Mechanisms and origin of multiferroicity

Motivated by the potential applications of their intrinsic cross-coupling properties, the interest in multiferroic materials has constantly increased recently, leading to significant experimental and theoretical advancements. From the theoretical point of view, recent progresses have allowed to identify different mechanisms responsible for the appearence of ferroelectric polarization coexisting with -- and coupled to -- magnetic properties. This chapter aims at reviewing the fundamental mechanisms devised so far, mainly in transition-metal oxides, which lie at the origin of multiferroicity

Superconductivity in the doped bilayer Hubbard model

We study by the Gutzwiller approximation the melting of the valence bond crystal phase of a bilayer Hubbard model at sufficiently large inter-layer hopping. We find that a superconducting domain, with order parameter $d_{z^2-r^2}$, $z$ being the inter-layer direction and $r$ the intra-layer one, is stabilized variationally close to the half-filled non-magnetic Mott insulator. Superconductivity exists at half-filling just at the border of the Mott transition and extends away from half-filling into a whole region till a critical doping, beyond which it gives way to a normal metal phase. This result suggests that superconductivity should be unavoidably met by liquefying a valence bond crystal, at least when each layer is an infinite coordination lattice and the Gutzwiller approximation becomes exact. Remarkably, this same behavior is well established in the other extreme of two-leg Hubbard ladders, showing it might be of quite general validity.Comment: 9 pages, 5 figure

Spontaneous skyrmionic lattice from anisotropic symmetric exchange in a Ni-halide monolayer

Topological spin structures, such as magnetic skyrmions, hold great promises for data storage applications, thanks to their inherent stability. In most cases, skyrmions are stabilized by magnetic fields in non-centrosymmetric systems displaying the chiral Dzyaloshinskii-Moriya exchange interaction, while spontaneous skyrmion lattices have been reported in centrosymmetric itinerant magnets with long-range interactions. Here, a spontaneous anti-biskyrmion lattice with unique topology and chirality is predicted in the monolayer of a semiconducting and centrosymmetric metal halide, NiI$_2$. Our first-principles and Monte Carlo simulations reveal that the anisotropies of the short-range symmetric exchange, when combined with magnetic frustration, can lead to an emergent chiral interaction that is responsible for the predicted topological spin structures. The proposed mechanism finds a prototypical manifestation in two-dimensional magnets, thus broadening the class of materials that can host spontaneous skyrmionic states.Comment: submitte

Ferroelectricity due to orbital ordering in E-type undoped rare-earth manganites

Aiming at understanding the origin of the electronic contribution to ferroelectric polarization in undoped manganites, we evaluate the Berry phase of orbital-polarizable Bloch electrons as an orbital ordering (OO) establishes in the background of an antiferromagnetic E-type configuration. The onset of OO is tuned by the Jahn-Teller (JT) interaction in a tight-binding model for interacting electrons moving along zigzag chains. A finite polarization is found as soon as the JT coupling is strong enough to induce OO, supporting the large electronic contribution predicted from first principles.Comment: 4 pages, 2 figures, figure and text substantially improved. Title change

Human Capital and Ferility Decisions: A Comparative Microeconometric Analysis in Europe

In this paper we have investigated the relationship between human capital proxied by the schooling level and fertility decisions for some EU countries. For this purpose we have resorted to a dynamic random effect probit model controlling for unobserved heterogeneity and initial problem, by using eight waves of 2001 European Community Household Panel (ECHP) data. The main objective has been to compare the effects of education on the probability of having a child for some EU countries. Since the probability of having a child is affected by the same probability related to the previous year, we have implemented a dynamic model instead of a static one. From the empirical results, we have found out a significant relationship between the schooling level and fertility decisions.

Giant Effective charges and Piezoelectricity in Gapped Graphene

Since the first realization of reversible charge doping in graphene via field-effect devices, it has become evident how the induction a gap could further enhance its potential for technological applications. Here we show that the gap opening due to a sublattice symmetry breaking has also a profound impact on the polar response of graphene. By combining ab-initio calculations and analytical modelling we show that for realistic band-gap values ($\Delta\lesssim 0.5$ eV) the piezoelectric coefficient and the Born effective charge of graphene attain a giant value, independent on the gap. In particular the piezoelectric coefficient per layer of gapped mono- and bilayer graphene is three times larger than that of a large-gap full polar insulator as hexagonal Boron Nitride (h-BN) monolayer, and 30\% larger than that of a polar semiconductor as MoS$_2$. This surprising result indicates that piezoelectric acoustic-phonons scattering can be relevant to model charge transport and charge-carrier relaxation in gated bilayer graphene. The independence of the piezoelectric coefficient and of the Born effective charge on the gap value follows from the connection between the polar response and the valley Chern number of gapped Dirac electrons, made possible by the effective gauge-field description of the electron-lattice/strain coupling in these systems. In the small gap limit, where the adiabatic ab-initio approximation fails, we implement analytically the calculation of the dynamical effective charge, and we establish a universal relation between the complex effective charge and the so-called Fano profile of the phonon optical peak. Our results provide a general theoretical framework to understand and compute the polar response in narrow-gap semiconductors, but may also be relevant for the contribution of piezoelectric scattering to the transport properties in Dirac-like systems