type of
the network reaction to congestion -- a sharp reduction of the input traffic
rate through congested routes which occurs on much shorter time scales. We
consider the onset of congestion in the Internet where local mismatch between
demand and capacity results in traffic losses and show that it can be described
as a phase transition characterized by strong non-Gaussian loss fluctuations at
a mesoscopic time scale. The fluctuations, caused by noise in input traffic,
are exacerbated by the heterogeneous nature of the network manifested in a
scale-free load distribution. They result in the network strongly overreacting
to the first signs of congestion by significantly reducing input traffic along
the communication paths where congestion is utterly negligible.Comment: 4 pages, 3 figure
Coherent State Description of the Ground State in the Tavis-Cummings Model and its Quantum Phase Transitions
Quantum phase transitions and observables of interest of the ground state in
the Tavis-Cummings model are analyzed, for any number of atoms, by using a
tensorial product of coherent states. It is found that this "trial" state
constitutes a very good approximation to the exact quantum solution, in that it
globally reproduces the expectation values of the matter and field observables.
These include the population and dipole moments of the two-level atoms and the
squeezing parameter. Agreement in the field-matter entanglement and in the
fidelity measures, of interest in quantum information theory, is also found.The
analysis is carried out in all three regions defined by the separatrix which
gives rise to the quantum phase transitions. It is argued that this agreement
is due to the gaussian structure of the probability distributions of the
constant of motion and the number of photons. The expectation values of the
ground state observables are given in analytic form, and the change of the
ground state structure of the system when the separatrix is crossed is also
studied.Comment: 38 pages, 16 figure
Energy landscape of a simple model for strong liquids
We calculate the statistical properties of the energy landscape of a minimal
model for strong network-forming liquids. Dynamics and thermodynamic properties
of this model can be computed with arbitrary precision even at low
temperatures. A degenerate disordered ground state and logarithmic statistics
for the energy distribution are the landscape signatures of strong liquid
behavior. Differences from fragile liquid properties are attributed to the
presence of a discrete energy scale, provided by the particle bonds, and to the
intrinsic degeneracy of topologically disordered networks.Comment: Revised versio
On the Saturation of Astrophysical Dynamos: Numerical Experiments with the No-cosines flow
In the context of astrophysical dynamos we illustrate that the no-cosines
flow, with zero mean helicity, can drive fast dynamo action and study the
dynamo's mode of operation during both the linear and non-linear saturation
regime: It turns out that in addition to a high growth rate in the linear
regime, the dynamo saturates at a level significantly higher than normal
turbulent dynamos, namely at exact equipartition when the magnetic Prandtl
number is on the order of unity. Visualization of the magnetic and velocity
fields at saturation will help us to understand some of the aspects of the
non-linear dynamo problem.Comment: 8 pages, 5 figures, submitted to the proceedings of "Space Climate 1"
to be peer-reviewed to Solar Physic
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