1,801 research outputs found
Breaking Symmetry with Light: Ultra-Fast Ferroelectricity and Magnetism from Three-Phonon Coupling
A theory describing how ferroic properties can emerge transiently in the
ultra-fast regime by breaking symmetry with light through three-phonon coupling
is presented. Particular emphasis is placed on the special case when two
exactly degenerate mid-infra-red or THz phonons are resonantly pumped, since
this situation can give rise to an exactly rectified ferroic response with
damping envelopes of ~ 1 ps or less. Light-induced ferroelectricity and
ferromagnetism are discussed in this context, and a number of candidate
materials that could display these phenomena are proposed. The same analysis is
also applied to the interpretation of previous femto-magnetism experiments,
performed in different frequency ranges (visible and near-infrared), but
sharing similar symmetry characteristics.Comment: 10 page
Ab initio calculation of spin fluctuation spectra using time dependent density functional perturbation theory, planewaves, and pseudopotentials
We present an implementation of time-dependent density functional
perturbation theory for spin fluctuations, based on planewaves and
pseudopotentials. We compute the dynamic spin susceptibility self-consistently
by solving the time-dependent Sternheimer equation, within the adiabatic local
density approximation to the exchange and correlation kernel. We demonstrate
our implementation by calculating the spin susceptibility of representative
elemental transition metals, namely bcc Fe, fcc Ni and bcc Cr. The calculated
magnon dispersion relations of Fe and Ni are in agreement with previous work.
The calculated spin susceptibility of Cr exhibits a soft-paramagnon
instability, indicating the tendency of the Cr spins to condense in a
incommensurate spin density wave phase, in agreement with experiment
Orbital ordering promotes weakly-interacting S=1/2 dimers in the triangular lattice compound Sr3Cr2O8
The weakly interacting S=1/2 dimers system Sr3Cr2O8 has been investigated by
powder neutron diffraction and inelastic neutron scattering. Our data reveal a
structural phase transition below room temperature corresponding to an
antiferro-orbital ordering with nearly 90 degrees arrangement of the occupied
3z^2-r^2 d-orbital. This configuration leads to a drastic reduction of the
inter-dimer exchange energies with respect to the high temperature
orbital-disorder state, as shown by a spin-dimer analysis of the
super-superexchange interactions performed using the Extended Huckel Tight
Binding method. Inelastic neutron scattering reveals the presence of a quasi
non-dispersive magnetic excitation at 5.4 meV, in agreement with the picture of
weakly-interacting dimers
First-principles study of multiferroic RbFe(MoO)
We have investigated the magnetic structure and ferroelectricity in
RbFe(MoO) via first-principles calculations. Phenomenological analyses
have shown that ferroelectricity may arise due to both the triangular chirality
of the magnetic structure, and through coupling between the magnetic helicity
and the ferroaxial structural distortion. Indeed, it was recently proposed that
the structural distortion plays a key role in stabilising the chiral magnetic
structure itself. We have determined the relative contribution of the two
mechanisms via \emph{ab-initio} calculations. Whilst the structural axiality
does induce the magnetic helix by modulating the symmetric exchange
interactions, the electric polarization is largely due to the in-plane spin
triangular chirality, with both electronic and ionic contributions being of
relativistic origin. At the microscopic level, we interpret the polarization as
a secondary steric consequence of the inverse Dzyaloshinskii-Moriya mechanism
and accordingly explain why the ferroaxial component of the electric
polarization must be small
Effect of Sr substitution on superconductivity in Hg2(Ba1-ySry)2YCu2O8-d (part2): bond valence sum approach of the hole distribution
The effects of Sr substitution on superconductivity, and more particulary the
changes induced in the hole doping mechanism, were investigated in
Hg2(Ba1-ySry)2YCu2O8-d by a "bond valence sum" analysis with Sr content from y
= 0.0 to y = 1.0. A comparison with CuBa2YCu2O7-d and Cu2Ba2YCu2O8 systems
suggests a possible explanation of the Tc enhancement from 0 K for y = 0.0 to
42 K for y = 1.0. The charge distribution among atoms of the unit cell was
determined from the refined structure, for y = 0.0 to 1.0. It shows a charge
transfer to the superconducting CuO2 plane via two doping channels pi(1) and
pi(2), i.e. through O2(apical)-Cu and Ba/Sr-O1 bonds respectively.Comment: 13 pages, 5 figures, accepted for publication in Journal of Physics:
Condensed Matte
A method for the location of noise-polluted area
This paper deals with the working out of a method suitable to locate the critical areas from an acoustic point of view inside the pertinence zones of the roads. We have applied our method to about one thousand kilometres of major roads managed by ANAS in Lombardia. The procedure is based on the coupling of a Geographic Information System (GIS) with an acoustics simulation model. In order to characterize the noise sources in the prediction software, it has been necessary to estimate in every significant section of the streets the day and night average fluxes of vehicles, the vehicle typology and their average velocity. This study started from a statistical analysis of the experimental data obtained by sound measurements and by counting the vehicle fluxes. As a result, we have obtained on a GIS the acoustics map of the whole Lombardia road network with information on where the noise limits classes are exceeded
Evolution of magneto-orbital order upon B-site electron doping in Na1-xCaxMn7O12 quadruple perovskite manganites
We present the discovery and refinement by neutron powder diffraction of a
new magnetic phase in the Na1-xCaxMn7O12 quadruple perovskite phase diagram,
which is the incommensurate analogue of the well-known pseudo-CE phase of the
simple perovskite manganites. We demonstrate that incommensurate magnetic order
arises in quadruple perovskites due to the exchange interactions between A and
B sites. Furthermore, by constructing a simple mean field Heisenberg exchange
model that generically describes both simple and quadruple perovskite systems,
we show that this new magnetic phase unifies a picture of the interplay between
charge, magnetic and orbital ordering across a wide range of compounds.Comment: Accepted for publication in Physical Review Letter
Errors evaluation in the estimate of the noise from the road traffic
Specific algorithms together with noise data acquired during a measurement campaign, consisting of approximately 80 one-hour records, were utilized to model the noise levels of a road network. Experimental measurements were used to evaluate the reliability of the model by analyzing the differences between the measured values and the estimated ones. We think that these differences have to
be especially ascribed to an imperfect representation of the combined effects of the attenuation due to acoustic wave diffraction and the attenuation produced by the
ground effect
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