6,546 research outputs found
A universal GRB photon energy-peak luminosity relation
The energetics and emission mechanism of GRBs are not well understood. Here
we demonstrate that the instantaneous peak flux or equivalent isotropic peak
luminosity, L_iso ergs s^-1, rather than the integrated fluence or equivalent
isotropic energy, E_iso ergs, underpins the known high-energy correlations.
Using new spectral/temporal parameters calculated for 101 bursts with redshifts
from BATSE, BeppoSAX, HETE-II and Swift we describe a parameter space which
characterises the apparently diverse properties of the prompt emission. We show
that a source frame characteristic-photon-energy/peak luminosity ratio, K_z,
can be constructed which is constant within a factor of 2 for all bursts
whatever their duration, spectrum, luminosity and the instrumentation used to
detect them. The new parameterization embodies the Amati relation but indicates
that some correlation between E_peak and E_iso follows as a direct mathematical
inference from the Band function and that a simple transformation of E_iso to
L_iso yields a universal high energy correlation for GRBs. The existence of K_z
indicates that the mechanism responsible for the prompt emission from all GRBs
is probably predominantly thermal.Comment: Submitted to Ap
Quantum Phase Transitions of Hard-Core Bosons in Background Potentials
We study the zero temperature phase diagram of hard core bosons in two
dimensions subjected to three types of background potentials: staggered,
uniform, and random. In all three cases there is a quantum phase transition
from a superfluid (at small potential) to a normal phase (at large potential),
but with different universality classes. As expected, the staggered case
belongs to the XY universality, while the uniform potential induces a mean
field transition. The disorder driven transition is clearly different from
both; in particular, we find z~1.4, \nu~1, and \beta~0.6.Comment: 4 pages (6 figures); published version-- 2 references added, minor
clarification
Time-resolved measurement of Landau--Zener tunneling in different bases
A comprehensive study of the tunneling dynamics of a Bose--Einstein
condensate in a tilted periodic potential is presented. We report numerical and
experimental results on time-resolved measurements of the Landau--Zener
tunneling of ultracold atoms introduced by the tilt, which experimentally is
realized by accelerating the lattice. The use of different protocols enables us
to access the tunneling probability, numerically as well as experimentally, in
two different bases, namely, the adiabatic basis and the diabatic basis. The
adiabatic basis corresponds to the eigenstates of the lattice, and the diabatic
one to the free-particle momentum eigenstates. Our numerical and experimental
results are compared with existing two-state Landau--Zener models
Low temperature electron transfer in strongly condensed phase
Electron transfer coupled to a collective vibronic degree of freedom is
studied in strongly condensed phase and at lower temperatures where quantum
fluctuations are essential. Based on an exact representation of the reduced
density matrix of the electronic+reaction coordinate compound in terms of path
integrals, recent findings on the overdamped limit in quantum dissipative
systems are employed. This allows to give for the first time a consistent
generalization of the well-known Zusman equations to the quantum domain.
Detailed conditions for the range of validity are specified. Using the Wigner
transform these results are also extended to the quantum dynamics in full phase
space. As an important application electronic transfer rates are derived that
comprise adiabatic and nonadiabatic processes in the low temperature regime
including nuclear tunneling. Accurate agreement with precise quantum Monte
Carlo data is observed.Comment: 16 pages, 6 figures, revised version with minor change
Simulating the Kibble-Zurek mechanism of the Ising model with a superconducting qubit system
The Kibble-Zurek mechanism (KZM) predicts the density of topological defects
produced in the dynamical processes of phase transitions in systems ranging
from cosmology to condensed matter and quantum materials. The similarity
between KZM and the Landau-Zener transition (LZT), which is a standard tool to
describe the dynamics of some non-equilibrium physics in contemporary physics,
is being extensively exploited. Here we demonstrate the equivalence between KZM
in the Ising model and LZT in a superconducting qubit system. We develop a
time-resolved approach to study quantum dynamics of LZT with nano-second
resolution. By using this technique, we simulate the key features of KZM in the
Ising model with LZT, e.g., the boundary between the adiabatic and impulse
regions, the freeze-out phenomenon in the impulse region, especially, the
scaling law of the excited state population as the square root of the quenching
rate. Our results supply the experimental evidence of the close connection
between KZM and LZT, two textbook paradigms to study the dynamics of the
non-equilibrium phenomena.Comment: Title changed, authors added, and some experimental data update
Minimizing Polarization and Disagreement in Social Networks
The rise of social media and online social networks has been a disruptive
force in society. Opinions are increasingly shaped by interactions on online
social media, and social phenomena including disagreement and polarization are
now tightly woven into everyday life. In this work we initiate the study of the
following question: given agents, each with its own initial opinion that
reflects its core value on a topic, and an opinion dynamics model, what is the
structure of a social network that minimizes {\em polarization} and {\em
disagreement} simultaneously?
This question is central to recommender systems: should a recommender system
prefer a link suggestion between two online users with similar mindsets in
order to keep disagreement low, or between two users with different opinions in
order to expose each to the other's viewpoint of the world, and decrease
overall levels of polarization? Our contributions include a mathematical
formalization of this question as an optimization problem and an exact,
time-efficient algorithm. We also prove that there always exists a network with
edges that is a approximation to the optimum.
For a fixed graph, we additionally show how to optimize our objective function
over the agents' innate opinions in polynomial time.
We perform an empirical study of our proposed methods on synthetic and
real-world data that verify their value as mining tools to better understand
the trade-off between of disagreement and polarization. We find that there is a
lot of space to reduce both polarization and disagreement in real-world
networks; for instance, on a Reddit network where users exchange comments on
politics, our methods achieve a -fold reduction in polarization
and disagreement.Comment: 19 pages (accepted, WWW 2018
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