2,605 research outputs found
Concentration Dependence of the Flory Chi Parameter within Two-State Models
The Flory chi parameter is typically assumed to depend only on the
temperature, T. Experimental results often require the replacement of this
chi(T) by chieff, that depends also on the monomer volume fraction, phi,
chieff(phi,T). Such chieff(phi,T) can arise from two state-models, proposed for
polyetheleneoxide (PEO) and other neutral water-soluble polymers. The predicted
phi dependence of chibar=chieff-(1-phi)\partial \chieff/\partial phi,
obtainable from colligative properties, differs qualitatively between the
various models: (i) The model of Karlstrom (J. Phys. Chem. 1985, 89, 4962)
yields \partial chibar/\partial phi > 0 while the model of Matsuyama and Tanaka
(Phys. Rev. Lett. 1990, 65, 341) and of Bekiranov et al (Phys. Rev. E 1997, 55,
577) allows for \partial chibar/\partial phi <0 (ii) chibar(phi) as calculated
from the Karlstrom model, utilizing the parameters used to fit the phase
diagram of PEO, agrees semiquantitatively with the experimental values. On the
other hand, chibar(phi) similarly calculated from the model of Bekiranov et al.
differs qualitatively from the measured results. Altogether, chibar(phi)
provides useful measure for the performance of a model.Comment: 8 pages, 5 figures, Macromolecules, in pres
Signatures of a Concentration Dependent Flory chi Parameter: Swelling and Collapse of Coils and Brushes
The quality of solvents of polymers is often described in terms of the Flory
chi parameter typically assumed to depend only on the temperature, T. In
certain polymer-solvent systems fitting the experimental data enforces the
replacement of chi(T) by a concentration dependent chieff. In turn, this
modifies the swelling and collapse behavior. These effects are studied, in the
framework of a mean-field theory, for isolated coils and for planar brushes.
The phi dependence of chieff gives rise to three main consequences: (i) Shift
in the cross-over between Gaussian and self-avoidance regimes; (ii) A
possibility of first-order collapse transition for isolated flexible coils;
(iii) The possibility of a first-order phase transition leading to a vertical
phase separation within the brush. The discussion relates these effects
directly to thermodynamic measurements and does not involve a specific
microscopic model. The implementation for the case of
Poly(N-isopropylacrylamide) (PNIPAM) is discussed.Comment: 12 pages, 9 figure
Screening of electrostatic potential in a composite fermion system
Screening of the electric field of a test charge by monolayer and
double-layer composite fermion systems is considered. It is shown that the
electric field of the test charge is partly screened at distances much large
then the magnetic length. The value of screening as a function of the distance
depends considerably on the filling factor. The effect of variation of the
value of screening in the double-layer system upon a transition to a state
described by the Halperin wave function is determined.Comment: 5 pages, 2 eps figures include
Interaction of quantum Hall systems with waveguide elastic modes
An interaction of non-uniform plane elastic modes of the waveguide type with
monolayer and double-layer quantum Hall systems is considered. It is shown,
that unlike the case of the surface acoustic wave propagation, the restriction
on maximal values of the wave vectors for which the velocity shift can be
observed experimentally does not take place for the waveguide modes. In case of
study of incompressible fractional quantum Hall states the effect can be used
for measuring a dependence of the effective magnetic length on the filling
factor and for observing phase transitions in double-layer system under the
interlayer distance variationComment: 6 pages, 3 eps figures included, Fig.1 and Fig.3 correcte
The Tomonaga-Luttinger Model and the Chern-Simons Theory for the Edges of Multi-layer Fractional Quantum Hall Systems
Wen's chiral Tomonaga-Luttinger model for the edge of an m-layer quantum Hall
system of total filling factor nu=m/(pm +- 1) with even p, is derived as a
random-phase approximation of the Chern-Simons theory for these states. The
theory allows for a description of edges both in and out of equilibrium,
including their collective excitation spectrum and the tunneling exponent into
the edge. While the tunneling exponent is insensitive to the details of a
nu=m/(pm + 1) edge, it tends to decrease when a nu=m/(pm - 1) edge is taken out
of equilibrium. The applicability of the theory to fractional quantum Hall
states in a single layer is discussed.Comment: 15 page
Magnetic degeneracy and hidden metallicity of the spin density wave state in ferropnictides
We analyze spin density wave (SDW) order in iron-based superconductors and
electronic structure in the SDW phase. We consider an itinerant model for
Fe-pnictides with two hole bands centered at and two electron bands
centered at and in the unfolded BZ. A SDW order in such a
model is generally a combination of two components with momenta and
, both yield order in the folded zone. Neutron
experiments, however, indicate that only one component is present. We show that
or order is selected if we assume that only one hole band
is involved in the SDW mixing with electron bands. A SDW order in such 3-band
model is highly degenerate for a perfect nesting and hole-electron interaction
only, but we show that ellipticity of electron pockets and interactions between
electron bands break the degeneracy and favor the desired or
order. We further show that stripe-ordered system remains a metal for
arbitrary coupling. We analyze electronic structure for parameters relevant to
the pnictides and argue that the resulting electronic structure is in good
agreement with ARPES experiments. We discuss the differences between our model
and model of localized spins.Comment: reference list updated, typos are correcte
Nonlocal Charge Transport Mediated by Spin Diffusion in the Spin-Hall Effect Regime
A nonlocal electric response in the spin-Hall regime, resulting from spin
diffusion mediating charge conduction, is predicted. The spin-mediated
transport stands out due to its long-range character, and can give dominant
contribution to nonlocal resistance. The characteristic range of nonlocality,
set by the spin diffusion length, can be large enough to allow detection of
this effect in materials such as GaAs despite its small magnitude. The
detection is facilitated by a characteristic nonmonotonic dependence of
transresistance on the external magnetic field, exhibiting sign changes and
decay.Comment: 4 pages, 2 figure
Steady States of a Microwave Irradiated Quantum Hall Gas
We consider effects of a long-wavelength disorder potential on the Zero
Conductance State (ZCS) of the microwave-irradiated 2D electron gas. Assuming a
uniform Hall conductivity, we construct a Lyapunov functional and derive
stability conditions on the domain structure of the photo-generated fields. We
solve the resulting equations for a general one-dimensional and certain
two-dimensional disorder potentials, and find non-zero conductances,
photo-voltages, and circulating dissipative currents. In contrast, weak white
noise disorder does not destroy the ZCS, but induces mesoscopic current
fluctuations.Comment: 4 pages, 2 colour figure
On Topological Susceptibility, Vacuum Energy and Theta Dependence in Gluodynamics
We suggest that the topological susceptibility in gluodynamics can be found
in terms of the gluon condensate using renormalizability and heavy fermion
representation of the anomaly. Analogous relations can be also obtained for
other zero momentum correlation functions involving the topological density
operator. Using these relations, we find the theta dependence of the
condensates , and of the partition function for small theta
and an arbitrary number of colors.Comment: Details of the derivation are clarified, changes in discussions, new
references are adde
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