14,003 research outputs found
Phase Transitions in Lyotropic Nematic Gels
In this paper, we discuss the equilibrium phases and collapse transitions of
a lyotropic nematic gel immersed in an isotropic solvent. A nematic gel
consists of a cross-linked polymer network with rod-like molecules embedded in
it. Upon decreasing the quality of the solvent, we find that a lyotropic
nematic gel undergoes a discontinuous volume change accompanied by an
isotropic-nematic transition. We also present phase diagrams that these systems
may exhibit. In particular, we show that coexistence of two isotropic phases,
of two nematic phases, or of an isotropic and a nematic phase can occur.Comment: 13 pages Revtex, 10 figures, submitted to EPJ
Non-equilibrium fluctuations and mechanochemical couplings of a molecular motor
We investigate theoretically the violations of Einstein and Onsager
relations, and the efficiency for a single processive motor operating far from
equilibrium using an extension of the two-state model introduced by Kafri {\em
et al.} [Biophys. J. {\bf 86}, 3373 (2004)]. With the aid of the Fluctuation
Theorem, we analyze the general features of these violations and this
efficiency and link them to mechanochemical couplings of motors. In particular,
an analysis of the experimental data of kinesin using our framework leads to
interesting predictions that may serve as a guide for future experiments.Comment: 4 pages, 4 figures, accepted to Phys. Rev. Let
Room temperature electron spin coherence in telecom-wavelength quaternary quantum wells
Time-resolved Kerr rotation spectroscopy is used to monitor the room
temperature electron spin dynamics of optical telecommunication wavelength
AlInGaAs multiple quantum wells lattice-matched to InP. We found that electron
spin coherence times and effective g-factors vary as a function of aluminum
concentration. The measured electron spin coherence times of these multiple
quantum wells, with wavelengths ranging from 1.26 microns to 1.53 microns,
reach approximately 100 ps at room temperature, and the measured electron
effective g-factors are in the range from -2.3 to -1.1.Comment: 4 pages, 4 figure
New variables, the gravitational action, and boosted quasilocal stress-energy-momentum
This paper presents a complete set of quasilocal densities which describe the
stress-energy-momentum content of the gravitational field and which are built
with Ashtekar variables. The densities are defined on a two-surface which
bounds a generic spacelike hypersurface of spacetime. The method used
to derive the set of quasilocal densities is a Hamilton-Jacobi analysis of a
suitable covariant action principle for the Ashtekar variables. As such, the
theory presented here is an Ashtekar-variable reformulation of the metric
theory of quasilocal stress-energy-momentum originally due to Brown and York.
This work also investigates how the quasilocal densities behave under
generalized boosts, i. e. switches of the slice spanning . It is
shown that under such boosts the densities behave in a manner which is similar
to the simple boost law for energy-momentum four-vectors in special relativity.
The developed formalism is used to obtain a collection of two-surface or boost
invariants. With these invariants, one may ``build" several different mass
definitions in general relativity, such as the Hawking expression. Also
discussed in detail in this paper is the canonical action principle as applied
to bounded spacetime regions with ``sharp corners."Comment: Revtex, 41 Pages, 4 figures added. Final version has been revised and
improved quite a bit. To appear in Classical and Quantum Gravit
Scanning Tunneling Spectroscopic Studies of the Effects of Dielectrics and Metallic Substrates on the Local Electronic Characteristics of Graphene
Atomically resolved imaging and spectroscopic characteristics of
graphene grown by chemical vapor deposition (CVD) on copper
foils are investigated and compared with those of mechanical
exfoliated graphene on SiO_2. For exfoliated graphene, the local
spectral deviations from ideal behavior may be attributed to strain
induced by the SiO_2 substrate. For CVD grown graphene, the
lattice structure appears strongly distorted by the underlying
copper, with regions in direct contact with copper showing nearly
square lattices whereas suspended regions from thermal relaxation
exhibiting nearly honeycomb or hexagonal lattice structures. The
electronic density of states (DOS) correlates closely with the
atomic arrangements of carbon, showing excess zero-bias
tunneling conductance and nearly energy-independent DOS for
strongly distorted graphene, in contrast to the linearly dispersive
DOS for suspended graphene. These results suggest that graphene
can interact strongly with both metallic and dielectric materials in
close proximity, leading to non-negligible modifications to the
electronic properties
Generating Spin Currents in Semiconductors with the Spin Hall Effect
We investigate electrically-induced spin currents generated by the spin Hall
effect in GaAs structures that distinguish edge effects from spin transport.
Using Kerr rotation microscopy to image the spin polarization, we demonstrate
that the observed spin accumulation is due to a transverse bulk electron spin
current, which can drive spin polarization nearly 40 microns into a region in
which there is minimal electric field. Using a model that incorporates the
effects of spin drift, we determine the transverse spin drift velocity from the
magnetic field dependence of the spin polarization.Comment: 4 pages, 4 figure
Entanglement and the nonlinear elastic behavior of forests of coiled carbon nanotubes
Helical or coiled nanostructures have been object of intense experimental and
theoretical studies due to their special electronic and mechanical properties.
Recently, it was experimentally reported that the dynamical response of
foamlike forest of coiled carbon nanotubes under mechanical impact exhibits a
nonlinear, non-Hertzian behavior, with no trace of plastic deformation. The
physical origin of this unusual behavior is not yet fully understood. In this
work, based on analytical models, we show that the entanglement among
neighboring coils in the superior part of the forest surface must be taken into
account for a full description of the strongly nonlinear behavior of the impact
response of a drop-ball onto a forest of coiled carbon nanotubes.Comment: 4 pages, 3 figure
Effects of counterion fluctuations in a polyelectrolyte brush
We investigate the effect of counterion fluctuations in a single
polyelectrolyte brush in the absence of added salt by systematically expanding
the counterion free energy about Poisson-Boltzmann mean field theory. We find
that for strongly charged brushes, there is a collapse regime in which the
brush height decreases with increasing charge on the polyelectrolyte chains.
The transition to this collapsed regime is similar to the liquid-gas
transition, which has a first-order line terminating at a critical point. We
find that for monovalent counterions the transition is discontinuous in theta
solvent, while for multivalent counterions the transition is generally
continuous. For collapsed brushes, the brush height is not independent of
grafting density as it is for osmotic brushes, but scales linear with it.Comment: 9 pages, 9 figure
Hamiltonians for a general dilaton gravity theory on a spacetime with a non-orthogonal, timelike or spacelike outer boundary
A generalization of two recently proposed general relativity Hamiltonians, to
the case of a general (d+1)-dimensional dilaton gravity theory in a manifold
with a timelike or spacelike outer boundary, is presented.Comment: 17 pages, 3 figures. Typos correcte
- …