2,462 research outputs found
Kerr effect as a tool for the investigation of dynamic heterogeneities
We propose a dynamic Kerr effect experiment for the distinction between
dynamic heterogeneous and homogeneous relaxation in glassy systems. The
possibility of this distinction is due to the inherent nonlinearity of the Kerr
effect signal. We model the slow reorientational molecular motion in
supercooled liquids in terms of non-inertial rotational diffusion. The Kerr
effect response, consisting of two terms, is calculated for heterogeneous and
for homogeneous variants of the stochastic model. It turns out that the
experiment is able to distinguish between the two scenarios. We furthermore
show that exchange between relatively 'slow' and 'fast' environments does not
affect the possibility of frequency-selective modifications. It is demonstrated
how information about changes in the width of the relaxation time distribution
can be obtained from experimental results.Comment: 23 pages incl. 6 figures accepted for publication in The Journal of
Chemical Physic
Ion and polymer dynamics in polymer electrolytes PPO-LiClO4: II. 2H and 7Li NMR stimulated-echo experiment
We use 2H NMR stimulated-echo spectroscopy to measure two-time correlation
functions characterizing the polymer segmental motion in polymer electrolytes
PPO-LiClO4 near the glass transition temperature Tg. To investigate effects of
the salt on the polymer dynamics, we compare results for different ether oxygen
to lithium ratios, namely, 6:1, 15:1, 30:1 and infinity. For all compositions,
we find nonexponential correlation functions, which can be described by a
Kohlrausch function. The mean correlation times show quantitatively that an
increase of the salt concentration results in a strong slowing down of the
segmental motion. Consistently, for the high 6:1 salt concentration, a high
apparent activation energy E_a=4.1eV characterizes the temperature dependence
of the mean correlation times at Tg < T< 1.1T_g, while smaller values E_a=2.5eV
are observed for moderate salt contents. The correlation functions are most
nonexponential for 15:1 PPO-LiClO4, whereas the stretching is reduced for
higher and lower salt concentrations. A similar dependence of the correlation
functions on the evolution time in the presence and in the absence of ions
indicates that addition of salt hardly affects the reorientational mechanism.
For all compositions, mean jump angles of about 15 degree characterize the
segmental reorientation. In addition, comparison of results from 2H and 7Li NMR
stimulated-echo experiments suggests a coupling of ion and polymer dynamics in
15:1 PPO-LiClO4.Comment: 14 pages, 12 figure
Experimental and computational analyses reveal that environmental restrictions shape HIV-1 spread in 3D cultures
Here, using an integrative experimental and computational approach, Imle et al. show how cell motility and density affect HIV cell-associated transmission in a three-dimensional tissue-like culture system of CD4+ T cells and collagen, and how different collagen matrices restrict infection by cell-free virions
Practical Implementations of Twirl Operations
Twirl operations, which convert impure singlet states into Werner states,
play an important role in many schemes for entanglement purification. In this
paper we describe strategies for implementing twirl operations, with an
emphasis on methods suitable for ensemble quantum information processors such
as nuclear magnetic resonance (NMR) quantum computers. We implement our twirl
operation on a general two-spin mixed state using liquid state NMR techniques,
demonstrating that we can obtain the singlet Werner state with high fidelity.Comment: 6 pages RevTex4 including 2 figures (fig 1 low quality to save space
Caracterização Espectroscópica da Matéria Orgânica do Solo.
bitstream/CNPDIA/10450/1/CiT24_2004.pd
Nonlinear response of dense colloidal suspensions under oscillatory shear: Mode-coupling theory and FT-rheology experiments
Using a combination of theory, experiment and simulation we investigate the
nonlinear response of dense colloidal suspensions to large amplitude
oscillatory shear flow. The time-dependent stress response is calculated using
a recently developed schematic mode-coupling-type theory describing colloidal
suspensions under externally applied flow. For finite strain amplitudes the
theory generates a nonlinear response, characterized by significant higher
harmonic contributions. An important feature of the theory is the prediction of
an ideal glass transition at sufficiently strong coupling, which is accompanied
by the discontinuous appearance of a dynamic yield stress. For the oscillatory
shear flow under consideration we find that the yield stress plays an important
role in determining the non linearity of the time-dependent stress response.
