7,087 research outputs found
Survival probability of surface excitations in a 2d lattice: non-Markovian effects and Survival Collapse
The evolution of a surface excitation in a two dimentional model is analyzed.
I) It starts quadratically up to a spreading time t_{S}. II) It follows an
exponential behavior governed by a self-consistent Fermi Golden Rule. III) At
longer times, the exponential is overrun by an inverse power law describing
return processes governed by quantum diffusion. At this last transition time
t_{R} a survival collapse becomes possible, bringing the survival probability
down by several orders of magnitude. We identify this strongly destructive
interference as an antiresonance in the time domain.Comment: 4 pages, 3 figures. Braz. Journ. of Phys., in press. Braz. Journ. of
Phys., in press. Braz. Journ. of Phys., in press. Braz. Journ. of Phys., in
press. Braz. Journ. of Phys., in press. Braz. Journ. of Phys., in press.
Braz. Journ. of Phys., in pres
Effective one-body dynamics in multiple-quantum NMR experiments
A suitable NMR experiment in a one-dimensional dipolar coupled spin system
allows one to reduce the natural many-body dynamics into effective one-body
dynamics. We verify this in a polycrystalline sample of hydroxyapatite (HAp) by
monitoring the excitation of NMR many-body superposition states: the
multiple-quantum coherences. The observed effective one-dimensionality of HAp
relies on the quasi 1d structure of the dipolar coupled network that, as we
show here, is dynamically enhanced by the quantum Zeno effect. Decoherence is
also probed through a Loschmidt echo experiment, where the time reversal is
implemented on the double-quantum Hamiltonian, I_{i,+}I_{j,+} + I_{i,-}I_{j,-}.
We contrast the decoherence of adamantane, a standard 3d system, with that of
HAp. While the first shows an abrupt Fermi-type decay, HAp presents a smooth
exponential law.Comment: 8 pages, 6 figure
LAS: a software platform to support oncological data management
The rapid technological evolution in the biomedical and molecular oncology fields is providing research laboratories with huge amounts of complex and heterogeneous data. Automated systems are needed to manage and analyze this knowledge, allowing the discovery of new information related to tumors and the improvement of medical treatments. This paper presents the Laboratory Assistant Suite (LAS), a software platform with a modular architecture designed to assist researchers throughout diverse laboratory activities. The LAS supports the management and the integration of heterogeneous biomedical data, and provides graphical tools to build complex analyses on integrated data. Furthermore, the LAS interfaces are designed to ease data collection and management even in hostile environments (e.g., in sterile conditions), so as to improve data qualit
The nucleation of monomeric parallel beta-sheet-like structures and their self-assembly in aqueous solution
The aromatic diacid residue 4,6-dibenzofuranbispropionic acid (1) was designed to nucleate a parallel beta-sheet-like structure in small peptides in aqueous solution via a hydrogen-bonded hydrophobic cluster. Even though a 14-membered ring hydrogen bond necessary for parallel beta-sheet formation is favored in simple amides composed of 1, this hydrogen bonding interaction does not appear to be sufficient to nucleate parallel beta-sheet formation in the absence of hydrophobic clustering between the dibenzofuran portion of 1 and the hydrophobic side chains of the flanking alpha-amino acids. The subsequence --hydrophobic residue-1-hydrophobic residue-- is required for folding in the context of a nucleated two-stranded parallel beta-sheet structure. In all cases where the peptidomimetics can fold into two diastereomeric parallel beta-sheet structures having different hydrogen bonding networks, these conformations appear to exchange rapidly. The majority of the parallel beta-sheet structures evaluated herein undergo linked intramolecular folding and self-assembly, affording a fibrillar beta-sheet quaternary structure. To unlink folding and assembly, asymmetric parallel beta-sheet structures incorporating N-methylated alpha-amino acid residues have been synthesized using a new solid phase approach. Residue 1 facilitates the folding of several peptides described within affording a monomeric parallel beta-sheet-like structure in aqueous solution, as ascertained by a variety of spectroscopic and biophysical methods, increasing our understanding of parallel beta-sheet structure
Characterization studies of 1-(4-cyano-2-oxo-1,2-dihydro-1-pyridyl)-3-(4-cyano-1,2-dihydro-1-pyridyl)propane formed from the reaction of hydroxide Ion with 1,3-Bis-(4-cyano pyridinium)propane
The aqueous alkaline reaction of 1,3-bis(4-cyanopyridinium)propane dibromide, a reactant constituted of two pyridinium rings linked by a three-methylene bridge, generates a novel compound,1 -(4-cyano-2-oxo-1,2-dihydro-1-pyridyl)-3-(4-cyano-1,2-dihydro-1-pyridyl)propane. The reaction pathway is attributed to the proximity of the OH- ion inserted between two pyridinium moieties, which occurs only in bis(pyridinium) derivatives connected by short methylene spacers, where charge-conformational effects are important.A reação em meio aquoso alcalino do dibrometo de 1,3-bis(4-cianopiridinium)propano, um composto constituído por dois anéis piridínicos conectados por uma ponte metilênica de três carbonos, gerou um novo composto, o 1-(4-ciano-2-oxo-1,2-diidro-1-piridil)-3-(4-ciano-1,2-diidro-1-piridil)propano. O resultado da reação é atribuído à proximidade do íon OH-, encapsulado entre os dois anéis piridínicos, fato este observado apenas em derivados bis-piridínicos conectados por pontes metilênicas de curta extensão, onde imperam efeitos de carga aliados à conformação.CNPqFAPESPFundação Araucári
Study of Thermal Properties of Graphene-Based Structures Using the Force Constant Method
The thermal properties of graphene-based materials are theoretically
investigated. The fourth-nearest neighbor force constant method for phonon
properties is used in conjunction with both the Landauer ballistic and the
non-equilibrium Green's function techniques for transport. Ballistic phonon
transport is investigated for different structures including graphene, graphene
antidot lattices, and graphene nanoribbons. We demonstrate that this particular
methodology is suitable for robust and efficient investigation of phonon
transport in graphene-based devices. This methodology is especially useful for
investigations of thermoelectric and heat transport applications.Comment: 23 pages, 9 figures, 1 tabl
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