Strain-gradient-induced switching of nanoscale domains in free-standing ultrathin films

We report first-principle atomistic simulations on the effect of local strain gradients on the nanoscale domain morphology of free-standing PbTiO$_3$ ultrathin films. First, the ferroelectric properties of free films at the atomic level are reviewed. For the explored thicknesses (10 to 23 unit cells), we find flux-closure domain structures whose morphology is thickness dependent. A critical value of 20 unit cells is observed: thinner films show structures with 90$^\circ$ domain loops, whereas thicker ones develop, in addition, 180$^\circ$ domain walls, giving rise to structures of the Landau-Lifshitz type. When a local and compressive strain gradient at the top surface is imposed, the gradient is able to switch the polarization of the downward domains, but not to the opposite ones. The evolution of the domain pattern as a function of the strain gradient strength consequently depends on the film thickness. Our simulations indicate that in thinner films, first the 90$^\circ$ domain loops migrate towards the strain-gradient region, and then the polarization in that zone is gradually switched. In thicker films, instead, the switching in the strain-gradient region is progressive, not involving domain-wall motion, which is attributed to less mobile 180$^\circ$ domain walls. The ferroelectric switching is understood based on the knowledge of the local atomic properties, and the results confirm that mechanical flexoelectricity provides a means to control the nanodomain pattern in ferroelectric systems.Comment: 9 pages, 6 figure

«Inversion» as focalization and related questions

The main empirical area studied in this article concerns so called subject «inversion» structures. The proposal is put forth that the postverbal subject is licensed in the specifier of a low Focus projection, internal to the functional clausal architecture. Focus is considered a morphosyntactic feature giving rise to its own projection and constituting a regular checking domain. Consequences of the hypothesis are investigated for other structures assumed to involve the same process of clause internal focalization such as: structures containing «emphatic» pronouns, structures undergoing complement reordering, unaccusative structures. Some speculative remarks are also made on the so called process of «marginalization» which is taken to constitute a case of clause internal «topicalization»; the process is also assumed to be involved in interrogative structures necessarily requiring a postverbal subject which displays behaviors different from the focalized postverbal subject of declarative sentences.La principal àrea empírica estudiada en aquest article té a veure amb les anomenades estructures d'«inversió» del subjecte. Es defensa la proposta que el subjecte postverbal es legitima a l'especificador d'una projecció de Focus incrustada, interna a l'estructura oracional funcional. El Focus es considera un tret morfosintàctic que dóna lloc a la seva pròpia projecció i que constitueix un domini de comprovació regular. S'investiguen les conseqüències de la hipòtesi per a altres estructures que es considera que impliquen el mateix procés de focalització interna a l'oració, com ara: estructures que contenen pronoms «emfàtics», estructures que pateixen reordenació dels complements, estructures inacusatives. També es fan algunes remarques especulatives sobre l'anomenat procés de «marginalització» que es considera un cas de «topicalització» interna a l'oració; s'assumeix que el procés també intervé en les estructures interrogatives que requereixen necessàriament un subjecte postverbal que presenta comportaments diferents del subjecte postverbal focalitzat de les oracions declaratives

Organocatalytic Enantioselective Vinylogous Aldol-Lactonization Cascade Reaction of 3-Alkylidene Oxindoles to Trifluoromethyl Ketones

In this work, an highly enantioselective vinylogous aldol-lactonization cascade reaction of 3-alkylidene oxindole to α,β-unsaturated trifluoromethyl ketone, promoted by bifunctional organocatalysts, is presented. The reaction proceed through 1,2-addition followed by cascade lactonization to afford an unsaturated lactone bearing a chiral trifluoromethylated tetrasubstituted carbon stereocenter with high enantioselectivity and moderate yield. Nevertheless, also the two E/Z isomers of the correspondent 1,2-addition product are obtained

The Emerging Role of the Peasant Economy at the End of the Industrial Age: Insights from Albania

AbstractAccording to the Intergovernmental Panel on Climate Change (2014), by 2050 the world society will need to reduce global greenhouse gas emissions by 40-70 percent compared to 2010, and reach the end goal of net zero emissions before 2100. At the same time, the global food system is now responsible for up to one-third of all human-caused greenhouse gas emissions (Gilbert, 2012). This means that, given the present conditions in terms of climate, demography and technology, the world society should be capable of progressing, not regressing, toward a sustainable rural economy before 2050, relying on a per capita availability of arable land, not matching the current world population distribution, which is around 0.20 hectares at present. On this premise, the present article claims that peasant agriculture oriented to family livelihood will be a key sector in the next wave of economic development. Therefore, in order to support this statement, the Albanian agriculture case study will be considered. This is because the case of Albania is a distinctive case in Europe of peasant agriculture, it being structurally oriented to family auto-consumption and local food markets. Referring to a rural economy based on peasant agriculture as a modern one seems to be a paradox. Nevertheless, climate change is pushing the ontological shift towards an agro-ecological paradigm in which an ecologically driven conception of value addressing societal reproduction rather than capital accumulation is emerging (McMichael, 2012). Thus, the present article aims to describe the role of peasant agriculture in economic development under the constraint of climate change, applying this topic to the empirical case of Albania

Asymptotic behavior of quantum invariants

In this thesis we address the problem of the rate of growth of quantum invariants, specifically the Turaev-Viro invariants of compact manifolds and the related Yokota invariants for embedded graphs. We prove the recent volume conjecture proposed by Chen and Yang in two interesting families of hyperbolic manifolds. Furthermore we propose a similar conjecture for the growth of a certain quantum invariant of planar graphs, and prove it in a large family of examples. This new conjecture naturally leads to the problem of finding the supremum of the volume function among all proper hyperbolic polyhedra with a fixed 1-skeleton; we prove that the supremum is always achieved at the rectification of the 1-skeleton