4,636 research outputs found
Small world yields the most effective information spreading
Spreading dynamics of information and diseases are usually analyzed by using
a unified framework and analogous models. In this paper, we propose a model to
emphasize the essential difference between information spreading and epidemic
spreading, where the memory effects, the social reinforcement and the
non-redundancy of contacts are taken into account. Under certain conditions,
the information spreads faster and broader in regular networks than in random
networks, which to some extent supports the recent experimental observation of
spreading in online society [D. Centola, Science {\bf 329}, 1194 (2010)]. At
the same time, simulation result indicates that the random networks tend to be
favorable for effective spreading when the network size increases. This
challenges the validity of the above-mentioned experiment for large-scale
systems. More significantly, we show that the spreading effectiveness can be
sharply enhanced by introducing a little randomness into the regular structure,
namely the small-world networks yield the most effective information spreading.
Our work provides insights to the understanding of the role of local clustering
in information spreading.Comment: 6 pages, 7 figures, accepted by New J. Phy
Efficient energy transfer in light-harvesting systems, I: optimal temperature, reorganization energy, and spatial-temporal correlations
Understanding the mechanisms of efficient and robust energy transfer in
light-harvesting systems provides new insights for the optimal design of
artificial systems. In this paper, we use the Fenna-Matthews-Olson (FMO)
protein complex and phycocyanin 645 (PC 645) to explore the general dependence
on physical parameters that help maximize the efficiency and maintain its
stability. With the Haken-Strobl model, the maximal energy transfer efficiency
(ETE) is achieved under an intermediate optimal value of dephasing rate. To
avoid the infinite temperature assumption in the Haken-Strobl model and the
failure of the Redfield equation in predicting the Forster rate behavior, we
use the generalized Bloch-Redfield (GBR) equation approach to correctly
describe dissipative exciton dynamics and find that maximal ETE can be achieved
under various physical conditions, including temperature, reorganization
energy, and spatial-temporal correlations in noise. We also identify regimes of
reorganization energy where the ETE changes monotonically with temperature or
spatial correlation and therefore cannot be optimized with respect to these two
variables
Universality in Systems with Power-Law Memory and Fractional Dynamics
There are a few different ways to extend regular nonlinear dynamical systems
by introducing power-law memory or considering fractional
differential/difference equations instead of integer ones. This extension
allows the introduction of families of nonlinear dynamical systems converging
to regular systems in the case of an integer power-law memory or an integer
order of derivatives/differences. The examples considered in this review
include the logistic family of maps (converging in the case of the first order
difference to the regular logistic map), the universal family of maps, and the
standard family of maps (the latter two converging, in the case of the second
difference, to the regular universal and standard maps). Correspondingly, the
phenomenon of transition to chaos through a period doubling cascade of
bifurcations in regular nonlinear systems, known as "universality", can be
extended to fractional maps, which are maps with power-/asymptotically
power-law memory. The new features of universality, including cascades of
bifurcations on single trajectories, which appear in fractional (with memory)
nonlinear dynamical systems are the main subject of this review.Comment: 23 pages 7 Figures, to appear Oct 28 201
Behavior of the breathing pyrochlore lattice Ba<sub>3</sub>Yb<sub>2</sub>Zn<sub>5</sub>O<sub>11</sub> in applied magnetic field
