8,141 research outputs found
Effective Spin Couplings in the Mott Insulator of the Honeycomb Lattice Hubbard Model
Motivated by the recent discovery of a spin liquid phase for the Hubbard
model on the honeycomb lattice at half-filling, we apply both perturbative and
non-perturbative techniques to derive effective spin Hamiltonians describing
the low-energy physics of the Mott-insulating phase of the system. Exact
diagonalizations of the so-derived models on small clusters are performed, in
order to assess the quality of the effective low-energy theory in the
spin-liquid regime. We show that six-spin interactions on the elementary loop
of the honeycomb lattice are the dominant sub-leading effective couplings. A
minimal spin model is shown to reproduce most of the energetic properties of
the Hubbard model on the honeycomb lattice in its spin-liquid phase.
Surprisingly, a more elaborate effective low-energy spin model obtained by a
systematic graph expansion rather disagrees beyond a certain point with the
numerical results for the Hubbard model at intermediate couplings.Comment: 20 pages, 10 figure
X, Y and Z States
Many new states in the charmonium mass region were recently discovered by
BaBar, Belle, CLEO-c, CDF, D0, BESIII, LHCb and CMS Collaborations. We use the
QCD Sum Rule approach to study the possible structure of some of these states.Comment: Contribution for the proceedings of the "XII Quark Confinement and
the Hadron Spectrum - CONF12" conferenc
Quantum Critical Scaling of Fidelity Susceptibility
The behavior of the ground-state fidelity susceptibility in the vicinity of a
quantum critical point is investigated. We derive scaling relations describing
its singular behavior in the quantum critical regime. Unlike it has been found
in previous studies, these relations are solely expressed in terms of
conventional critical exponents. We also describe in detail a quantum Monte
Carlo scheme that allows for the evaluation of the fidelity susceptibility for
a large class of many-body systems and apply it in the study of the quantum
phase transition for the transverse-field Ising model on the square lattice.
Finite size analysis applied to the so obtained numerical results confirm the
validity of our scaling relations. Furthermore, we analyze the properties of a
closely related quantity, the ground-state energy's second derivative, that can
be numerically evaluated in a particularly efficient way. The usefulness of
both quantities as alternative indicators of quantum criticality is examined.Comment: 13 pages, 7 figures. Published versio
Geographic patterns of vertebrate diversity and identification of relevant areas for conservation in Europe
The âEU Council conclusions on biodiversity postâ2010âČ reâenforced Europeâs commitment to halt biodiversity loss by 2020. Identifying areas of highâvalue for biodiversity conservation is an important issue to meet this target. We investigated the geographic pattern of terrestrial vertebrate diversity status in Europe by assessing the species richness, rarity, vulnerability (according to IUCN criteria), and a combined index of the three former for the amphibians, reptiles, bird and mammals of this region. We also correlated the value of all indices with climate and human influence variables. Overall, clear geographic gradients of species diversity were found. The combined biodiversity index indicated that highâvalue biodiversity areas were mostly located in the Mediterranean basin and the highest vulnerability was found in the Iberian peninsula for most taxa. Across all indexes, the proportion of variance explained by climate and human influence factors was moderate to low. The results obtained in this study have the potential to provide valuable support for nature conservation policies in Europe and, consequently, might contribute to mitigate biodiversity decline in this region
Neutrino Telescopes as a Direct Probe of Supersymmetry Breaking
We consider supersymmetric models where the scale of supersymmetry breaking
lies between 5 GeV and 5 GeV. In this class of
theories, which includes models of gauge mediated supersymmetry breaking, the
lightest supersymmetric particle is the gravitino. The next to lightest
supersymmetric particle is typically a long lived charged slepton with a
lifetime between a microsecond and a second, depending on its mass. Collisions
of high energy neutrinos with nucleons in the earth can result in the
production of a pair of these sleptons. Their very high boost means they
typically decay outside the earth. We investigate the production of these
particles by the diffuse flux of high energy neutrinos, and the potential for
their observation in large ice or water Cerenkov detectors. The relatively
small cross-section for the production of supersymmetric particles is partially
compensated for by the very long range of heavy particles. The signal in the
detector consists of two parallel charged tracks emerging from the earth about
100 meters apart, with very little background. A detailed calculation using the
Waxman-Bahcall limit on the neutrino flux and realistic spectra shows that
km experiments could see as many as 4 events a year. We conclude that
neutrino telescopes will complement collider searches in the determination of
the supersymmetry breaking scale, and may even give the first evidence for
supersymmetry at the weak scale.Comment: 4 pages, 3 figure
QCD Sum Rule Study for a Possible Charmed Pentaquark \Theta c(3250)
We use QCD sum rules to study the possible existence of a \Theta c(3250)
charmed pentaquark. We consider the contributions of condensates up to
dimension-10 and work at leading order in \alpha_s. We obtain m(\Theta c) =
(3.21 +/- 0.13) GeV, compatible with the mass of the structure seen by BaBar
Collaboration in the decay channel B- -> p- \Sigma c++ pi- pi-. The proposed
state is compatible with a previous proposed pentaquark state in the
anti-charmed sector.Comment: 8 pages, 7 figures, 1 tabl
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