111 research outputs found
Ab initio phonon structure of h-YMnO3 in low-symmetry ferroelectric phase
We present a systematic first-principles study of the phonon spectrum of
hexagonal YMnO3 in ferroelectric (multiferroic) phase. We investigated in
detail the low-energy phonon modes, their dispersion, symmetry, as well as the
infrared optical properties of the crystal and determined the phonon density of
states
Neutron scattering study of ferroelectric Sn2P2S6 under pressure
Ferroelectric phase transition in the semiconductor Sn2P2S6 single crystal
has been studied by means of neutron scattering in the pressure-temperature
range adjacent to the anticipated tricritical Lifshitz point (p=0.18GPa,
T=296K). The observations reveal a direct ferroelectric-paraelectric phase
transition in the whole investigated pressure range (0.18 - 0.6GPa). These
results are in a clear disagreement with phase diagrams assumed in numerous
earlier works, according to which a hypothetical intermediate incommensurate
phase extends over several or even tens of degrees in the 0.5GPa pressure
range. Temperature dependence of the anisotropic quasielastic diffuse
scattering suggests that polarization fluctuations present above TC are
strongly reduced in the ordered phase. Still, the temperature dependence of the
(200) Bragg reflection intensity at p=0.18GPa can be remarkably well modeled
assuming the order-parameter amplitude growth according to the power law with
logarithmic corrections predicted for a uniaxial ferroelectric transition at
the tricritical Lifshitz point
First-principles prediction of oxygen octahedral rotations in perovskite-structure EuTiO3
We present a systematic first-principles study of the structural and
vibrational properties of perovskite-structure EuTiO3. Our calculated phonon
spectrum of the high-symmetry cubic structural prototype shows strong M- and
R-point instabilities, indicating a tendency to symmetry-lowering structural
deformations composed of rotations and tilts of the oxygen octahedra.
Subsequent explicit study of 14 different octahedral tilt-patterns showed that
the I4/mcm, Imma, and R\bar{3}c structures, all with antiferrodistortive
rotations of the octahedra, have significantly lower total energy than the
prototype Pm\bar{3}m structure. We discuss the dynamical stability of these
structures, and the influence of the antiferrodistortive structural distortions
on the vibrational, optical, and magnetic properties of EuTiO3, in the context
of recent unexplained experimental observations
Interface Engineering to Create a Strong Spin Filter Contact to Silicon
Integrating epitaxial and ferromagnetic Europium Oxide (EuO) directly on
silicon is a perfect route to enrich silicon nanotechnology with spin filter
functionality.
To date, the inherent chemical reactivity between EuO and Si has prevented a
heteroepitaxial integration without significant contaminations of the interface
with Eu silicides and Si oxides.
We present a solution to this long-standing problem by applying two
complementary passivation techniques for the reactive EuO/Si interface:
() an hydrogen-Si passivation and () the
application of oxygen-protective Eu monolayers --- without using any additional
buffer layers.
By careful chemical depth profiling of the oxide-semiconductor interface via
hard x-ray photoemission spectroscopy, we show how to systematically minimize
both Eu silicide and Si oxide formation to the sub-monolayer regime --- and how
to ultimately interface-engineer chemically clean, heteroepitaxial and
ferromagnetic EuO/Si in order to create a strong spin filter contact to
silicon.Comment: 11 pages of scientific paper, 10 high-resolution color figures.
Supplemental information on the thermodynamic problem available (PDF).
