75 research outputs found
Frustrated magnets in three dimensions: a nonperturbative approach
Frustrated magnets exhibit unusual critical behaviors: they display scaling
laws accompanied by nonuniversal critical exponents. This suggests that these
systems generically undergo very weak first order phase transitions. Moreover,
the different perturbative approaches used to investigate them are in conflict
and fail to correctly reproduce their behavior. Using a nonperturbative
approach we explain the mismatch between the different perturbative approaches
and account for the nonuniversal scaling observed.Comment: 7 pages, 1 figure. IOP style files included. To appear in Journal of
Physics: Condensed Matter. Proceedings of the conference HFM 2003, Grenoble,
Franc
Regenerative approaches for V/UHTS feeder links: system analysis and on-board complexity reduction
The dramatically increasing demand for high data rates necessitates the proper dimensioning of the feeder links of very or ultra high throughput satellite (V/UHTS) systems. However, because most of the current solutions rely on transparent payloads, the deployment of a very large number of spatially separated ground stations is necessary to support the total system bandwidth by enabling a full reuse of the scarce available uplink bandwidth. This approach has a significant impact on the complexity and the costs of the ground segment infrastructure. Regenerative payloads could be considered to avoid this design bottleneck. By allowing demodulation and decoding on-board the satellite, the favourable link budget conditions of feeder links compared to the user links can be exploited. Using a spectral efficient transmission technique, the number of ground stations required to support a target sum throughput can be notably reduced. Meanwhile, regenerative solutions have until now barely been used in V/UHTS payloads due to their high on-board power consumption. As a consequence, candidate solutions are proposed in this work to overcome this limitation. A non-coherent modulation technique, known as Differential Amplitude Phase Shift Keying (DAPSK), is introduced to avoid on-board carrier synchronization. Moreover, polar codes are considered to minimize the power consumption of the channel decoder. A preliminary analysis of the expected on-board power consumption compared to that of a standard DVB-S2 approach is conducted using available results in the open literature. Link performance is also evaluated via numerical simulations
Functional renormalization group in the broken symmetry phase: momentum dependence and two-parameter scaling of the self-energy
We include spontaneous symmetry breaking into the functional renormalization
group (RG) equations for the irreducible vertices of Ginzburg-Landau theories
by augmenting these equations by a flow equation for the order parameter, which
is determined from the requirement that at each RG step the vertex with one
external leg vanishes identically. Using this strategy, we propose a simple
truncation of the coupled RG flow equations for the vertices in the broken
symmetry phase of the Ising universality class in D dimensions. Our truncation
yields the full momentum dependence of the self-energy Sigma (k) and
interpolates between lowest order perturbation theory at large momenta k and
the critical scaling regime for small k. Close to the critical point, our
method yields the self-energy in the scaling form Sigma (k) = k_c^2 sigma^{-}
(k | xi, k / k_c), where xi is the order parameter correlation length, k_c is
the Ginzburg scale, and sigma^{-} (x, y) is a dimensionless two-parameter
scaling function for the broken symmetry phase which we explicitly calculate
within our truncation.Comment: 9 pages, 4 figures, puplished versio
Fixed points in frustrated magnets revisited
We analyze the validity of perturbative renormalization group estimates
obtained within the fixed dimension approach of frustrated magnets. We
reconsider the resummed five-loop beta-functions obtained within the minimal
subtraction scheme without epsilon-expansion for both frustrated magnets and
the well-controlled ferromagnetic systems with a cubic anisotropy. Analyzing
the convergence properties of the critical exponents in these two cases we find
that the fixed point supposed to control the second order phase transition of
frustrated magnets is very likely an unphysical one. This is supported by its
non-Gaussian character at the upper critical dimension d=4. Our work confirms
the weak first order nature of the phase transition occuring at three
dimensions and provides elements towards a unified picture of all existing
theoretical approaches to frustrated magnets.Comment: 18 pages, 8 figures. This article is an extended version of
arXiv:cond-mat/060928
Non-perturbative renormalization-group approach to zero-temperature Bose systems
We use a non-perturbative renormalization-group technique to study
interacting bosons at zero temperature. Our approach reveals the instability of
the Bogoliubov fixed point when and yields the exact infrared
behavior in all dimensions within a rather simple theoretical framework.
It also enables to compute the low-energy properties in terms of the parameters
of a microscopic model. In one-dimension and for not too strong interactions,
it yields a good picture of the Luttinger-liquid behavior of the superfluid
phase.Comment: v1) 6 pages, 8 figures; v2) added references; v3) corrected typo
Equivalent Fixed-Points in the Effective Average Action Formalism
Starting from a modified version of Polchinski's equation, Morris'
fixed-point equation for the effective average action is derived. Since an
expression for the line of equivalent fixed-points associated with every
critical fixed-point is known in the former case, this link allows us to find,
for the first time, the analogous expression in the latter case.Comment: 30 pages; v2: 29 pages - major improvements to section 3; v3:
published in J. Phys. A - minor change
Heisenberg frustrated magnets: a nonperturbative approach
Frustrated magnets are a notorious example where the usual perturbative
methods are in conflict. Using a nonperturbative Wilson-like approach, we get a
coherent picture of the physics of Heisenberg frustrated magnets everywhere
between and . We recover all known perturbative results in a single
framework and find the transition to be weakly first order in . We compute
effective exponents in good agreement with numerical and experimental data.Comment: 5 pages, Revtex, technical details available at
http://www.lpthe.jussieu.fr/~tissie
Critical behavior of frustrated systems: Monte Carlo simulations versus Renormalization Group
We study the critical behavior of frustrated systems by means of Pade-Borel
resummed three-loop renormalization-group expansions and numerical Monte Carlo
simulations. Amazingly, for six-component spins where the transition is second
order, both approaches disagree. This unusual situation is analyzed both from
the point of view of the convergence of the resummed series and from the
possible relevance of non perturbative effects.Comment: RevTex, 10 pages, 3 Postscript figure
A non perturbative approach of the principal chiral model between two and four dimensions
We investigate the principal chiral model between two and four dimensions by
means of a non perturbative Wilson-like renormalization group equation. We are
thus able to follow the evolution of the effective coupling constants within
this whole range of dimensions without having recourse to any kind of small
parameter expansion. This allows us to identify its three dimensional critical
physics and to solve the long-standing discrepancy between the different
perturbative approaches that characterizes the class of models to which the
principal chiral model belongs.Comment: 5 pages, 1 figure, Revte
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