2,583 research outputs found
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
, being the inter-layer direction and 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. 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
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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
( 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. 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
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