713 research outputs found
Spin anisotropy effects in dimer single molecule magnets
We present a model of equal spin dimer single molecule magnets. The
spins within each dimer interact via the Heisenberg and the most general set of
four quadratic anisotropic spin interactions with respective strengths and
, and with the magnetic induction . We solve the model
exactly for , and for antiferromagnetic Heisenberg couplings
(), present curves at low for these cases. Low-
curves for and electron paramagnetic susceptibility
for are also provided. For weak anisotropy
interactions, we employ a perturbative treatment, and show that the Hartree and
extended Hartree approximations lead to reliable analytic results at low
and large for these quantities and for the inelastic neutron scattering
cross-section . Our results are discussed with
regard to existing experiments on Fe dimer
single molecule magnets, and suggest that one of them contains a substantial
amount of single-ion anisotropy, without any sizeable global spin anisotropy.
We urge further experiments of the above types on single crystals of Fe and
on some [Mn] dimers, in order to elucidate the precise values
of the various microscopic interactions.Comment: 30 pages, 25 figures, submitted to Phys. Rev.
All null supersymmetric backgrounds of N=2, D=4 gauged supergravity coupled to abelian vector multiplets
The lightlike supersymmetric solutions of N=2, D=4 gauged supergravity
coupled to an arbitrary number of abelian vector multiplets are classified
using spinorial geometry techniques. The solutions fall into two classes,
depending on whether the Killing spinor is constant or not. In both cases, we
give explicit examples of supersymmetric backgrounds. Among these BPS
solutions, which preserve one quarter of the supersymmetry, there are
gravitational waves propagating on domain walls or on bubbles of nothing that
asymptote to AdS_4. Furthermore, we obtain the additional constraints obeyed by
half-supersymmetric vacua. These are divided into four categories, that include
bubbles of nothing which are asymptotically AdS_4, pp-waves on domain walls,
AdS_3 x R, and spacetimes conformal to AdS_3 times an interval.Comment: 55 pages, uses JHEP3.cls. v2: Minor errors corrected, small changes
in introductio
Matone's Relation in the Presence of Gravitational Couplings
The prepotential in N=2 SUSY Yang-Mills theories enjoys remarkable
properties. One of the most interesting is its relation to the coordinate on
the quantum moduli space that results into recursion
equations for the coefficients of the prepotential due to instantons. In this
work we show, with an explicit multi-instanton computation, that this relation
holds true at arbitrary winding numbers. Even more interestingly we show that
its validity extends to the case in which gravitational corrections are taken
into account if the correlators are suitably modified. These results apply also
to the cases in which matter in the fundamental and in the adjoint is included.
We also check that the expressions we find satisfy the chiral ring relations
for the gauge case and compute the first gravitational correction.Comment: 21 page
Nonequilibrium phase transition in a model for the propagation of innovations among economic agents
We characterize the different morphological phases that occur in a simple
one-dimensional model of propagation of innovations among economic agents [X.\
Guardiola, {\it et. al.}, Phys. Rev E {\bf 66}, 026121 (2002)]. We show that
the model can be regarded as a nonequilibrium surface growth model. This allows
us to demonstrate the presence of a continuous roughening transition between a
flat (system size independent fluctuations) and a rough phase (system size
dependent fluctuations). Finite-size scaling studies at the transition strongly
suggest that the dynamic critical transition does not belong to directed
percolation and, in fact, critical exponents do not seem to fit in any of the
known universality classes of nonequilibrium phase transitions. Finally, we
present an explanation for the occurrence of the roughening transition and
argue that avalanche driven dynamics is responsible for the novel critical
behavior
Improved matrix-model calculation of the N=2 prepotential
We present a matrix-model expression for the sum of instanton contributions
to the prepotential of an N=2 supersymmetric U(N) gauge theory, with matter in
various representations. This expression is derived by combining the
renormalization-group approach to the gauge theory prepotential with
matrix-model methods. This result can be evaluated order-by-order in
matrix-model perturbation theory to obtain the instanton corrections to the
prepotential. We also show, using this expression, that the one-instanton
prepotential assumes a universal form.Comment: 20 pages, LaTeX, 2 figure
Direct Integration of the Topological String
We present a new method to solve the holomorphic anomaly equations governing
the free energies of type B topological strings. The method is based on direct
integration with respect to the non-holomorphic dependence of the amplitudes,
and relies on the interplay between non-holomorphicity and modularity
properties of the topological string amplitudes. We develop a formalism valid
for any Calabi-Yau manifold and we study in detail two examples, providing
closed expressions for the amplitudes at low genus, as well as a discussion of
the boundary conditions that fix the holomorphic ambiguity. The first example
is the non-compact Calabi-Yau underlying Seiberg-Witten theory and its
gravitational corrections. The second example is the Enriques Calabi-Yau, which
we solve in full generality up to genus six. We discuss various aspects of this
model: we obtain a new method to generate holomorphic automorphic forms on the
Enriques moduli space, we write down a new product formula for the fiber
amplitudes at all genus, and we analyze in detail the field theory limit. This
allows us to uncover the modularity properties of SU(2), N=2 super Yang-Mills
theory with four massless hypermultiplets.Comment: 75 pages, 3 figure
Vanishing Preons in the Fifth Dimension
We examine supersymmetric solutions of N=2, D=5 gauged supergravity coupled
to an arbitrary number of abelian vector multiplets using the spinorial
geometry method. By making use of methods developed in hep-th/0606049 to
analyse preons in type IIB supergravity, we show that there are no solutions
preserving exactly 3/4 of the supersymmetry.Comment: 19 pages, latex. Reference added, and further modification to the
introductio
New stable phase of non uniform black strings in
We consider the non uniform black string equations in arbitrary number
of dimension in a perturbative approach up to order 2 and in a non
perturbative. We restrict the study in the perturbative approach to the
backreacting modes, since they provide the first relevant corrections on the
thermodynamical quantities of the solutions. We also present some preliminary
results in the construction of non-perturbative solutions, in particular, we
present a first part of the non uniform - uniform black string phase diagram.
Our results suggests the existence of a new stable phase for non uniform
black strings, namely long non uniform black string, with the extra direction
length of the order of the curvature.Comment: Results extended. 14 pages, 5 figure
Nonperturbative Renormalization Group Equation and Beta Function in N=2 SUSY Yang-Mills
We obtain the exact beta function for SUSY Yang-Mills theory
and prove the nonperturbative Renormalization Group Equation Comment: LaTex, 10 pg. Expanded introduction, references added, to appear in
Phys. Rev. Let
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