The breathing pyrochlore lattice material Ba3Yb2Zn5O11 exists in the nearly decoupled limit, in contrast to most other well-studied breathing pyrochlore compounds. As a result, it constitutes a useful platform to benchmark theoretical calculations of exchange interactions in insulating Yb3+ magnets. Here we study Ba3Yb2Zn5O11 at low temperatures in applied magnetic fields as a further probe of the physics of this model system. Experimentally, we consider the behavior of polycrystalline samples of Ba3Yb2Zn5O11 with a combination of inelastic neutron scattering and heat capacity measurements down to 75 mK and up to fields of 10 T. Consistent with previous work, inelastic neutron scattering finds a level crossing near 3 T, but no significant dispersion of the spin excitations is detected up to the highest applied fields. Refinement of the theoretical model previously determined at zero field can reproduce much of the inelastic neutron scattering spectra and specific heat data. A notable exception is a low temperature peak in the specific heat at similar to 0.1 K. This may indicate the scale of interactions between tetrahedra or may reflect undetected disorder in Ba3Yb2Zn5O11
The Physical Basis for Long-lived Electronic Coherence in Photosynthetic Light Harvesting Systems
The physical basis for observed long-lived electronic coherence in
photosynthetic light-harvesting systems is identified using an analytically
soluble model. Three physical features are found to be responsible for their
long coherence lifetimes: i) the small energy gap between excitonic states, ii)
the small ratio of the energy gap to the coupling between excitonic states, and
iii) the fact that the molecular characteristics place the system in an
effective low temperature regime, even at ambient conditions. Using this
approach, we obtain decoherence times for a dimer model with FMO parameters of
160 fs at 77 K and 80 fs at 277 K. As such, significant
oscillations are found to persist for 600 fs and 300 fs, respectively, in
accord with the experiment and with previous computations. Similar good
agreement is found for PC645 at room temperature, with oscillations persisting
for 400 fs. The analytic expressions obtained provide direct insight into the
parameter dependence of the decoherence time scales.Comment: 5 figures; J. Phys. Chem. Lett. (2011
A Study in Depth of f0(1370)
Claims have been made that f0(1370) does not exist. The five primary sets of
data requiring its existence are refitted. Major dispersive effects due to the
opening of the 4pi threshold are included for the first time; the sigma -> 4pi
amplitude plays a strong role. Crystal Barrel data on pbar-p -> 3pizero at rest
require f0(1370) signals of at least 32 and 33 standard deviations in 1S0 and
3P1 annihilation respectively. Furthermore, they agree within 5 MeV for mass
and width. Data on pbar-p -> eta-eta-pizero agree and require at least a 19
standard deviation contribution. This alone is sufficient to demonstrate the
existence of f0(1370). BES II data for J/Psi -> phi-pi-pi contain a visible
f0(1370) signal > 8 standard devations. In all cases, a resonant phase
variation is required. The possibility of a second pole in the sigma amplitude
due to the opening of the 4pi channel is excluded. Cern-Munich data for pi-pi
elastic scattering are fitted well with the inclusion of some mixing between
sigma, f0(1370) and f0(1500). The pi-pi widths for f2(1565), rho3(1690),
rho3(1990) and f4(2040) are determined.Comment: 25 pages, 22 figures. Typos corrected in Eqs 2 and 7. Introduction
rewritten. Conclusions unchange
Control of magnetic anisotropy by orbital hybridization in (La0.67Sr0.33MnO3)n/(SrTiO3)n superlattice
The asymmetry of chemical nature at the hetero-structural interface offers an
unique opportunity to design desirable electronic structure by controlling
charge transfer and orbital hybridization across the interface. However, the
control of hetero-interface remains a daunting task. Here, we report the
modulation of interfacial coupling of (La0.67Sr0.33MnO3)n/(SrTiO3)n
superlattices by manipulating the periodic thickness with n unit cells of
SrTiO3 and n unit cells La0.67Sr0.33MnO3. The easy axis of magnetic anisotropy
rotates from in-plane (n = 10) to out-of-plane (n = 2) orientation at 150 K.
Transmission electron microscopy reveals enlarged tetragonal ratio > 1 with
breaking of volume conservation around the (La0.67Sr0.33MnO3)n/(SrTiO3)n
interface, and electronic charge transfer from Mn to Ti 3d orbitals across the
interface. Orbital hybridization accompanying the charge transfer results in
preferred occupancy of 3d3z2-r2 orbital at the interface, which induces a
stronger electronic hopping integral along the out-of-plane direction and
corresponding out-of-plane magnetic easy axis for n = 2. We demonstrate that
interfacial orbital hybridization in superlattices of strongly correlated
oxides may be a promising approach to tailor electronic and magnetic properties
in device applications
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