Our theoretical findings are strongly supported by both large amplitude
oscillatory (LAOS) experiments (with FT-rheology analysis) on suspensions of
thermosensitive core-shell particles dispersed in water and Brownian dynamics
simulations performed on a two-dimensional binary hard-disc mixture. In
particular, theory predicts nontrivial values of the exponents governing the
final decay of the storage and loss moduli as a function of strain amplitude
which are in excellent agreement with both simulation and experiment. A
consistent set of parameters in the presented schematic model achieves to
jointly describe linear moduli, nonlinear flow curves and large amplitude
oscillatory spectroscopy
1/T_1 nuclear relaxation time of \kappa-(BEDT-TTF)_ 2 Cu [N(CN)_2] Cl : effects of magnetic frustration
We study the role played by the magnetic frustration in the antiferromagnetic
phase of the organic salt \kappa-(BEDT-TTF)_ 2 Cu [N(CN)_2] Cl. Using the
spatially anisotropic triangular Heisenberg model we analyze previous and new
performed NMR experiments. We compute the 1/T_1 relaxation time by means of the
modified spin wave theory. The strong suppression of the nuclear relaxation
time observed experimentally under varying pressure and magnetic field is
qualitatively well reproduced by the model. Our results suggest the existence
of a close relation between the effects of pressure and magnetic frustration.Comment: 10 pages, 9 figures, to appear in Journal of Phys.: Condens Matte
Dynamical Heterogeneities Below the Glass Transition
We present molecular dynamics simulations of a binary Lennard-Jones mixture
at temperatures below the kinetic glass transition. The ``mobility'' of a
particle is characterized by the amplitude of its fluctuation around its
average position. The 5% particles with the largest/smallest mean amplitude are
thus defined as the relatively most mobile/immobile particles. We investigate
for these 5% particles their spatial distribution and find them to be
distributed very heterogeneously in that mobile as well as immobile particles
form clusters. The reason for this dynamic heterogeneity is traced back to the
fact that mobile/immobile particles are surrounded by fewer/more neighbors
which form an effectively wider/narrower cage. The dependence of our results on
the length of the simulation run indicates that individual particles have a
characteristic mobility time scale, which can be approximated via the
non-Gaussian parameter.Comment: revtex, 10 pages, 20 postscript figure
Dynamics of Quasi-ordered Structure in a Regio-regulated pi-Conjugated Polymer:Poly(4-methylthiazole-2,5-diyl)
Dynamics of regio-regulated Poly(4-methylthiazole-2,5-diyl) [HH-P4MeTz] was
inves tigated by solid-state 1H, 2D, 13C NMR spectroscopies, and differential
scanning calorimetry(DSC) measurements. DSC, 2D quadrupolar echo NMR, 13C
cross-polarization and magic-angle spinning(CPMAS) NMR, and 2D spin-echo(2DSE)
CPMAS NMR spectroscopy suggest existence of a quasi-ordered phase in which
backbone twists take place with weakened pi-stackings. Two-dimensional exchange
2D NMR(2DEX) detected slow dynamics with a rate of an order of 10^2Hz for the
CD_3 group in d_3-HH-P4MeTz at 288K. The frequency dependence of proton
longitudinal relaxation rate at 288K shows a omega^-1/2 dependence, which is
due to the one-dimensional diffusion-like motion of backbone conformational
modulation waves. The diffusion rate was estimated as 3+/-2 GHz, which was
approximately 10^7 times larger than that estimated by 2DEX NMR measurements.
These results suggest that there exists anomalous dispersion of modulation
waves in HH-P4MeTz. The one-dimensional group velocity of the wave packet is
responsible for the behavior of proton longitudinal relaxation time. On the
other hand, the 2DEX NMR is sensitive to phase velocity of the nutation of
methyl groups that is associated with backbone twists. From proton T_1 and T_2
measurements, the activation energy was estimated as 2.9 and 3.4 kcal/mol,
respectively. These were in agreement with 3.0 kcal/mol determined by
Moller-Plesset(MP2) molecular orbital(MO) calculation. We also performed
chemical shielding calculation of the methyl-carbon in order to understand
chemical shift tensor behavior, leading to the fact that a quasi-ordered phase
coexist with the crystalline phase.Comment: 14 pages, 11 figures, to appear in Phys.Rev.
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