High-resolution abstract graphic available (PNG). Original research (2016
Structure similarity and lattice dynamics of InSe and In₄Se₃ crystals
In the work it was shown that the In₄Se₃ crystal is similar by its dynamical
properties to the InSe modulated crystal. The transformation of the D⁴₆h symmetry group of β-InSe crystal to D⁹₂h group of the In₄Se₃ structure
and the D¹²₂h group of In₄Se₃ structure has been investigated. In spite of
structure corrugation of In₄Se₃ layer, it is shown that the region of the weak
bond in this crystal by the dynamical characteristics is similar to the weak
bond region in InSe crystal.У роботі показано, що кристал In₄Se₃ за своїми динамічними властивостями близький до модульованого кристалу InSe. Досліджено перетворення групи симетрії D⁴₆h кристалу β-InSe до групи D⁹₂h структури In₄Se₄ і до групи D¹²₂h In₄Se₃. Показано, що незважаючи на гофрованість структури шару In₄Se₃, область слабкого зв’язку у ньому за
динамічними властивостями є подібною до області слабкого зв’язку
у кристалі InSe
Phonon spectra of Cu₆PS₅Br superionic ferroelastic: experimental and theoretical studies
The anomalous temperature behavior of the Raman scattering bands at superionic phase transition of Cu₆PS₅Br is revealed. Using density functional perturbation theory within local density approximation, ab initio long-wavelength phonons have been obtained for high-symmetry cubic structure. In the calculated phonon spectrum two unstable modes indicate the experimentally observed sequence of ferroelastic and superionic phase transitions.Експериментально виявлено аномальну температурну поведiнку пiкiв раманiвського розсіювання свiтла при суперiонному фазовому переходi в Cu₆PS₅Br. Для високосиметричної кубiчної фази теоретично розраховано першопринципнi частоти довгохвильових фононiв з використанням теорії збурень функцiонала густини у локальному наближеннi. У фононному спектрi спостерiгаються дві нестабiльнi моди, якi вказують на експериментально спостережувану послiдовнiсть сегнетоеластичного i суперiонного фазових переходiв
Magnetodielectric coupling and phonon properties of compressively strained EuTiO3 thin films deposited on LSAT
Compressively strained epitaxial (001) EuTiO3 thin films of tetragonal
symmetry have been deposited on (001) (LaAlO3)_0.29-(SrAl_{1/2}Ta_{1/2}O3)_0.71
(LSAT) substrates by reactive molecular-beam epitaxy. Enhancement of the Neel
temperature by 1 K with 0.9% compressive strain was revealed. The polar phonons
ofthe films have been investigated as a function of temperature and magnetic
field by means of infrared reflectance spectroscopy. All three infrared active
phonons show strongly stiffened frequencies compared to bulk EuTiO3 in
accordance with first principles calculations. The phonon frequencies exhibit
gradual softening on cooling leading to an increase in static permittivity. A
new polar phonon with frequency near the TO1 soft mode was detected below 150
K. The new mode coupled with the TO1 mode was assigned as the optical phonon
from the Brillouin zone edge, which is activated in infrared spectra due to an
antiferrodistortive phase transition and due to simultaneous presence of polar
and/or magnetic nanoclusters. In the antiferromagnetic phase we have observed a
remarkable softening of the lowest-frequency polar phonon under an applied
magnetic field, which qualitatively agrees with first principles calculations.
This demonstrates the strong spin-phonon coupling in EuTiO3, which is
responsible for the pronounced dependence of its static permittivity on
magnetic field in the antiferromagnetic phase.Comment: Submitted to Phys. Rev.
Terahertz and infrared spectroscopic evidence of phonon-paramagnon coupling in hexagonal piezomagnetic YMnO3
Terahertz and far-infrared electric and magnetic responses of hexagonal
piezomagnetic YMnO3 single crystals are investigated. Antiferromagnetic
resonance is observed in the spectra of magnetic permeability mu_a [H(omega)
oriented within the hexagonal plane] below the Neel temperature T_N. This
excitation softens from 41 to 32 cm-1 on heating and finally disappears above
T_N. An additional weak and heavily-damped excitation is seen in the spectra of
complex dielectric permittivity epsilon_c within the same frequency range. This
excitation contributes to the dielectric spectra in both antiferromagnetic and
paramagnetic phases. Its oscillator strength significantly increases on heating
towards room temperature thus providing evidence of piezomagnetic or
higher-order couplings to polar phonons. Other heavily-damped dielectric
excitations are detected near 100 cm-1 in the paramagnetic phase in both
epsilon_c and epsilon_a spectra and they exhibit similar temperature behavior.
These excitations appearing in the frequency range of magnon branches well
below polar phonons could remind electromagnons; however, their temperature
dependence is quite different. We have used density functional theory for
calculating phonon dispersion branches in the whole Brillouin zone. A detailed
analysis of these results and of previously published magnon dispersion
branches brought us to the conclusion that the observed absorption bands stem
from phonon-phonon and phonon- paramagnon differential absorption processes.
The latter is enabled by a strong short-range in-plane spin correlations in the
paramagnetic phase.Comment: subm. to PR
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