1,859 research outputs found
Basin of attraction for turbulent thermalization and the range of validity of classical-statistical simulations
Different thermalization scenarios for systems with large fields have been
proposed in the literature based on classical-statistical lattice simulations
approximating the underlying quantum dynamics. We investigate the range of
validity of these simulations for condensate driven as well as fluctuation
dominated initial conditions for the example of a single component scalar field
theory. We show that they lead to the same phenomenon of turbulent
thermalization for the whole range of (weak) couplings where the
classical-statistical approach is valid. In the turbulent regime we establish
the existence of a dual cascade characterized by universal scaling exponents
and scaling functions. This complements previous investigations where only the
direct energy cascade has been studied for the single component theory. A
proposed alternative thermalization scenario for stronger couplings is shown to
be beyond the range of validity of classical-statistical simulations.Comment: 11 pages, 10 figures; version published in JHEP (minor revisions
Trade Liberalization and Market Access: Analyzing Dominican Export Performance during the Twentieth Century
Leading management thinker C.K. Prahalad argues that selling consumer goods to four billion poor people at the bottom of the economic pyramid (BoP) both generates sizeable profits for large businesses and eliminates poverty. A welcome, innovative and influential perspective, but an opportunity missed, I argue here. First, selling to the poor may do little to eradicate poverty, but potentially hurts small businesses and threatens local jobs and incomes. Second, a more precise analysis using household surveys shows a much smaller BoP market size, less than 5% of previous estimates. Third, virtually everyone in developing countries is classified as a 'poor' consumer in much of the BoP literature. The focus and the bulk of Prahalad's new purchasing power rests with the emerging middle class in India, China and Brazil, while the 2 billion people below $2 a day, especially those in Sub-Saharan Africa, are marginalised in this debate. Data for consumer prices confirms that the true challenge is to serve the latter group, those that are completely cut off from the global marketplace. This paper concludes that big businesses have a central role in shaping and expanding these future markets by generating employment and incomes.
Simulating nonequilibrium quantum fields with stochastic quantization techniques
We present lattice simulations of nonequilibrium quantum fields in
Minkowskian space-time. Starting from a non-thermal initial state, the
real-time quantum ensemble in 3+1 dimensions is constructed by a stochastic
process in an additional (5th) ``Langevin-time''. For the example of a
self-interacting scalar field we show how to resolve apparent unstable Langevin
dynamics, and compare our quantum results with those obtained in classical
field theory. Such a direct simulation method is crucial for our understanding
of collision experiments of heavy nuclei or other nonequilibrium phenomena in
strongly coupled quantum many-body systems.Comment: 4 pages, 4 figures, PRL version, minor change
Perturbative and Nonperturbative Kolmogorov Turbulence in a Gluon Plasma
In numerical simulations of nonabelian plasma instabilities in the hard-loop
approximation, a turbulent spectrum has been observed that is characterized by
a phase-space density of particles with exponent , which is larger than expected from relativistic
scatterings. Using the approach of Zakharov, L'vov and Falkovich, we analyse
possible Kolmogorov coefficients for relativistic -particle
processes, which give at most perturbatively for an energy cascade.
We discuss nonperturbative scenarios which lead to larger values. As an extreme
limit we find the result generically in an inherently nonperturbative
effective field theory situation, which coincides with results obtained by
Berges et al.\ in large- scalar field theory. If we instead assume that
scaling behavior is determined by Schwinger-Dyson resummations such that the
different scaling of bare and dressed vertices matters, we find that
intermediate values are possible. We present one simple scenario which would
single out .Comment: published versio
Numerically Exact Long Time Behavior of Nonequilibrium Quantum Impurity Models
A Monte Carlo sampling of diagrammatic corrections to the non-crossing
approximation is shown to provide numerically exact estimates of the long-time
dynamics and steady state properties of nonequilibrium quantum impurity models.
This `bold' expansion converges uniformly in time and significantly ameliorates
the sign problem that has heretofore limited the power of real-time Monte Carlo
approaches to strongly interacting real-time quantum problems. The new approach
enables the study of previously intractable problems ranging from generic long
time nonequilibrium transport characteristics in systems with large onsite
repulsion to the direct description of spectral functions on the real frequency
axis in Dynamical Mean Field Theory
Bosonic effective action for interacting fermions
We compare different versions of a bosonic description for systems of
interacting fermions, with particular emphasis on the free energy functional.
The bosonic effective action makes the issue of symmetries particularly
transparent and we present for the Hubbard model an exact mapping between
repulsive and attractive interactions. A systematic expansion for the bosonic
effective action starts with a solution to the lowest order Schwinger-Dyson or
gap equation. We propose a two particle irreducible formulation of an exact
functional renormalization group equation for computations beyond leading
order. On this basis we suggest a renormalized gap equation. This approach is
compared with functional renormalization in a partially bosonized setting.Comment: new sections on exact mapping between attractive and repulsive
Hubbard model and relation between two-particle-irreducible formalism, 32
pages,1 figure,LaTe
Incommensurate antiferromagnetic fluctuations in the two-dimensional Hubbard model
Commensurate and incommensurate antiferromagnetic fluctuations in the
two-dimensional repulsive t-t'-Hubbard model are investigated using functional
renormalization group equations. For a sufficient deviation from half filling
we establish the existence of local incommensurate order below a pseudocritical
temperature T_{pc}. Fluctuations not accounted for in the mean field
approximation are important--they lower T_{pc} by a factor \approx2.5.Comment: 7 pages, 8 figures, some changes due to referees' comments,
equivalent to published versio
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