209 research outputs found
Bicritical and tetracritical phenomena and scaling properties of the SO(5) theory
By large scale Monte Carlo simulations it is shown that the stable fixed
point of the SO(5) theory is either bicritical or tetracritical depending on
the effective interaction between the antiferromagnetism and superconductivity
orders. There are no fluctuation-induced first-order transitions suggested by
epsilon expansions. Bicritical and tetracritical scaling functions are derived
for the first time and critical exponents are evaluated with high accuracy.
Suggestions on experiments are given.Comment: 11 pages, 8 postscript figures, Revtex, revised versio
Comment on "Bicritical and Tetracritical Phenomena and Scaling Properties of the SO(5) Theory"
The multicritical point at which both a 3-component and a 2-component order
parameters order simultaneously in 3 dimensions is shown to have the critical
behavior of the decoupled fixed point, with separate n=3 and n=2 behavior. This
contradicts both the extrapolation of the epsilon-expansion at leading order,
which yields the biconical point, and recent Monte Carlo simulations, which
gave isotropic SO(5) behavior. Thus, this tetracritical point carries no
information on the relevance of the so-called SO(5) theory of high-T
superconductivity.Comment: 1 pag
Power-law correlations and orientational glass in random-field Heisenberg models
Monte Carlo simulations have been used to study a discretized Heisenberg
ferromagnet (FM) in a random field on simple cubic lattices. The spin variable
on each site is chosen from the twelve [110] directions. The random field has
infinite strength and a random direction on a fraction x of the sites of the
lattice, and is zero on the remaining sites. For x = 0 there are two phase
transitions. At low temperatures there is a [110] FM phase, and at intermediate
temperature there is a [111] FM phase. For x > 0 there is an intermediate phase
between the paramagnet and the ferromagnet, which is characterized by a
|k|^(-3) decay of two-spin correlations, but no true FM order. The [111] FM
phase becomes unstable at a small value of x. At x = 1/8 the [110] FM phase has
disappeared, but the power-law correlated phase survives.Comment: 8 pages, 12 Postscript figure
Interplay of quantum and classical fluctuations near quantum critical points
For a system near a quantum critical point (QCP), above its lower critical
dimension , there is in general a critical line of second order phase
transitions that separates the broken symmetry phase at finite temperatures
from the disordered phase. The phase transitions along this line are governed
by thermal critical exponents that are different from those associated with the
quantum critical point. We point out that, if the effective dimension of the
QCP, ( is the Euclidean dimension of the system and the
dynamic quantum critical exponent) is above its upper critical dimension ,
there is an intermingle of classical (thermal) and quantum critical
fluctuations near the QCP. This is due to the breakdown of the generalized
scaling relation between the shift exponent of the critical
line and the crossover exponent , for by a \textit{dangerous
irrelevant interaction}. This phenomenon has clear experimental consequences,
like the suppression of the amplitude of classical critical fluctuations near
the line of finite temperature phase transitions as the critical temperature is
reduced approaching the QCP.Comment: 10 pages, 6 figures, to be published in Brazilian Journal of Physic
The effective potential, critical point scaling and the renormalization group
The desirability of evaluating the effective potential in field theories near
a phase transition has been recognized in a number of different areas. We show
that recent Monte Carlo simulations for the probability distribution for the
order parameter in an equilibrium Ising system, when combined with low-order
renormalization group results for an ordinary system, can be used to
extract the effective potential. All scaling features are included in the
process.Comment: REVTEX file, 22 pages, three figures, submitted to Phys. Rev.
Critical structure factors of bilinear fields in O(N)-vector models
We compute the two-point correlation functions of general quadratic operators
in the high-temperature phase of the three-dimensional O(N) vector model by
using field-theoretical methods. In particular, we study the small- and
large-momentum behavior of the corresponding scaling functions, and give
general interpolation formulae based on a dispersive approach. Moreover, we
determine the crossover exponent associated with the traceless
tensorial quadratic field, by computing and analyzing its six-loop perturbative
expansion in fixed dimension. We find: ,
, and for respectively.Comment: 27 page
Quantum phase transitions in the Triangular-lattice Bilayer Heisenberg Model
We study the triangular lattice bilayer Heisenberg model with
antiferromagnetic interplane coupling and nearest neighbour
intraplane coupling , which can be ferro- or
antiferromagnetic, by expansions in . For negative a phase
transition is found to an ordered phase at a critical which is in the 3D classical Heisenberg universality class. For
, we find a transition at a rather large . The
universality class of the transition is consistent with that of Kawamura's 3D
antiferromagnetic stacked triangular lattice. The spectral weight for the
triplet excitations, at the ordering wavevector, remains finite at the
transition, suggesting that a phase with free spinons does not exist in this
model.Comment: revtex, 4 pages, 3 figure
Galactic Bulge Microlensing Optical Depth from EROS-2
We present a new EROS-2 measurement of the microlensing optical depth toward
the Galactic Bulge. Light curves of clump-giant stars
distributed over of the Bulge were monitored during seven Bulge
seasons. 120 events were found with apparent amplifications greater than 1.6
and Einstein radius crossing times in the range 5 {\rm d}.
This is the largest existing sample of clump-giant events and the first to
include northern Galactic fields. In the Galactic latitude range
1.4\degr<|b|<7.0\degr, we find with . These results are in good
agreement with our previous measurement, with recent measurements of the MACHO
and OGLE-II groups, and with predictions of Bulge models.Comment: accepted A&A, minor revision
Ising Universality in Three Dimensions: A Monte Carlo Study
We investigate three Ising models on the simple cubic lattice by means of
Monte Carlo methods and finite-size scaling. These models are the spin-1/2
Ising model with nearest-neighbor interactions, a spin-1/2 model with
nearest-neighbor and third-neighbor interactions, and a spin-1 model with
nearest-neighbor interactions. The results are in accurate agreement with the
hypothesis of universality. Analysis of the finite-size scaling behavior
reveals corrections beyond those caused by the leading irrelevant scaling
field. We find that the correction-to-scaling amplitudes are strongly dependent
on the introduction of further-neighbor interactions or a third spin state. In
a spin-1 Ising model, these corrections appear to be very small. This is very
helpful for the determination of the universal constants of the Ising model.
The renormalization exponents of the Ising model are determined as y_t = 1.587
(2), y_h = 2.4815 (15) and y_i = -0.82 (6). The universal ratio Q =
^2/ is equal to 0.6233 (4) for periodic systems with cubic symmetry.
The critical point of the nearest-neighbor spin-1/2 model is K_c=0.2216546
(10).Comment: 25 pages, uuencoded compressed PostScript file (to appear in Journal
of Physics A
Critical Indices as Limits of Control Functions
A variant of self-similar approximation theory is suggested, permitting an
easy and accurate summation of divergent series consisting of only a few terms.
The method is based on a power-law algebraic transformation, whose powers play
the role of control functions governing the fastest convergence of the
renormalized series. A striking relation between the theory of critical
phenomena and optimal control theory is discovered: The critical indices are
found to be directly related to limits of control functions at critical points.
The method is applied to calculating the critical indices for several difficult
problems. The results are in very good agreement with accurate numerical data.Comment: 1 file, 5 pages, RevTe
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