82 research outputs found
Rotation of a spheroid in a simple shear at small Reynolds number
We derive an effective equation of motion for the orientational dynamics of a
neutrally buoyant spheroid suspended in a simple shear flow, valid for
arbitrary particle aspect ratios and to linear order in the shear Reynolds
number. We show how inertial effects lift the degeneracy of the Jeffery orbits
and determine the stabilities of the log-rolling and tumbling orbits at
infinitesimal shear Reynolds numbers. For prolate spheroids we find stable
tumbling in the shear plane, log-rolling is unstable. For oblate particles, by
contrast, log-rolling is stable and tumbling is unstable provided that the
aspect ratio is larger than a critical value. When the aspect ratio is smaller
than this value tumbling turns stable, and an unstable limit cycle is born.Comment: 25 pages, 5 figure
Aperiodic tumbling of microrods advected in a microchannel flow
We report on an experimental investigation of the tumbling of microrods in
the shear flow of a microchannel (40 x 2.5 x 0.4 mm). The rods are 20 to 30
microns long and their diameters are of the order of 1 micron. Images of the
centre-of-mass motion and the orientational dynamics of the rods are recorded
using a microscope equipped with a CCD camera. A motorised microscope stage is
used to track individual rods as they move along the channel. Automated image
analysis determines the position and orientation of a tracked rods in each
video frame. We find different behaviours, depending on the particle shape, its
initial position, and orientation. First, we observe periodic as well as
aperiodic tumbling. Second, the data show that different tumbling trajectories
exhibit different sensitivities to external perturbations. These observations
can be explained by slight asymmetries of the rods. Third we observe that after
some time, initially periodic trajectories lose their phase. We attribute this
to drift of the centre of mass of the rod from one to another stream line of
the channel flow.Comment: 14 pages, 8 figures, as accepted for publicatio
Superconducting transition temperatures and coherence length in non s-wave pairing materials correlated with spin-fluctuation mediated interaction
Following earlier work on electron or hole liquids flowing through assemblies
with magnetic fluctuations, we have recently exposed a marked correlation of
the superconducting temperature Tc, for non s-wave pairing materials, with
coherence length xi and effective mass m*. The very recent study of Abanov et
al. [Europhys. Lett. 54, 488 (2001)] and the prior investigation of Monthoux
and Lonzarich [Phys. Rev. B 59, 14598 (1999)] have each focussed on the concept
of a spin-fluctuation temperature T_sf, which again is intimately related to
Tc. For the d-wave pairing via antiferromagnetic spin fluctuations in the
cuprates, these studies are brought into close contact with our own work, and
the result is that k_B T_sf ~ hbar^2 / m* xi^2. This demonstrates that xi is
also determined by such antiferromagnetic spin-fluctuation mediated pair
interaction. The coherence length in units of the lattice spacing is then
essentially given in the cuprates as the square root of the ratio of two
characteristic energies, namely: the kinetic energy of localization of a charge
carrier of mass m* in a specified magnetic correlation length to the hopping
energy. The quasi-2D ruthenate Sr_2RuO_4, with Tc ~ 1.3 K, has p-wave
spin-triplet pairing and so is also briefly discussed here.Comment: Accepted for publication in Phys. Rev.
Pairing symmetry of superconducting graphene
The possibility of intrinsic superconductivity in alkali-coated graphene
monolayers has been recently suggested theoretically. Here, we derive the
possible pairing symmetries of a carbon honeycomb lattice and discuss their
phase diagram. We also evaluate the superconducting local density of states
(LDOS) around an isolated impurity. This is directly related to scanning
tunneling microscopy experiments, and may evidence the occurrence of
unconventional superconductivity in graphene.Comment: Eur. Phys. J. B, to appea
A new constant-pressure molecular dynamics method for finite system
In this letter, by writing the volume as a function of coordinates of atoms,
we present a new constant-pressure molecular dynamics method with parameters
free. This method is specially appropriate for the finite system in which the
periodic boundary condition does not exist. Simulations on the carbon nanotube
and the Ni nanoparticle clearly demonstrate the validity of the method. By
using this method, one can easily obtain the equation of states for the finite
system under the external pressure.Comment: RevTex, 5 pages, 3 figures, submitted to Phys. Rev. Let
Effects of proximity to an electronic topological transition on normal state transport properties of the high-Tc superconductors
Within the time dependent Ginzburg-Landau theory, the effects of the
superconducting fluctuations on the transport properties above the critical
temperature are characterized by a non-zero imaginary part of the relaxation
rate gamma of the order parameter. Here, we evaluate Im gamma for an
anisotropic dispersion relation typical of the high-Tc cuprate superconductors
(HTS), characterized by a proximity to an electronic topological transition
(ETT). We find that Im gamma abruptly changes sign at the ETT as a function of
doping, in agreement with the universal behavior of the HTS. We also find that
an increase of the in-plane anisotropy, as is given by a non-zero value of the
next-nearest to nearest hopping ratio r=t'/t, increases the value of | Im gamma
| close to the ETT, as well as its singular behavior at low temperature,
therefore enhancing the effect of superconducting fluctuations. Such a result
is in qualitative agreement with the available data for the excess Hall
conductivity for several cuprates and cuprate superlattices.Comment: to appear in Phys. Rev.
Ballistic transport properties across nonuniform strain barriers in graphene
We study the effect of uniaxial strain on the transmission and the
conductivity across a strain-induced barrier in graphene. At variance with
conventional studies, which consider sharp barriers, we consider a more
realistic, smooth barrier, characterized by a nonuniform, continuous strain
profile. Our results are instrumental towards a better understanding of the
transport properties in corrugated graphene.Comment: High Press. Res., to appea
The role of inertia for the rotation of a nearly spherical particle in a general linear flow
We analyse the angular dynamics of a neutrally buoyant nearly spherical
particle immersed in a steady general linear flow. The hydrodynamic torque
acting on the particle is obtained by means of a reciprocal theorem, regular
perturbation theory exploiting the small eccentricity of the nearly spherical
particle, and assuming that inertial effects are small, but finite.Comment: 7 pages, 1 figur
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