270 research outputs found
Maximum relative excitation of a specific vibrational mode via optimum laser pulse duration
For molecules and materials responding to femtosecond-scale optical laser
pulses, we predict maximum relative excitation of a Raman-active vibrational
mode with period T when the pulse has an FWHM duration of 0.42 T. This result
follows from a general analytical model, and is precisely confirmed by detailed
density-functional-based dynamical simulations for C60 and a carbon nanotube,
which include anharmonicity, nonlinearity, no assumptions about the
polarizability tensor, and no averaging over rapid oscillations within the
pulse. The mode specificity is, of course, best at low temperature and for
pulses that are electronically off-resonance, and the energy deposited in any
mode is proportional to the fourth power of the electric field.Comment: 5 pages, 4 figure
Neutron stars within the SU(2) parity doublet model
The equation of state of beta-stable and charge neutral nucleonic matter is
computed within the SU(2) parity doublet model in mean field and in the
relativistic Hartree approximation. The mass of the chiral partner of the
nucleon is assumed to be 1200 MeV. The transition to the chiral restored phase
turns out to be a smooth crossover in all the cases considered, taking place at
a baryon density of just . The mass-radius relations of compact stars
are calculated to constrain the model parameters from the maximum mass limit of
neutron stars. It is demonstrated that chiral symmetry starts to be restored,
which in this model implies the appearance of the chiral partners of the
nucleons, in the center of neutron stars. However, the analysis of the decay
width of the assumed chiral partner of the nucleon poses limits on the validity
of the present version of the model to describe vacuum properties.Comment: 14 pages, 9 figures, 2 tables, version accepted for publication in
EJP
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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Ultrafast Dynamics of Photoexcited Carriers in HWCVD a-Si:H and a-SiGe:H
We present femtosecond studies of photoexcited carrier dynamics in hydrogenated amorphous silicon and silicon-germanium alloys grown by the hot-wire assisted chemical vapor deposition (HWCVD) technique, which is promising for producing high-quality device-grade materials
Communications: Polarity fluctuations of the protic ionic liquid ethylammonium nitrate in the terahertz regime
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