17 research outputs found
Raman and Infrared-Active Phonons in Hexagonal HoMnO Single Crystals: Magnetic Ordering Effects
Polarized Raman scattering and infrared reflection spectra of hexagonal
HoMnO single crystals in the temperature range 10-300 K are reported.
Group-theoretical analysis is performed and scattering selection rules for the
second order scattering processes are presented. Based on the results of
lattice dynamics calculations, performed within the shell model, the observed
lines in the spectra are assigned to definite lattice vibrations. The magnetic
ordering of Mn ions, which occurs below T=76 K, is shown to effect both
Raman- and infrared-active phonons, which modulate Mn-O-Mn bonds and,
consequently, Mn exchange interaction.Comment: 8 pages, 6 figure
Phase Transition in a One-Dimensional Extended Peierls-Hubbard Model with a Pulse of Oscillating Electric Field: I. Threshold Behavior in Ionic-to-Neutral Transition
Photoinduced dynamics of charge density and lattice displacements is
calculated by solving the time-dependent Schr\"odinger equation for a
one-dimensional extended Peierls-Hubbard model with alternating potentials for
the mixed-stack organic charge-transfer complex, TTF-CA. A pulse of oscillating
electric field is incorporated into the Peierls phase of the transfer integral.
The frequency, the amplitude, and the duration of the pulse are varied to study
the nonlinear and cooperative character of the photoinduced transition. When
the dimerized ionic phase is photoexcited, the threshold behavior is clearly
observed by plotting the final ionicity as a function of the increment of the
total energy. Above the threshold photoexcitation, the electronic state reaches
the neutral one with equidistant molecules after the electric field is turned
off. The transition is initiated by nucleation of a metastable neutral domain,
for which an electric field with frequency below the linear absorption peak is
more effective than that at the peak. When the pulse is strong and short, the
charge transfer takes place on the same time scale with the disappearance of
dimerization. As the pulse becomes weak and long, the dimerization-induced
polarization is disordered to restore the inversion symmetry on average before
the charge transfer takes place to bring the system neutral. Thus, a
paraelectric ionic phase is transiently realized by a weak electric field. It
is shown that infrared light also induces the ionic-to-neutral transition,
which is characterized by the threshold behavior.Comment: 24 pages, 11 figure
Effects of Lattice and Molecular Phonons on Photoinduced Neutral-to-Ionic Transition Dynamics in Tetrathiafulvalene--Chloranil
For electronic states and photoinduced charge dynamics near the neutral-ionic
transition in the mixed-stack charge-transfer complex
tetrathiafulvalene--chloranil (TTF-CA), we review the effects of Peierls
coupling to lattice phonons modulating transfer integrals and Holstein
couplings to molecular vibrations modulating site energies. The former
stabilizes the ionic phase and reduces discontinuities in the phase transition,
while the latter stabilizes the neutral phase and enhances the discontinuities.
To reproduce the experimentally observed ionicity, optical conductivity and
photoinduced charge dynamics, both couplings are quantitatively important. In
particular, strong Holstein couplings to form the highly-stabilized neutral
phase are necessary for the ionic phase to be a Mott insulator with large
ionicity. A comparison with the observed photoinduced charge dynamics indicates
the presence of strings of lattice dimerization in the neutral phase above the
transition temperature.Comment: 9 pages, 7 figures, accepted for publication in J. Phys. Soc. Jp
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