56,735 research outputs found
Simulation and theory of fluid demixing and interfacial tension of mixtures of colloids and non-ideal polymers
An extension of the Asakura-Oosawa-Vrij model of hard sphere colloids and
non-adsorbing polymers, that takes polymer non-ideality into account through a
repulsive stepfunction pair potential between polymers, is studied with grand
canonical Monte Carlo simulations and density functional theory. Simulation
results validate previous theoretical findings for the shift of the bulk fluid
demixing binodal upon increasing strength of polymer-polymer repulsion,
promoting the tendency to mix. For increasing strength of the polymer-polymer
repulsion, simulation and theory consistently predict the interfacial tension
of the free colloidal liquid-gas interface to decrease significantly for fixed
colloid density difference in the coexisting phases, and to increase for fixed
polymer reservoir packing fraction.Comment: 10 pages, 4 figure
Full counting statistics of spin transfer through the Kondo dot
We calculate the spin current distribution function for a Kondo dot in two
different regimes. In the exactly solvable Toulouse limit the linear response,
zero temperature statistics of the spin transfer is trinomial, such that all
the odd moments vanish and the even moments follow a binomial distribution. On
the contrary, the corresponding spin-resolved distribution turns out to be
binomial. The combined spin and charge statistics is also determined. In
particular, we find that in the case of a finite magnetic field or an
asymmetric junction the spin and charge measurements become statistically
dependent. Furthermore, we analyzed the spin counting statistics of a generic
Kondo dot at and around the strong-coupling fixed point (the unitary limit).
Comparing these results with the Toulouse limit calculation we determine which
features of the latter are generic and which ones are artifacts of the spin
symmetry breaking.Comment: 9 pages, 3 eps figure
Vanishing conductivity of quantum solitons in polyacetylene
Quantum solitons or polarons are supposed to play a crucial role in the
electric conductivity of polyacetylene, in the intermediate doping regime. We
present an exact fully quantized calculation of the quantum soliton
conductivity in polyacetylene and show that it vanishes exactly. This is
obtained by applying a general method of soliton quantization, based on
order-disorder duality, to a Z(2)-symmetric complex extension of the TLM
dimerization effective field theory. We show that, in this theory,
polyacetylene solitons are sine-Gordon solitons in the phase of the complex
field.Comment: To appear in J. Phys. A: Math. Theor., 15 page
Excitation spectra and rf-response near the polaron-to-molecule transition from the functional renormalization group
A light impurity in a Fermi sea undergoes a transition from a polaron to a
molecule for increasing interaction. We develop a new method to compute the
spectral functions of the polaron and molecule in a unified framework based on
the functional renormalization group with full self-energy feedback. We discuss
the energy spectra and decay widths of the attractive and repulsive polaron
branches as well as the molecular bound state and confirm the scaling of the
excited state decay rate near the transition. The quasi-particle weight of the
polaron shifts from the attractive to the repulsive branch across the
transition, while the molecular bound state has a very small residue
characteristic for a composite particle. We propose an experimental procedure
to measure the repulsive branch in a Li6 Fermi gas using rf-spectroscopy and
calculate the corresponding spectra.Comment: 15 pages, 13 figures; v2: version published in Phys. Rev.
Correctness of an STM Haskell implementation
A concurrent implementation of software transactional memory in Concurrent Haskell using a call-by-need functional language with processes and futures is given. The description of the small-step operational semantics is precise and explicit, and employs an early abort of conflicting transactions. A proof of correctness of the implementation is given for a contextual semantics with may- and should-convergence. This implies that our implementation is a correct evaluator for an abstract specification equipped with a big-step semantics
Cosmological Simulations of Normal-Branch Braneworld Gravity
We introduce a cosmological model based on the normal branch of DGP
braneworld gravity with a smooth dark energy component on the brane. The
expansion history in this model is identical to LambdaCDM, thus evading all
geometric constraints on the DGP cross-over scale r_c. This model can serve as
a first approximation to more general braneworld models whose cosmological
solutions have not been obtained yet. We study the formation of large scale
structure in this model in the linear and non-linear regime using N-body
simulations for different values of r_c. The simulations use the code presented
in (F.S., arXiv:0905.0858) and solve the full non-linear equation for the
brane-bending mode in conjunction with the usual gravitational dynamics. The
brane-bending mode is attractive rather than repulsive in the DGP normal
branch, hence the sign of the modified gravity effects is reversed compared to
those presented in arXiv:0905.0858. We compare the simulation results with
those of ordinary LambdaCDM simulations run using the same code and initial
conditions. We find that the matter power spectrum in this model shows a
characteristic enhancement peaking at k ~ 0.7 h/Mpc. We also find that the
abundance of massive halos is significantly enhanced. Other results presented
here include the density profiles of dark matter halos, and signatures of the
brane-bending mode self-interactions (Vainshtein mechanism) in the simulations.
Independently of the expansion history, these results can be used to place
constraints on the DGP model and future generalizations through their effects
on the growth of cosmological structure.Comment: 17 pages, 10 figures; v2: minor changes; v3: references added; v4:
added appendix on comparison with previous results; matches published
version; v5: corrected Eqs. (2.4-2.5) and Fig. 1 following Ref. [28]; all
following results unchange
Magnetically induced Ferroelectricity in BiCuO
The tetragonal copper oxide BiCuO has an unusual crystal structure
with a three-dimensional network of well separated CuO plaquettes. This
material was recently predicted to host electronic excitations with an
unconventional spectrum and the spin structure of its magnetically ordered
state appearing at T 43 K remains controversial. Here we present the
results of detailed studies of specific heat, magnetic and dielectric
properties of BiCuO single crystals grown by the floating zone
technique, combined with the polarized neutron scattering and high-resolution
X-ray measurements. Our polarized neutron scattering data show Cu spins are
parallel to the plane. Below the onset of the long range antiferromagnetic
ordering we observe an electric polarization induced by an applied magnetic
field, which indicates inversion symmetry breaking by the ordered state of Cu
spins. For the magnetic field applied perpendicular to the tetragonal axis, the
spin-induced ferroelectricity is explained in terms of the linear
magnetoelectric effect that occurs in a metastable magnetic state. A relatively
small electric polarization induced by the field parallel to the tetragonal
axis may indicate a more complex magnetic ordering in BiCuO
Self-Consistent Cosmological Simulations of DGP Braneworld Gravity
We perform cosmological N-body simulations of the Dvali-Gabadadze-Porrati
braneworld model, by solving the full non-linear equations of motion for the
scalar degree of freedom in this model, the brane bending mode. While coupling
universally to matter, the brane-bending mode has self-interactions that become
important as soon as the density field becomes non-linear. These
self-interactions lead to a suppression of the field in high-density
environments, and restore gravity to General Relativity. The code uses a
multi-grid relaxation scheme to solve the non-linear field equation in the
quasi-static approximation. We perform simulations of a flat self-accelerating
DGP model without cosmological constant. The results of the DGP simulations are
compared with standard gravity simulations assuming the same expansion history,
and with DGP simulations using the linearized equation for the brane bending
mode. This allows us to isolate the effects of the non-linear self-couplings of
the field which are noticeable already on quasi-linear scales. We present
results on the matter power spectrum and the halo mass function, and discuss
the behavior of the brane bending mode within cosmological structure formation.
We find that, independently of CMB constraints, the self-accelerating DGP model
is strongly constrained by current weak lensing and cluster abundance
measurements.Comment: 21 pages; 10 figures. Revised version matching published versio
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