333 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
Competition between fluctuations and disorder in frustrated magnets
We investigate the effects of impurities on the nature of the phase
transition in frustrated magnets, in d=4-epsilon dimensions. For sufficiently
small values of the number of spin components, we find no physically relevant
stable fixed point in the deep perturbative region (epsilon << 1), contrarily
to what is to be expected on very general grounds. This signals the onset of
important physical effects.Comment: 4 pages, 3 figures, published versio
Optimization of the derivative expansion in the nonperturbative renormalization group
We study the optimization of nonperturbative renormalization group equations
truncated both in fields and derivatives. On the example of the Ising model in
three dimensions, we show that the Principle of Minimal Sensitivity can be
unambiguously implemented at order of the derivative expansion.
This approach allows us to select optimized cut-off functions and to improve
the accuracy of the critical exponents and . The convergence of the
field expansion is also analyzed. We show in particular that its optimization
does not coincide with optimization of the accuracy of the critical exponents.Comment: 13 pages, 9 PS figures, published versio
Spin-stiffness and topological defects in two-dimensional frustrated spin systems
Using a {\it collective} Monte Carlo algorithm we study the low-temperature
and long-distance properties of two systems of two-dimensional classical tops.
Both systems have the same spin-wave dynamics (low-temperature behavior) as a
large class of Heisenberg frustrated spin systems. They are constructed so that
to differ only by their topological properties. The spin-stiffnesses for the
two systems of tops are calculated for different temperatures and different
sizes of the sample. This allows to investigate the role of topological defects
in frustrated spin systems. Comparisons with Renormalization Group results
based on a Non Linear Sigma model approach and with the predictions of some
simple phenomenological model taking into account the topological excitations
are done.Comment: RevTex, 25 pages, 14 figures, Minor changes, final version. To appear
in Phys.Rev.
Critical thermodynamics of three-dimensional chiral model for N > 3
The critical behavior of the three-dimensional -vector chiral model is
studied for arbitrary . The known six-loop renormalization-group (RG)
expansions are resummed using the Borel transformation combined with the
conformal mapping and Pad\'e approximant techniques. Analyzing the fixed point
location and the structure of RG flows, it is found that two marginal values of
exist which separate domains of continuous chiral phase transitions and where such
transitions are first-order. Our calculations yield and
. For the structure of RG flows is identical to
that given by the and 1/N expansions with the chiral fixed point
being a stable node. For the chiral fixed point turns out to be a
focus having no generic relation to the stable fixed point seen at small
and large . In this domain, containing the physical values and , phase trajectories approach the fixed point in a spiral-like
manner giving rise to unusual crossover regimes which may imitate varying
(scattered) critical exponents seen in numerous physical and computer
experiments.Comment: 12 pages, 3 figure
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
Partial loss of function of the GHRH Receptor leads to mild Growth Hormone Deficiency
OBJECTIVE: Recessive mutations in GHRHR are associated with severe isolated growth hormone deficiency (IGHD), with a final height in untreated patients of 130 cm ± 10 cm (-7.2 ± 1.6 SDS; males) and 114 ± 0.7 cm (-8.3 ± 0.1 SDS; females). DESIGN: We hypothesized that a consanguineous Pakistani family with IGHD in three siblings (two males, one female) would have mutations in GH1 or GHRHR. RESULTS: Two novel homozygous missense variants [c.11G>A (p.R4Q), c.236C>T (p.P79L)] at conserved residues were identified in all three siblings. Both were absent from control databases, aside from pR4Q appearing once in heterozygous form in the Exome Aggregation Consortium Browser. The brothers were diagnosed with GH deficiency at 9.8 and 6.0 years (height SDS: -2.24 and -1.23, respectively), with a peak GH of 2.9 μg/liter with low IGF-1/IGF binding protein 3. Their sister presented at 16 years with classic GH deficiency (peak GH <0.1 μg/liter, IGF-1 <3.3 mmol/liter) and attained an untreated near-adult height of 144 cm (-3.0 SDS); the tallest untreated patient with GHRHR mutations reported. An unrelated Pakistani female IGHD patient was also compound homozygous. All patients had a small anterior pituitary on magnetic resonance imaging. Functional analysis revealed a 50% reduction in maximal cAMP response to stimulation with GHRH by the p.R4Q/p.P79L double mutant receptor, with a 100-fold increase in EC50. CONCLUSION: We report the first coexistence of two novel compound homozygous GHRHR variants in two unrelated pedigrees associated with a partial loss of function. Surprisingly, the patients have a relatively mild IGHD phenotype. Analysis revealed that the pP79L mutation is associated with the compromise in function, with the residual partial activity explaining the mild phenotype
Spin Stiffness of Stacked Triangular Antiferromagnets
We study the spin stiffness of stacked triangular antiferromagnets using both
heat bath and broad histogram Monte Carlo methods. Our results are consistent
with a continuous transition belonging to the chiral universality class first
proposed by Kawamura.Comment: 5 pages, 7 figure
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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