105 research outputs found
First-principles theory of magnetically induced ferroelectricity in TbMnO3
We study the polarization induced via spin-orbit interaction by a magnetic
cycloidal order in orthorhombic TbMnO3 using first-principle methods. The case
of magnetic spiral lying in the b-c plane is analyzed, in which the pure
electronic contribution to the polarization is shown to be small. We focus our
attention on the lattice-mediated contribution, and study it's dependence on
the Coulomb interaction parameter U in the LDA+U method and on the wave-vector
of the spin spiral. The role of the spin-orbit interaction on different sites
is also analyzed.Comment: 4 pages, 2 figures, submitted to EPJ B (MEIPIC6 proceedings
First-principles DFT+GW study of oxygen vacancies in rutile TiO2
We perform first-principles calculations of the quasiparticle defect states,
charge transition levels, and formation energies of oxygen vacancies in rutile
titanium dioxide. The calculations are done within the recently developed
combined DFT+GW formalism, including the necessary electrostatic corrections
for the supercells with charged defects. We find the oxygen vacancy to be a
negative U defect, where U is the defect electron addition energy. For the
values of Fermi level below 2.8 eV (relative to the valence band maximum) we
find the +2 charge state of the vacancy to be the most stable, while above 2.8
eV we find that the neutral charge state is the most stable
Orbital magnetoelectric coupling at finite electric field
We extend the band theory of linear orbital magnetoelectric coupling to treat
crystals under finite electric fields. Previous work established that the
orbital magnetoelectric response of a generic insulator at zero field comprises
three contributions that were denoted as local circulation, itinerant
circulation, and Chern-Simons. We find that the expression for each of them is
modified by the presence of a dc electric field. Remarkably, the sum of the
three correction terms vanishes, so that the total coupling is still given by
the same formula as at zero field. This conclusion is confirmed by numerical
tests on a tight-binding model, for which we calculate the field-induced change
in the linear magnetoelectric coefficient.Comment: 4 pages, 2 figure
Full magnetoelectric response of Cr2O3 from first principles
The linear magnetoelectric response of Cr2O3 at zero temperature is
calculated from first principles by tracking the change in magnetization under
a macroscopic electric field. Both the spin and the orbital contributions to
the induced magnetization are computed, and in each case the response is
decomposed into lattice and electronic parts. We find that the transverse
response is dominated by the spin-lattice and spin-electronic contributions,
whose calculated values are consistent with static and optical magnetoelectric
measurements. In the case of the longitudinal response, orbital contributions
dominate over spin contributions, but the net calculated longitudinal response
remains much smaller than the experimentally measured one at low temperatures.
We also discuss the absolute sign of the magnetoelectric coupling in the two
time-reversed magnetic domains of Cr2O3.Comment: 7 pages, 1 figur
Chern-Simons orbital magnetoelectric coupling in generic insulators
We present a Wannier-based method to calculate the Chern-Simons orbital
magnetoelectric coupling in the framework of first-principles
density-functional theory. In view of recent developments in connection with
strong Z2 topological insulators, we anticipate that the Chern-Simons
contribution to the magnetoelectric coupling could, in special cases, be as
large or larger than the total magnetoelectric coupling in known
magnetoelectrics like Cr2O3. The results of our calculations for the ordinary
magnetoelectrics Cr2O3, BiFeO3 and GdAlO3 confirm that the Chern-Simons
contribution is quite small in these cases. On the other hand, we show that if
the spatial inversion and time-reversal symmetries of the Z2 topological
insulator Bi2Se3 are broken by hand, large induced changes appear in the
Chern-Simons magnetoelectric coupling.Comment: 13 pages, 8 figures, 1 tabl
Dependence of electronic polarization on octahedral rotations in TbMnO3 from first principles
The electronic contribution to the magnetically induced polarization in
orthorhombic TbMnO3 is studied from first principles. We compare the cases in
which the spin cycloid, which induces the electric polarization via the
spin-orbit interaction, is in either the b-c or a-b plane. We find that the
electronic contribution is negligible in the first case, but much larger, and
comparable to the lattice-mediated contribution, in the second case. However,
we how that this behavior is an artifact of the particular pattern of
octahedral rotations characterizing the structurally relaxed Pbnm crystal
structure. To do so, we explore how the electronic contribution varies for a
structural model of rigidly rotated MnO6 octahedra, and demonstrate that it can
vary over a wide range, comparable with the lattice-mediated contribution, for
both b-c and a-b spirals. We introduce a phenomenological model that is capable
of describing this behavior in terms of sums of symmetry-constrained
contributions arising from the displacements of oxygen atoms from the centers
of the Mn-Mn bonds.Comment: 8 pages, 5 figures, 3 table
Natural optical activity and its control by electric field in electrotoroidic systems
We propose the existence, via analytical derivations, novel phenomenologies,
and first-principles-based simulations, of a new class of materials that are
not only spontaneously optically active, but also for which the sense of
rotation can be switched by an electric field applied to them-- via an induced
transition between the dextrorotatory and laevorotatory forms. Such systems
possess electric vortices that are coupled to a spontaneous electrical
polarization. Furthermore, our atomistic simulations provide a deep microscopic
insight into, and understanding of, this class of naturally optically active
materials.Comment: 3 figure
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