67 research outputs found
Giant infrared intensity of the Peierls mode at the neutral-ionic phase transition
We present exact diagonalization results on a modified Peierls-Hubbard model
for the neutral-ionic phase transition. The ground state potential energy
surface and the infrared intensity of the Peierls mode point to a strong,
non-linear electron-phonon coupling, with effects that are dominated by the
proximity to the electronic instability rather than by electronic correlations.
The huge infrared intensity of the Peierls mode at the ferroelectric
transition is related to the temperature dependence of the dielectric constant
of mixed-stack organic crystals.Comment: 4 pages, 4 figure
Electronic and Lattice Dynamics in The Photoinduced Ionic-to-Neutral Phase Transition in a One-Dimensional Extended Peierls-Hubbard Model
Real-time dynamics of charge density and lattice displacements is studied
during photoinduced ionic-to-neutral phase transitions by using a
one-dimensional extended Peierls-Hubbard model with alternating potentials for
the one-dimensional mixed-stack charge-transfer complex, TTF-CA. The
time-dependent Schr\"odinger equation and the classical equation of motion are
solved for the electronic and lattice parts, respectively. We show how neutral
domains grow in the ionic background. As the photoexcitation becomes intense,
more neutral domains are created. Above threshold intensity, the neutral phase
is finally achieved. After the photoexcitation, ionic domains with wrong
polarization also appear. They quickly reduce the averaged staggered lattice
displacement, compared with the averaged ionicity. As the degree of initial
lattice disorder increases, more solitons appear between these ionic domains
with different polarizations, which obstruct the growth of neutral domains and
slow down the transition.Comment: 9 pages, 10 figures, submitted to J. Phys. Soc. Jp
Advances in ab-initio theory of Multiferroics. Materials and mechanisms: modelling and understanding
Within the broad class of multiferroics (compounds showing a coexistence of
magnetism and ferroelectricity), we focus on the subclass of "improper
electronic ferroelectrics", i.e. correlated materials where electronic degrees
of freedom (such as spin, charge or orbital) drive ferroelectricity. In
particular, in spin-induced ferroelectrics, there is not only a {\em
coexistence} of the two intriguing magnetic and dipolar orders; rather, there
is such an intimate link that one drives the other, suggesting a giant
magnetoelectric coupling. Via first-principles approaches based on density
functional theory, we review the microscopic mechanisms at the basis of
multiferroicity in several compounds, ranging from transition metal oxides to
organic multiferroics (MFs) to organic-inorganic hybrids (i.e. metal-organic
frameworks, MOFs)Comment: 22 pages, 9 figure
Título: Nouveau traité de la guerre
Sign.: []\p1\s, a-b\p12\s, c\p2\s, A-O\p12\s, P\p11\sTexto con apostillas marxinaisA f. de grav. preg. calc. "Retraite des Dix-Mille..." asinada por "M.A. Elizab.\ph\s Hardy sculp."A outra f. de grav. preg. calc. é "Bataille de Cunaza" e non está asinad
Título: Nouveau traité de la guerre
Sign.: []\p1\s, A-Z\p12\s, 2A\p12\s, 2B\p4\sTexto con apostillas marxinai
Physics of Neutral-to-Ionic Phase Transition in Organic Charge Transfer Semiconducting Compounds
An uncommon excitonic instability takes place in some exotic semiconducting compounds. Indeed, the equilibrium neutral-to-ionic (N-I) phase transition, as well as the non-equilibrium photo-induced phase transformation, observed in some organic charge-transfer complexes, originate from intra- and inter-chain cooperative effects between structurally relaxed charge-transfer excitations. This electronic-structural phase transition manifests itself by a change of the degree of charge-transfer and a dimerization distortion with the formation of donor-acceptor pairs along the stacking axis in the I phase. Thermal charge-transfer excitations associated with the formation of I strings along N chains are at the heart of the mechanism of this phase transition. These relaxed electronic excitations, which are an intrinsic feature of low-dimensional systems with strong electron-phonon coupling, can be described in terms of self-trapping and self-multiplication of charge-transfer excitons. Precise structural studies on the prototype compound, tetrathiafulvalene-p-chloranil allow to highlight the respective role taken by the ionicity and the dimerization. Symmetry and thermodynamics analysis of the N-I transition, based on recent determination of the pressure-temperature phase diagram, make possible to present a consistent picture of this phase transition. Supported by theoretical considerations taking into account the interplay between quantum and thermal effects, the experimental observations show that the N-I transition results from the condensation and the ordering (crystallization) of charge-transfer excitations, following a phase diagram analogous to the solid-liquid-gas one
- …