93 research outputs found
Structure and properties of the stable two-dimensional conducting polymer Mg5C60
We present a study on the structural, spectroscopic, conducting,
and
magnetic properties of Mg5C60, which is a two-dimensional (2D)
fulleride polymer. The polymer phase is stable up to the
exceptionally
high temperature of 823 K. The infrared and Raman studies
suggest the
formation of single bonds between the fulleride ions and
possibly
Mg-C-60 covalent bonds. Mg5C60 is a metal at ambient
temperature, as
shown by electron spin resonance and microwave conductivity
measurements. The smooth transition from a metallic to a
paramagnetic
insulator state below 200 K is attributed to Anderson
localization
driven by structural disorder
Light-induced instabilities in photo-oriented liquid crystal cells
In a planar liquid crystal sample sandwiched between a photosensitive and a
reference plate instabilities occurred, when the cell was illuminated from the
reference side. The instabilities were induced both by polarized white light
source and monochromatic laser beams. Static and dynamic regimes were found;
for laser irradiation dynamic instability was found only in a range of
polarization directions. A model, developed for monochromatic excitation,
predicts that at certain thicknesses dynamic instability is forbidden.
Experiments on a wedge-like cell confirmed this conclusion.Comment: to appear in Mol. Cryst. Liq. Crys
Fluctuating-friction molecular motors
We show that the correlated stochastic fluctuation of the friction
coefficient can give rise to long-range directional motion of a particle
undergoing Brownian random walk in a constant periodic energy potential
landscape. The occurrence of this motion requires the presence of two
additional independent bodies interacting with the particle via friction and
via the energy potential, respectively, which can move relative to each other.
Such three-body system generalizes the classical Brownian ratchet mechanism,
which requires only two interacting bodies. In particular, we describe a simple
two-level model of fluctuating-friction molecular motor that can be solved
analytically. In our previous work [M.K., L.M and D.P. 2000 J. Nonlinear Opt.
Phys. Mater. vol. 9, 157] this model has been first applied to understanding
the fundamental mechanism of the photoinduced reorientation of dye-doped liquid
crystals. Applications of the same idea to other fields such as molecular
biology and nanotechnology can however be envisioned. As an example, in this
paper we work out a model of the actomyosin system based on the
fluctuating-friction mechanism.Comment: to be published in J. Physics Condensed Matter
(http://www.iop.org/Journals/JPhysCM
Spin-stretching modes in anisotropic magnets: spin-wave excitations in the multiferroic Ba2CoGe2O7
We studied spin excitations of the multiferroic Ba2CoGe2O7 in high magnetic
fields up to 33 T. In the electron spin resonance and far infrared absorption
spectra we found several spin excitations beyond the two conventional magnon
modes expected for such a two-sublattice antiferromagnet. We show that a
multi-boson spin-wave theory can capture these unconventional modes, that
include spin-stretching modes associated with an oscillating magnetic dipole
(or only quadrupole) moment. The lack of the inversion symmetry allows these
modes to become electric dipole active. We expect that the spin-stretching
modes can be generally observed in inelastic neutron scattering and light
absorption experiments in a broad class of ordered S > 1/2 spin systems with
strong single-ion anisotropy and/or non-centrosymmetric lattice structure.Comment: 5+4 pages, 3 figures, supplement added, manuscript revise
Light-induced rotation of dye-doped liquid crystal droplets
We investigate both theoretically and experimentally the rotational dynamics
of micrometric droplets of dye-doped and pure liquid crystal induced by
circularly and elliptically polarized laser light. The droplets are dispersed
in water and trapped in the focus of the laser beam. Since the optical torque
acting on the molecular director is known to be strongly enhanced in
light-absorbing dye-doped materials, the question arises whether a similar
enhancement takes place also for the overall optical torque acting on the whole
droplets. We searched for such enhancement by measuring and comparing the
rotation speed of dye-doped droplets induced by a laser beam having a
wavelength either inside or outside the dye absorption band, and also comparing
it with the rotation of pure liquid crystal droplets. No enhancement was found,
confirming that photoinduced dye effects are only associated with an internal
exchange of angular momentum between orientational and translational degrees of
freedom of matter. Our result provides also the first direct experimental proof
of the existence of a photoinduced stress tensor in the illuminated dye-doped
liquid crystal. Finally, peculiar photoinduced dynamical effects are predicted
to occur in droplets in which the molecular director is not rigidly locked to
the flow, but so far they could not be observed
Spintronics: Fundamentals and applications
Spintronics, or spin electronics, involves the study of active control and
manipulation of spin degrees of freedom in solid-state systems. This article
reviews the current status of this subject, including both recent advances and
well-established results. The primary focus is on the basic physical principles
underlying the generation of carrier spin polarization, spin dynamics, and
spin-polarized transport in semiconductors and metals. Spin transport differs
from charge transport in that spin is a nonconserved quantity in solids due to
spin-orbit and hyperfine coupling. The authors discuss in detail spin
decoherence mechanisms in metals and semiconductors. Various theories of spin
injection and spin-polarized transport are applied to hybrid structures
relevant to spin-based devices and fundamental studies of materials properties.
Experimental work is reviewed with the emphasis on projected applications, in
which external electric and magnetic fields and illumination by light will be
used to control spin and charge dynamics to create new functionalities not
feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes
from the published versio
Shear flow induced propagating domains in cholesterics
It is shown, that shear flow destabilizes the planar texture of cholesterics of pitch small compared to sample thickness above a threshold shear rate. In addition, in the presence of the shear, perturbations of the planar configuration propagate with a velocity proportional to the shear rate. In this paper we present a simplified theory of these effects and compare our results with experimental data.Il est montré, qu'au-dessus du seuil du taux de cisaillement, un écoulement déstabilise la texture planaire des cholestériques dont le pas est inférieur à l'épaisseur de l'échantillon. Les perturbations de la configuration planaire se propagent à une vitesse proportionnelle au taux de cisaillement. Dans cet article, nous présentons une théorie simplifiée de ces phénomènes qui est comparée aux résultats expérimentaux
Diffuso-mechanical coupling in cholesteric liquid crystals
It has been observed that in cholesterics shear flow induces a transverse drift of small particles suspended in the liquid crystal. Quantitative measurements of the drift velocity are presented. The effect is interpreted using the analogy of the thermomechanical coupling, predicted for cholesterics.L'étude d'une suspension de petites particules dans un cholestérique en écoulement de cisaillement met en évidence une dérive transversale de ces petites particules. Des mesures de la valeur de la vitesse de dérive sont présentées. Ce phénomène est interprété en utilisant l'analogie du couplage thermomécanique prévu pour les cholestériques
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