Understanding the ferroelectrocity in magnetic ferroelectric oxides is of both fundamental and technological importance. Here, we identify the nature of the ferroelectric phase transition in the hexagonal manganite, YMnO(3), using a combination of single-crystal X-ray diffraction, thorough structure analysis and first-principles density-functional calculations. The ferroelectric phase is characterized by a buckling of the layered MnO(5) polyhedra, accompanied by displacements of the Y ions, which lead to a net electric polarization. Our calculations show that the mechanism is driven entirely by electrostatic and size effects, rather than the usual changes in chemical bonding associated with ferroelectric phase transitions in perovskite oxides. As a result, the usual indicators of structural instability, such as anomalies in Born effective charges on the active ions, do not hold. In contrast to the chemically stabilized ferroelectrics, this mechanism for ferroelectricity permits the coexistence of magnetism and ferroelectricity, and so suggests an avenue for designing novel magnetic ferroelectrics.
An interatomic model potential for
molecular dynamics is derived
from first-principles and used to study the molecular rotations and
relaxation times in methylammonium lead halide, here considered the
prototypical example of a hybrid crystal with a strong reorientational
dynamics. Within the limits of a simple ionic scheme, the potential
is able to catch the main qualitative features of the material at
zero and finite temperature and opens the way to the development of
classical potentials for hybrid perovskites. In agreement with experiments
and previous theoretical findings, the molecule trajectories exhibit
a transition from a dynamics dominated by high symmetry directions
at low temperature to a fast dynamics at room temperature in which
the molecule can reorient quasi-randomly. By computing the angular
time correlation function we discuss the reorientational time as a
function of temperature in comparison with existing literature, providing
a simple model and a clear attribution of the relaxation times in
terms of their temperature dependence. This work clarifies the temperature
dependence of the relaxation times and the interpretation of the experimental
data in terms of the different mechanisms contributing to the molecule
dynamics.
All density-functional calculations of single-molecule transport to date have used continuous exchange-correlation approximations. The lack of derivative discontinuity in such calculations leads to the erroneous prediction of metallic transport for insulating molecules. A simple and computationally undemanding atomic self-interaction correction (SIC) opens conduction gaps in I-V characteristics that otherwise are predicted metallic, as in the case of the prototype Au/ditholated-benzene/Au junction.
We describe the intrinsic mechanism of 2-dimensional electron confinement at the n-type SrTiO3/LaAlO3 interface as a function of the sheet carrier density n(s) via advanced first-principles calculations. Electrons localize spontaneously in Ti 3d(xy) levels within a thin (≲2 nm) interface-adjacent SrTiO3 region for n(s) lower than a threshold value n(c)∼10(14) cm(-2). For n(s)>n(c) a portion of charge flows into Ti 3d(xz)-d(yz) levels extending farther from the interface. This intrinsic confinement can be attributed to the interface-induced symmetry breaking and localized nature of Ti 3d t(2g) states. The sheet carrier density directly controls the binding energy and the spatial extension of the conductive region. A direct, quantitative relation of these quantities with n(s) is provided.
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