Multiferroic BiFeO3-BiMnO3 Nanocheckerboard From First Principles

We present a first principles study of an unusual heterostructure, an atomic-scale checkerboard of BiFeO3-BiMnO3, and compare its properties to the two bulk constituent materials, BiFeO3 and BiMnO3. The "nanocheckerboard" is found to have a multiferroic ground state with the desired properties of each constituent: polar and ferrimagnetic due to BiFeO3 and BiMnO3, respectively. The effect of B-site cation ordering on magnetic ordering in the BiFeO3-BiMnO3 system is studied. The checkerboard geometry is seen to give rise to a a novel magnetostructural effect that is neither present in the bulk constituent materials, nor in the layered BiFeO3-BiMnO3 superlattice.Comment: 15 pages, 14 figure

Chemical control of polar behavior in bicomponent short-period superlattices

Using first-principles density functional calculations, we study the interplay of ferroelectricity and polar discontinuities in a range of 1-1 oxide superlattices, built out of ferroelectric and paraelectric components. Studies have been carried out for a varied choice of chemical composition of the components. We find that, when polar interfaces are present, the polar discontinuities induce off- centric movements in the ferroelectric layers, even though the ferroelectric is only one unit cell thick. The distortions yield non-switchable polarizations, with magnitudes comparable to those of the corresponding bulk ferroelectrics. In contrast, in superlattices with no polar discontinuity at the interfaces, the off-centric movements in the ferroelectric layer are usually suppressed. The details of the behavior and functional properties are, however, found to be sensitive to epitaxial strain, rotational instabilities and second-order Jahn-Teller activity, and are therefore strongly in uenced by the chemical composition of the paraelectric layer.Comment: 7 pages, 2 figure

Ground-State Decay Rate for the Zener Breakdown in Band and Mott Insulators

Non-linear transport of electrons in strong electric fields, as typified by dielectric breakdown, is re-formulated in terms of the ground-state decay rate originally studied by Schwinger in non-linear QED. We discuss the effect of electron interaction on Zener tunneling by comparing the dielectric breakdown of the band insulator and the Mott insulator, where the latter is studied by the time-dependent density-matrix renormalization group (DMRG). The relation with the Berry's phase theory of polarization is also established.Comment: 5 pages 2 figures, revised text, version to appear in Phys. Rev. Let

POLITICAL PARTIES AS COMBINERS: TRANSITIONS IN TUNISIA AND EGYPT BETWEEN DEMOCRATIZATION AND AUTHORITARIAN RESILIENCE

In marking an historical change in large parts of the Arab region, the Arab uprisings have also triggered a process of soul searching in the scholarship of Arab politics. The events unfolding in the region since 2011 prove indeed that the two paradigms dividing the academic community, that of authoritarian resilience (also known as the post-democratization paradigm) and that of democratization, are alone insufficient in accounting for the political developments of the area. In this regard, extant contributions explaining the different paths undertook by Tunisia and Egypt between 2011 and 2013 enlighten the point in case. On the one hand, democratization studies pinpoint to the different choices taken by transitional actors but remain only partial in that don\u2019t go further than the observed behavior. On the other hand, post-democratization studies focus on the structures and the mechanisms underpinning the previous authoritarian regimes but fall short in providing the causal link between them and the observed outcome. In line with the transitology approach, this research moves from the acknowledgment that the failure or the success of the transitions in Egypt and Tunisia is to be attributed to political parties\u2019 choice about whether to cooperate or not with the other ones within the phase of installation. Yet, unlike any other kind of transitional actor, parties\u2019 strategy profiles result from the interplay among: i) extant social divisions; ii) power resources and iii) ideological polarization. By breaking down the concept of agency in this way, the different structures of competition underneath the previous authoritarian rule in Tunisia and Egypt account for much of the variation across the two countries. First, they are responsible for the uneven distribution of seats\u2019 share inside the transitional national assemblies encouraging stronger parties to defect from cooperation in order to institutionalize their power advantage. Second, they are responsible for the different degrees of ideological polarization which is deemed to reduce the common ground available for the constitutional bargaining. While offering new insights accounting for the political developments in Tunisia and Egypt following the ousting of the previous dictators, this work combines the two contending paradigms into a unified reading fulfilling their respective shortcomings. On the one hand, the concept of agency so formulated allows democratization studies to go deeper than \u2018what ought to be\u2019. On the other hand, it provides post-democratization studies a causal link, i.e. parties\u2019 agency, connecting previous regimes\u2019 structures with the outcome of transition

