130 research outputs found
Attractive Hubbard Model on a Honeycomb Lattice
We study the attractive fermionic Hubbard model on a honeycomb lattice using
determinantal quantum Monte Carlo simulations. By increasing the interaction
strength U (relative to the hopping parameter t) at half-filling and zero
temperature, the system undergoes a quantum phase transition at 5.0 < U_c/t <
5.1 from a semi-metal to a phase displaying simultaneously superfluid behavior
and density order. Doping away from half-filling, and increasing the
interaction strength at finite but low temperature T, the system always appears
to be a superfluid exhibiting a crossover between a BCS and a molecular regime.
These different regimes are analyzed by studying the spectral function. The
formation of pairs and the emergence of phase coherence throughout the sample
are studied as U is increased and T is lowered
Extreme events driven glassy behaviour in granular media
Motivated by recent experiments on the approach to jamming of a weakly forced
granular medium using an immersed torsion oscillator [Nature 413 (2001) 407],
we propose a simple model which relates the microscopic dynamics to macroscopic
rearrangements and accounts for the following experimental facts: (1) the
control parameter is the spatial amplitude of the perturbation and not its
reduced peak acceleration; (2) a Vogel-Fulcher-Tammann-like form for the
relaxation time. The model draws a parallel between macroscopic rearrangements
in the system and extreme events whose probability of occurrence (and thus the
typical relaxation time) is estimated using extreme-value statistics. The range
of validity of this description in terms of the control parameter is discussed
as well as the existence of other regimes.Comment: 7 pages, to appear in Europhys. Let
Statistics of S-matrix poles in Few-Channel Chaotic Scattering: Crossover from Isolated to Overlapping Resonances
We derive the explicit expression for the distribution of resonance widths in
a chaotic quantum system coupled to continua via M equivalent open channels. It
describes a crossover from the distribution (regime of isolated
resonances) to a broad power-like distribution typical for the regime of
overlapping resonances. The first moment is found to reproduce exactly the
Moldauer-Simonius relation between the mean resonance width and the
transmission coefficient. This fact may serve as another manifestation of
equivalence between the spectral and the ensemble averaging.Comment: 4 two-column pages, RevTex. text is slightly modified; some misprints
are correcte
Electronic structure and optical properties of lightweight metal hydrides
We study the electronic structures and dielectric functions of the simple
hydrides LiH, NaH, MgH2 and AlH3, and the complex hydrides Li3AlH6, Na3AlH6,
LiAlH4, NaAlH4 and Mg(AlH4)2, using first principles density functional theory
and GW calculations. All these compounds are large gap insulators with GW
single particle band gaps varying from 3.5 eV in AlH3 to 6.5 eV in the MAlH4
compounds. The valence bands are dominated by the hydrogen atoms, whereas the
conduction bands have mixed contributions from the hydrogens and the metal
cations. The electronic structure of the aluminium compounds is determined
mainly by aluminium hydride complexes and their mutual interactions. Despite
considerable differences between the band structures and the band gaps of the
various compounds, their optical responses are qualitatively similar. In most
of the spectra the optical absorption rises sharply above 6 eV and has a strong
peak around 8 eV. The quantitative differences in the optical spectra are
interpreted in terms of the structure and the electronic structure of the
compounds.Comment: 13 pages, 10 figure
A Model for the Development of the Rhizobial and Arbuscular Mycorrhizal Symbioses in Legumes and Its Use to Understand the Roles of Ethylene in the Establishment of these two Symbioses
We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both processes are dissected into many steps, using Pisum sativum L. nodulation mutants as a guideline. For nodulation, we distinguish two main developmental programs, one epidermal and one cortical. Whereas Nod factors alone affect the cortical program, bacteria are required to trigger the epidermal events. We propose that the two programs of the rhizobial symbiosis evolved separately and that, over time, they came to function together. The distinction between these two programs does not exist for arbuscular mycorrhizae development despite events occurring in both root tissues. Mutations that affect both symbioses are restricted to the epidermal program. We propose here sites of action and potential roles for ethylene during the formation of the two symbioses with a specific hypothesis for nodule organogenesis. Assuming the epidermis does not make ethylene, the microsymbionts probably first encounter a regulatory level of ethylene at the epidermis–outermost cortical cell layer interface. Depending on the hormone concentrations there, infection will either progress or be blocked. In the former case, ethylene affects the cortex cytoskeleton, allowing reorganization that facilitates infection; in the latter case, ethylene acts on several enzymes that interfere with infection thread growth, causing it to abort. Throughout this review, the difficulty of generalizing the roles of ethylene is emphasized and numerous examples are given to demonstrate the diversity that exists in plants
Identificação de QTL associados à simbiose entre Bradyrhizobium japonicum, B. elkanii e soja
Uncovering the intrinsic size dependence of hydriding phase transformations in nanocrystals
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