358 research outputs found
RVB description of the low-energy singlets of the spin 1/2 kagome antiferromagnet
{Extensive calculations in the short-range RVB (Resonating valence bond)
subspace on both the trimerized and the regular (non-trimerized) Heisenberg
model on the kagome lattice show that short-range dimer singlets capture the
specific low-energy features of both models. In the trimerized case the singlet
spectrum splits into bands in which the average number of dimers lying on one
type of bonds is fixed. These results are in good agreement with the mean field
solution of an effective model recently introduced. For the regular model one
gets a continuous, gapless spectrum, in qualitative agreement with exact
diagonalization results.Comment: 10 pages, 13 figures, 3 tables. Submitted to EPJ
The spin gap of CaV4O9 revisited
The large-plaquette scenario of the spin gap in CaV4O9 is investigated on the
basis of extensive exact diagonalizations. We confirm the existence of a
large-plaquette phase in a wide range of parameters, and we show that the most
recent neutron scattering data actually require an intra-plaquette second
neighbor exchange integral much larger than the inter-plaquette one, thus
justifying the perturbative calculation used in the interpretation of the
neutron scattering experiments.Comment: 2 pages with 3 figure
Exotic phenomena in doped quantum magnets
We investigate the properties of the two-dimensional frustrated quantum
antiferromagnet on the square lattice, especially at infinitesimal doping. We
find that next nearest neighbor (N.N.) J2 and next-next N.N. J3 interactions
together destroy the antiferromagnetic long range order and stabilize a quantum
disordered valence bond crystalline plaquette phase. A static vacancy or a
dynamic hole doped into this phase liberates a spinon. From the profile of the
spinon wavefunction around the (static) vacancy we identify an intermediate
behavior between complete deconfinement (behavior seen in the kagome lattice)
and strong confinement (behavior seen in the checkerboard lattice) with the
emergence of two length scales, a spinon confinement length larger than the
magnetic correlation length. When a finite hole hopping is introduced, this
behavior translates into an extended (mobile) spinon-holon boundstate with a
very small quasiparticle weight. These features provide clear evidence for a
nearby "deconfined critical point" in a doped microscopic model. Finally, we
give arguments in favor of superconducting properties of the doped plaquette
phase.Comment: Submitted to J. of Phys. Condens. Matter (Proceedings of
International Conference "Highly Frustrated Magnets", Osaka (Japan), August
2006). 6 pages, 5 figures Display problems with Figure 2 fixe
Static impurities in the kagome lattice: dimer freezing and mutual repulsion
We consider the effects of doping the S = 1/2 kagome lattice with static
impurities. We demonstrate that impurities lower the number of low-lying
singlet states, induce dimer-dimer correlations of considerable spatial extent,
and do not generate free spin degrees of freedom. Most importantly, they
experience a highly unconventional mutual repulsion as a direct consequence of
the strong spin frustration. These properties are illustrated by exact
diagonalization, and reproduced to semi-quantitative accuracy within a dimer
resonating-valence-bond description which affords access to longer length
scales. We calculate the local magnetization induced by doped impurities, and
consider its implications for nuclear magnetic resonance measurements on known
kagome systems.Comment: 9 pages, 12 figure
Valence Bond Entanglement Entropy
We introduce for SU(2) quantum spin systems the Valence Bond Entanglement
Entropy as a counting of valence bond spin singlets shared by two subsystems.
For a large class of antiferromagnetic systems, it can be calculated in all
dimensions with Quantum Monte Carlo simulations in the valence bond basis. We
show numerically that this quantity displays all features of the von Neumann
entanglement entropy for several one-dimensional systems. For two-dimensional
Heisenberg models, we find a strict area law for a Valence Bond Solid state and
multiplicative logarithmic corrections for the Neel phase.Comment: 4 pages, 3 figures, v2: small corrections, published versio
Supersymmetry Breaking due to Moduli Stabilization in String Theory
We consider the phenomenological consequences of fixing compactification
moduli. In the simplest KKLT constructions, stabilization of internal
dimensions is rather soft: weak scale masses for moduli are generated, and are
of order m_\sigma ~ m_{3/2}. As a consequence one obtains a pattern of soft
supersymmetry breaking masses found in gravity and/or anomaly mediated
supersymmetry breaking (AMSB) models. These models may lead to destabilization
of internal dimensions in the early universe, unless the Hubble constant during
inflation is very small. Fortunately, strong stabilization of compactified
dimensions can be achieved by a proper choice of the superpotential (e.g in the
KL model with a racetrack superpotential). This allows for a solution of the
cosmological moduli problem and for a successful implementation of inflation in
supergravity. We show that strong moduli stabilization leads a very distinct
pattern of soft supersymmetry breaking masses. In general, we find that soft
scalar masses remain of order the gravitino mass, while gaugino masses nearly
vanish at the tree level, i.e. they are of order m_{3/2}^2/m_\sigma. Radiative
corrections generate contributions to gaugino masses reminiscent of AMSB models
and a decoupled spectrum of scalars reminiscent of split-supersymmetry. This
requires a relatively large gravitino mass ~ O(100) TeV, resolving the
cosmological gravitino problem and problems with tachyonic staus in AMSB
models.Comment: 10 pages, 2 figures, some references and comments added. This is the
version to appear in Phys. Rev.
Interaction between static holes in a quantum dimer model on the kagome lattice
A quantum dimer model (QDM) on the kagome lattice with an extensive
ground-state entropy was recently introduced [Phys. Rev. B 67, 214413 (2003)].
The ground-state energy of this QDM in presence of one and two static holes is
investigated by means of exact diagonalizations on lattices containing up to
144 kagome sites. The interaction energy between the holes (at distances up to
7 lattice spacings) is evaluated and the results show no indication of
confinement at large hole separations.Comment: 6 pages, 3 figures. IOP style files included. To appear in J. Phys.:
Condens. Matter, Proceedings of the HFM2003 conference, Grenobl
Adiabatic compression and indirect detection of supersymmetric dark matter
Recent developments in the modelling of the dark matter distribution in our
Galaxy point out the necessity to consider some physical processes to satisfy
observational data. In particular, models with adiabatic compression, which
include the effect of the baryonic gas in the halo, increase significantly the
dark matter density in the central region of the Milky Way. On the other hand,
the non-universality in scalar and gaugino sectors of supergravity models can
also increase significantly the neutralino annihilation cross section. We show
that the combination of both effects gives rise to a gamma-ray flux arising
from the Galactic Center largely reachable by future experiments like GLAST. We
also analyse in this framework the EGRET excess data above 1 GeV, as well as
the recent data from CANGAROO and HESS. The analysis has been carried out
imposing the most recent experimental constraints, such as the lower bound on
the Higgs mass, the \bsg branching ratio, and the muon . In addition, the
recently improved upper bound on has also been taken
into account. The astrophysical (WMAP) bounds on the dark matter density have
also been imposed on the theoretical computation of the relic neutralino
density through thermal production.Comment: 32 pages, 11 figures, final version to appear in JCA
Metastable Vacua in Flux Compactifications and Their Phenomenology
In the context of flux compactifications, metastable vacua with a small
positive cosmological constant are obtained by combining a sector where
supersymmetry is broken dynamically with the sector responsible for moduli
stabilization, which is known as the F-uplifting. We analyze this procedure in
a model-independent way and study phenomenological properties of the resulting
vacua.Comment: 21 pages, 19 figures; v2: matches version published in JHE
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