23,584 research outputs found
On the universality of distribution of ranked cluster masses at critical percolation
The distribution of masses of clusters smaller than the infinite cluster is
evaluated at the percolation threshold. The clusters are ranked according to
their masses and the distribution of the scaled masses M for any
rank r shows a universal behaviour for different lattice sizes L (D is the
fractal dimension). For different ranks however, there is a universal
distribution function only in the large rank limit, i.e., (y and are defined in the text), where the
universal scaling function g is found to be Gaussian in nature.Comment: 4 pages, to appear in J. Phys.
Product Representation of Dyon Partition Function in CHL Models
A formula for the exact partition function of 1/4 BPS dyons in a class of CHL
models has been proposed earlier. The formula involves inverse of Siegel
modular forms of subgroups of Sp(2,Z). In this paper we propose product
formulae for these modular forms. This generalizes the result of Borcherds and
Gritsenko and Nikulin for the weight 10 cusp form of the full Sp(2,Z) group.Comment: LaTeX file, 39 pages; minor change
Non-Supersymmetric Attractors in String Theory
We find examples of non-supersymmetric attractors in Type II string theory
compactified on a Calabi Yau three-fold. For a non-supersymmetric attractor the
fixed values to which the moduli are drawn at the horizon must minimise an
effective potential. For Type IIA at large volume, we consider a configuration
carrying D0, D2, D4 and D6 brane charge. When the D6 brane charge is zero, we
find for some range of the other charges, that a non-supersymmetric attractor
solution exists. When the D6 brane charge is non-zero, we find for some range
of charges, a supersymmetry breaking extremum of the effective potential.
Closer examination reveals though that it is not a minimum of the effective
potential and hence the corresponding black hole solution is not an attractor.
Away from large volume, we consider the specific case of the quintic in CP^4.
Working in the mirror IIB description we find non-supersymmetric attractors
near the Gepner point.Comment: Added a few clarification
Ferromagnetism in nanoscale BiFeO3
A remarkably high saturation magnetization of ~0.4mu_B/Fe along with room
temperature ferromagnetic hysteresis loop has been observed in nanoscale (4-40
nm) multiferroic BiFeO_3 which in bulk form exhibits weak magnetization
(~0.02mu_B/Fe) and an antiferromagnetic order. The magnetic hysteresis loops,
however, exhibit exchange bias as well as vertical asymmetry which could be
because of spin pinning at the boundaries between ferromagnetic and
antiferromagnetic domains. Interestingly, like in bulk BiFeO_3, both the
calorimetric and dielectric permittivity data in nanoscale BiFeO_3 exhibit
characteristic features at the magnetic transition point. These features
establish formation of a true ferromagnetic-ferroelectric system with a
coupling between the respective order parameters in nanoscale BiFeO_3.Comment: 13 pages including 4 figures; pdf only; submitted to Appl. Phys. Let
Black Hole Microstates and Attractor Without Supersymmetry
Due to the attractor mechanism, the entropy of an extremal black hole does
not vary continuously as we vary the asymptotic values of various moduli
fields. Using this fact we argue that the entropy of an extremal black hole in
string theory, calculated for a range of values of the asymptotic moduli for
which the microscopic theory is strongly coupled, should match the statistical
entropy of the same system calculated for a range of values of the asymptotic
moduli for which the microscopic theory is weakly coupled. This argument does
not rely on supersymmetry and applies equally well to nonsupersymmetric
extremal black holes. We discuss several examples which support this argument
and also several caveats which could invalidate this argument.Comment: 50 pages; references adde
Non-Supersymmetric Attractors in Gravities
We investigate the attractor mechanism for spherically symmetric extremal
black holes in a theory of general gravity in 4-dimensions, coupled to
gauge fields and moduli fields. For the general theory, we look for
solutions which are analytic near the horizon, show that they exist and enjoy
the attractor behavior. The attractor point is determined by extremization of
an effective potential at the horizon. This analysis includes the backreaction
and supports the validity of non-supersymmetric attractors in the presence of
higher derivative interactions. To include a wider class of solutions, we
continue our analysis for the specific case of a Gauss-Bonnet theory which is
non-topological, due to the coupling of Gauss-Bonnet terms to the moduli
fields. We find that the regularity of moduli fields at the horizon is
sufficient for attractor behavior. For the non-analytic sector, this regularity
condition in turns implies the minimality of the effective potential at the
attractor point.Comment: 19 pages, 2 figure
Fundamental Strings in Open String Theory at the Tachyonic Vacuum
We show that the world-volume theory on a D-p-brane at the tachyonic vacuum
has solitonic string solutions whose dynamics is governed by the Nambu-Goto
action of a string moving in (25+1) dimensional space-time. This provides
strong evidence for the conjecture that at this vacuum the full (25+1)
dimensional Poincare invariance is restored. We also use this result to argue
that the open string field theory at the tachyonic vacuum must contain closed
string excitations.Comment: LaTeX file, 16 pages, references and clarification adde
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