Nanoparticle-doped electrospun fiber random lasers with spatially extended light modes

Complex assemblies of light-emitting polymer nanofibers with molecular materials exhibiting optical gain can lead to important advance to amorphous photonics and to random laser science and devices. In disordered mats of nanofibers, multiple scattering and waveguiding might interplay to determine localization or spreading of optical modes as well as correlation effects. Here we study electrospun fibers embedding a lasing fluorene-carbazole-fluorene molecule and doped with titania nanoparticles, which exhibit random lasing with sub-nm spectral width and threshold of about 9 mJ cm^-2 for the absorbed excitation fluence. We focus on the spatial and spectral behavior of optical modes in the disordered and non-woven networks, finding evidence for the presence of modes with very large spatial extent, up to the 100 micrometer-scale. These findings suggest emission coupling into integrated nanofiber transmission channels as effective mechanism for enhancing spectral selectivity in random lasers and correlations of light modes in the complex and disordered material.Comment: 22 pages, 6 figure

A classification model for product-service offerings

Organisations have been approaching servitisation in an unstructured fashion. This is partially because there is insufficient understanding of the different types of Product-Service offerings. Therefore, a more detailed understanding of Product-Service types might advance the collective knowledge and assist organisations that are considering a servitisation strategy. Current models discuss specific aspects on the basis of few (or sometimes single) dimensions. In this paper, we develop a comprehensive model for classifying traditional and green Product-Service offerings, thus combining business and green offerings in a single model. We describe the model building process and its practical application in a case study. The model reveals the various traditional and green options available to companies and identifies how to compete between services; it allows servitisation positions to be identified such that a company may track its journey over time. Finally it fosters the introduction of innovative Product-Service Systems as promising business models to address environmental and social challenges.EPSRC, EPSRC supported the Cranfield IMRCThis is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.jclepro.2013.11.03

Grapevine cvs Primitivo, Zinfandel and Crljenak kastelanski: Molecular analysis by AFLP

Research Not

Diverse regimes of mode intensity correlation in nanofiber random lasers through nanoparticle doping

Random lasers are based on disordered materials with optical gain. These devices can exhibit either intensity or resonant feedback, relying on diffusive or interference behaviour of light, respectively, which leads to either coupling or independent operation of lasing modes. We study for the first time these regimes in complex, solid-state nanostructured materials. The number of lasing modes and their intensity correlation features are found to be tailorable in random lasers made of light-emitting, electrospun polymer fibers upon nanoparticle doping. By material engineering, directional waveguiding along the length of fibers is found to be relevant to enhance mode correlation in both intensity feedback and resonant feedback random lasing. The here reported findings can be used to establish new design rules for tuning the emission of nano-lasers and correlation properties by means of the compositional and morphological properties of complex nanostructured materials.Comment: 30 pages, 10 figure

Analytic and geometric properties of photoinduced effects in noncentrosymmetric crystals: photovoltaic current and optical rectification

An original dispersion relation between the stationary coherent nonlinear optical responses by current and polarisation is obtained. The dispersion relation provides a new complimentary tool that can be employed to study light-induced charge transport models and facilitate experimental data analysis. It is shown that the origin of the coherent current and the dc-polarisation induced in a noncentrosymmetric crystal under illumination is related to the theory of the Berry phase and can be represented in terms of the renormalised geometric potentials. This renormalisation originates from the extra phase difference acquired by a carrier in the light field on the quantum transition between the electronic bands. The gauge invariance of the corresponding expressions for the current and the polarisation is demonstrated.Comment: 7 page