405 research outputs found
Direct visualization of aging in colloidal glasses
We use confocal microscopy to directly visualize the dynamics of aging
colloidal glasses. We prepare a colloidal suspension at high density, a simple
model system which shares many properties with other glasses, and initiate
experiments by stirring the sample. We follow the motion of several thousand
colloidal particles after the stirring and observe that their motion
significantly slows as the sample ages. The aging is both spatially and
temporally heterogeneous. Furthermore, while the characteristic relaxation time
scale grows with the age of the sample, nontrivial particle motions continue to
occur on all time scales.Comment: submitted to proceedings for Liquid Matter Conference 200
Nonlocal Effects on the Magnetic Penetration Depth in d-wave Superconductors
We show that, under certain conditions, the low temperature behavior of the
magnetic penetration depth of a pure d-wave superconductor is
determined by nonlocal electrodynamics and, contrary to the general belief, the
deviation is proportional to T^2 and
not T. We predict that the dependence, due to
nonlocality, should be observable experimentally in nominally clean high-T_c
superconductors below a crossover temperature . Possible complications due to impurities, surface quality and
crystal axes orientation are discussed.Comment: REVTeX3.0; 4 pages, 1 EPS figure (included); Submitted to Phys. Rev.
Let
Coherence, incoherence and scaling along the c axis of YBa_2Cu_3O_{6+x}
The optical properties of single crystals of YBa_2Cu_3O_{6+x} have been
examined along the c axis above and below the critical temperature (T_c) for a
wide range of oxygen dopings. The temperature dependence of the
optically-determined value of the dc conductivity (\sigma_{dc}) in the normal
state suggests a crossover from incoherent (hopping-type) transport at lower
oxygen dopings (x \lesssim 0.9) to more coherent anisotropic three-dimensional
behavior in the overdoped (x \approx 0.99) material at temperatures close to
T_c. The assumption that superconductivity occurs along the c axis through the
Josephson effect yields a scaling relation between the strength of the
superconducting condensate (\rho_{s,c}, a measure of the number of
superconducting carriers), the critical temperature, and the normal-state
c-axis value for \sigma_{dc} just above T_c; \rho_{s,c} \propto \sigma_{dc}
T_c. This scaling relation is observed along the c axis for all oxygen dopings,
as well as several other cuprate materials. However, the agreement with the
Josephson coupling model does not necessarily imply incoherent transport,
suggesting that these materials may indeed be tending towards coherent behavior
at the higher oxygen dopings.Comment: Six pages with four figures and one tabl
Intrinsic route to melt fracture in polymer extrusion: a weakly nonlinear subcritical instability of viscoelastic Poiseuille flow
As is well known, the extrusion rate of polymers from a cylindrical tube or slit (a die ) is in practice limited by the appearance of melt fracture instabilities which give rise to unwanted distortions or even fracture of the extrudate. We present the results of a weakly nonlinear analysis which gives evidence for an intrinsic generic route to melt fracture via a weakly nonlinear subcritical instability of viscoelastic Poiseuille flow. This instability and the onset of associated melt fracture phenomena appear at a well-defined ratio of the elastic stresses to viscous stresses of the polymer solution
A preference for a non-zero neutrino mass from cosmological data
We present results from the analysis of cosmic microwave background (CMB),
large scale structure (galaxy redshift survey) and X-ray galaxy cluster (baryon
fraction and X-ray luminosity function) data, assuming a geometrically flat
cosmological model and allowing for tensor components and a non-negligible
neutrino mass. From a combined analysis of all data, assuming three degenerate
neutrinos species, we measure a contribution of neutrinos to the energy density
of the universe, Omega_nu h^2=0.0059^{+0.0033}_{-0.0027} (68 per cent
confidence limits), with zero falling on the 99 per cent confidence limit. This
corresponds to ~4 per cent of the total mass density of the Universe and
implies a species-summed neutrino mass \sum_i m_i =0.56^{+0.30}_{-0.26} eV, or
m_nu~0.2 eV per neutrino. We examine possible sources of systematic uncertainty
in the results. Combining the CMB, large scale structure and cluster baryon
fraction data, we measure an amplitude of mass fluctuations on 8h^{-1} Mpc
scales of sigma_8=0.74^{+0.12}_{-0.07}, which is consistent with measurements
based on the X-ray luminosity function and other studies of the number density
and evolution of galaxy clusters. This value is lower than that obtained when
fixing a negligible neutrino mass (sigma_8=0.86^{+0.08}_{-0.07}). The
combination of CMB, large scale structure and cluster baryon fraction data also
leads to remarkably tight constraints on the Hubble constant,
H_0=68.4^{+2.0}_{-1.4} km/s/Mpc, mean matter density, Omega_m =0.31\pm0.02 and
physical baryon density, Omega_b h^2=0.024\pm0.001, of the Universe.Comment: Final version. MNRAS, in press (9 pages, 6 figures, 1 table).
Includes small modification to the neutrino mass calculation and comment on
quintessence. Conclusions unchange
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