34,357 research outputs found
Shear flow induced isotropic to nematic transition in a suspension of active filaments
We study the effects of externally applied shear flow on a model of
suspensions of motors and filaments, via the equations of active hydrodynamics
[PRL {\bf 89} (2002) 058101; {\bf 92} (2004) 118101]. In the absence of shear,
the orientationally ordered phase of {\it both} polar and apolar active
particles is always unstable at zero-wavenumber. An imposed steady shear large
enough to overcome the active stresses stabilises both apolar and moving polar
phases. Our work is relevant to {\it in vitro} studies of active filaments, the
reorientation of endothelial cells subject to shear flow and shear-induced
motility of attached cells.Comment: 8 pages, 4 figures submitted to Europhysics Letter
Probing The Dust-To-Gas Ratio of z > 0 Galaxies Through Gravitational Lenses
We report the detection of differential gas column densities in three
gravitational lenses, MG0414+0534, HE1104-1805, and PKS1830-211. Combined with
the previous differential column density measurements in B1600+434 and
Q2237+0305 and the differential extinction measurements of these lenses, we
probe the dust-to-gas ratio of a small sample of cosmologically distant normal
galaxies. We obtain an average dust-to-gas ratio of E(B-V)/NH =(1.4\pm0.5) e-22
mag cm^2/atoms with an estimated intrinsic dispersion in the ratio of ~40%.
This average dust-to-gas ratio is consistent with the average Galactic value of
1.7e-22 mag cm^2/atoms and the estimated intrinsic dispersion is also
consistent with the 30% observed in the Galaxy.Comment: 14 pages, 4 figures, Accepted by Ap
Droplet Fluctuations in the Morphology and Kinetics of Martensites
We derive a coarse grained, free-energy functional which describes droplet
configurations arising on nucleation of a product crystal within a parent. This
involves a new `slow' vacancy mode that lives at the parent-product interface.
A mode-coupling theory suggests that a {\it slow} quench from the parent phase
produces an equilibrium product, while a {\it fast} quench produces a
metastable martensite. In two dimensions, the martensite nuclei grow as
`lens-shaped' strips having alternating twin domains, with well-defined front
velocities. Several empirically known structural and kinetic relations drop out
naturally from our theory.Comment: 4 pages, REVTEX, and 3 .eps figures, compressed and uuencoded,
Submitted to Phys. Rev. Let
Interplay of p-d and d-d charge transfer transitions in rare-earth perovskite manganites
We have performed both theoretical and experimental study of optical response
of parent perovskite manganites RMnO_3 with a main goal to elucidate nature of
clearly visible optical features. Starting with a simple cluster model approach
we addressed the both one-center (p-d) and two-center (d-d) charge transfer
(CT) transitions, their polarization properties, the role played by structural
parameters, orbital mixing, and spin degree of freedom. Optical complex
dielectric function of single crystalline samples of RMnO_3 (R=La, Pr, Nd, Sm,
Eu) was measured by ellipsometric technique at room temperature in the spectral
range from 1.0 to 5.0 eV for two light polarizations: E \parallel c and E \perp
c. The comparative analysis of the spectral behavior of \varepsilon _1 and
\varepsilon _2 is believed to provide a more reliable assignment of spectral
features. We have found an overall agreement between experimental spectra and
theoretical predictions based on the theory of one-center p-d CT transitions
and inter-site d-d CT transitions. Our experimental data and theoretical
analysis evidence a dual nature of the dielectric gap in nominally
stoichiometric matrix of perovskite manganites RMnO_3, it is formed by a
superposition of forbidden or weak dipole allowed p-d CT transitions and
inter-site d-d CT transitions. In fact, the parent perovskite manganites RMnO_3
should rather be sorted neither into the CT insulator nor the Mott-Hubbard
insulator in the Zaanen, Sawatzky, Allen scheme.Comment: 20 pages, 6 figure
Renormalization group study of the Kondo problem at a junction of several Luttinger wires
We study a system consisting of a junction of N quantum wires, where the
junction is characterized by a scalar S-matrix, and an impurity spin is coupled
to the electrons close to the junction. The wires are modeled as weakly
interacting Tomonaga-Luttinger liquids. We derive the renormalization group
equations for the Kondo couplings of the spin to the electronic modes on
different wires, and analyze the renormalization group flows and fixed points
for different values of the initial Kondo couplings and of the junction
S-matrix (such as the decoupled S-matrix and the Griffiths S-matrix). We
generally find that the Kondo couplings flow towards large and
antiferromagnetic values in one of two possible ways. For the Griffiths
S-matrix, we study one of the strong coupling flows by a perturbative expansion
in the inverse of the Kondo coupling; we find that at large distances, the
system approaches the ferromagnetic fixed point of the decoupled S-matrix. For
the decoupled S-matrix with antiferromagnetic Kondo couplings and weak
inter-electron interactions, the flows are to one of two strong coupling fixed
points in which all the channels are strongly coupled to each other through the
impurity spin. But strong inter-electron interactions, with K_\rho < N/(N+2),
stabilize a multi-channel fixed point in which the coupling between different
channels goes to zero. We have also studied the temperature dependence of the
conductance at the decoupled and Griffiths S-matrices.Comment: Revtex4, 16 pages including 6 figure
Investigation of HIV-1 Gag binding with RNAs and Lipids using Atomic Force Microscopy
Atomic Force Microscopy was utilized to study the morphology of Gag,
{\Psi}RNA, and their binding complexes with lipids in a solution environment
with 0.1{\AA} vertical and 1nm lateral resolution. TARpolyA RNA was used as a
RNA control. The lipid used was phospha-tidylinositol-(4,5)-bisphosphate
(PI(4,5)P2). The morphology of specific complexes Gag-{\Psi}RNA, Gag-TARpolyA
RNA, Gag-PI(4,5)P2 and PI(4,5)P2-{\Psi}RNA-Gag were studied. They were imaged
on either positively or negatively charged mica substrates depending on the net
charges carried. Gag and its complexes consist of monomers, dimers and
tetramers, which was confirmed by gel electrophoresis. The addition of specific
{\Psi}RNA to Gag is found to increase Gag multimerization. Non-specific
TARpolyA RNA was found not to lead to an increase in Gag multimerization. The
addition PI(4,5)P2 to Gag increases Gag multimerization, but to a lesser extent
than {\Psi}RNA. When both {\Psi}RNA and PI(4,5)P2 are present Gag undergoes
comformational changes and an even higher degree of multimerization
Transport in quantum wires
With a brief introduction to one-dimensional channels and conductance
quantisation in mesoscopic systems, we discuss some recent experimental puzzles
in these systems, which include reduction of quantised conductances and an
interesting {\it odd-even} effect in the presence of an in-plane magnetic
field. We then discuss a recent non-homogeneous Luttinger liquid model proposed
by us, which addresses and gives an explanation for the reduced conductances
and the {\it odd-even} effect. We end with a brief summary and discussion of
future projects.Comment: Talk presented at the International Discussion Meeting on Mesoscopic
and Disordered systems, December, 2000, 16 pages, 2 figure
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