Negative Differential Resistivity and Positive Temperature Coefficient of Resistivity effect in the diffusion limited current of ferroelectric thin film capacitors

We present a model for the leakage current in ferroelectric thin- film capacitors which explains two of the observed phenomena that have escaped satisfactory explanation, i.e. the occurrence of either a plateau or negative differential resistivity at low voltages, and the observation of a Positive Temperature Coefficient of Resistivity (PTCR) effect in certain samples in the high-voltage regime. The leakage current is modelled by considering a diffusion-limited current process, which in the high-voltage regime recovers the diffusion-limited Schottky relationship of Simmons already shown to be applicable in these systems

Topology of the polarization field in ferroelectric nanowires from first principles

The behaviour of the cross-sectional polarization field is explored for thin nanowires of barium titanate from first-principles calculations. Topological defects of different winding numbers have been obtained, beyond the known textures in ferroelectric nanostructures. They result from the inward accommodation of the polarization patterns imposed at the surface of the wire by surface and edge effects. Close to a topological defect the polarization field orients out of the basal plane in some cases, maintaining a close to constant magnitude, whereas it virtually vanishes in other cases.Comment: 4 pages, 3 figure

Fully Constrained Majorana Neutrino Mass Matrices Using Σ(72×3)\Sigma(72\times 3)

In 2002, two neutrino mixing ansatze having trimaximally-mixed middle ($\nu_2$) columns, namely tri-chi-maximal mixing ($\text{T}\chi\text{M}$) and tri-phi-maximal mixing ($\text{T}\phi\text{M}$), were proposed. In 2012, it was shown that $\text{T}\chi\text{M}$ with $\chi=\pm \frac{\pi}{16}$ as well as $\text{T}\phi\text{M}$ with $\phi = \pm \frac{\pi}{16}$ leads to the solution, $\sin^2 \theta_{13} = \frac{2}{3} \sin^2 \frac{\pi}{16}$, consistent with the latest measurements of the reactor mixing angle, $\theta_{13}$. To obtain $\text{T}\chi\text{M}_{(\chi=\pm \frac{\pi}{16})}$ and $\text{T}\phi\text{M}_{(\phi=\pm \frac{\pi}{16})}$, the type~I see-saw framework with fully constrained Majorana neutrino mass matrices was utilised. These mass matrices also resulted in the neutrino mass ratios, $m_1:m_2:m_3=\frac{\left(2+\sqrt{2}\right)}{1+\sqrt{2(2+\sqrt{2})}}:1:\frac{\left(2+\sqrt{2}\right)}{-1+\sqrt{2(2+\sqrt{2})}}$. In this paper we construct a flavour model based on the discrete group $\Sigma(72\times 3)$ and obtain the aforementioned results. A Majorana neutrino mass matrix (a symmetric $3\times 3$ matrix with 6 complex degrees of freedom) is conveniently mapped into a flavon field transforming as the complex 6 dimensional representation of $\Sigma(72\times 3)$. Specific vacuum alignments of the flavons are used to arrive at the desired mass matrices.Comment: 20 pages, 1 figure. arXiv admin note: substantial text overlap with arXiv:1402.085

Multiferroic behavior of Aurivillius Bi4Mn3O12 from first-principles

The multiferroic behavior of the hypothetical Aurivillius compound Bi4Mn3O12 has been explored on the basis of density functional calculations. We find that the tetragonal paraelectric phase of this material is ferromagnetic, showing ferroelectric and antiferrodistortive instabilities similar to the ones observed in its ferroelectric parent compound Bi4Ti3O12 . Our results indicate, however, that the presence of Mn+4 ions at the B-sites shrinks the cell volume and consequently the unstable polar mode, associated with the ferroelectric polarization, is overcame by an antiferrodistortive distortion. In this way, Bi4Mn3O12 exhibits incipient ferroelectricity at its equilibrium volume. We show that the ferroelectric state can be favored by strain or partial substitution of Mn with Ti.Comment: 6 pages, 5 figure

Deviations from Tribimaximal Neutrino Mixing using a Model with Δ(27)\Delta(27) Symmetry

We present a model of neutrino mixing based on the flavour group $\Delta(27)$ in order to account for the observation of a non-zero reactor mixing angle ($\theta_{13}$). The model provides a common flavour structure for the charged-lepton and the neutrino sectors, giving their mass matrices a `circulant-plus-diagonal' form. Mass matrices of this form readily lead to mixing patterns with realistic deviations from tribimaximal mixing, including non-zero $\theta_{13}$. With the parameters constrained by existing measurements, our model predicts an inverted neutrino mass hierarchy. We obtain two distinct sets of solutions in which the atmospheric mixing angle lies in the first and the second octants. The first (second) octant solution predicts the lightest neutrino mass, $m_3 \sim 29~\text{meV}$ ($m_3 \sim 65~\text{meV}$) and the $CP$ phase, $\delta_{CP} \sim -\frac{\pi}{4}$ ($\delta_{CP} \sim \frac{\pi}{2}$), offering the possibility of large observable $CP$ violating effects in future experiments.Comment: 9 pages, 3 figure