5,963 research outputs found
Structural phase transitions in Ruddlesden-Popper phases of strontium titanate: {\em ab initio} and inhomogeneous Ginzburg-Landau approaches
We present the first systematic {\em ab initio} study of anti-ferrodistortive
(AFD) order in Ruddlesden-Popper (RP) phases of strontium titanate,
SrTiO, as a function of both compressive epitaxial strain
and phase number . We find all RP phases to exhibit AFD order under a
significant range of strains, recovering the bulk AFD order as . A
Ginzburg-Landau Hamiltonian generalized to include inter-octahedral
interactions reproduces our {\em ab initio} results well, opening a pathway to
understanding other nanostructured perovskite systems
Fatigue crack propagation in a quasi one-dimensional elasto-plastic model
Fatigue crack advance induced by the application of cyclic quasistatic loads
is investigated both numerically and analytically using a lattice spring model.
The system has a quasi-one-dimensional geometry, and consists in two
symmetrical chains that are pulled apart, thus breaking springs which connect
them, and producing the advance of a crack. Quasistatic crack advance occurs as
a consequence of the plasticity included in the springs which form the chains,
and that implies a history dependent stress-strain curve for each spring. The
continuous limit of the model allows a detailed analytical treatment that gives
physical insight of the propagation mechanism. This simple model captures key
features that cause well known phenomenology in fatigue crack propagation, in
particular a Paris-like law of crack advance under cyclic loading, and the
overload retardation effect.Comment: To be published in the International Journal of Solids and Structure
Dynamical transitions in a pollination--herbivory interaction
Plant-pollinator associations are often seen as purely mutualistic, while in
reality they can be more complex. Indeed they may also display a diverse array
of antagonistic interactions, such as competition and victim--exploiter
interactions. In some cases mutualistic and antagonistic interactions are
carried-out by the same species but at different life-stages. As a consequence,
population structure affects the balance of inter-specific associations, a
topic that is receiving increased attention. In this paper, we developed a
model that captures the basic features of the interaction between a flowering
plant and an insect with a larval stage that feeds on the plant's vegetative
tissues (e.g. leaves) and an adult pollinator stage. Our model is able to
display a rich set of dynamics, the most remarkable of which involves
victim--exploiter oscillations that allow plants to attain abundances above
their carrying capacities, and the periodic alternation between states
dominated by mutualism or antagonism. Our study indicates that changes in the
insect's life cycle can modify the balance between mutualism and antagonism,
causing important qualitative changes in the interaction dynamics. These
changes in the life cycle could be caused by a variety of external drivers,
such as temperature, plant nutrients, pesticides and changes in the diet of
adult pollinators.
Abstract Keywords: mutualism, pollination, herbivory, insects,
stage-structure, oscillationsComment: 20 pages, 7 main figures, 2 appendix figure
Silvopastoral agroforestry for rural environment sustainability and valorization of the region of Guarda and Serra da Estrela, Portugal
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