3,450 research outputs found

### The effect of the lateral interactions on the critical behavior of long straight rigid rods on two-dimensional lattices

Using Monte Carlo simulations and finite-size scaling analysis, the critical
behavior of attractive rigid rods of length k (k-mers) on square lattices at
intermediate density has been studied. A nematic phase, characterized by a big
domain of parallel k-mers, was found. This ordered phase is separated from the
isotropic state by a continuous transition occurring at a intermediate density
\theta_c, which increases linearly with the magnitude of the lateral
interactions.Comment: 7 pages, 6 figure

### Critical behavior of long straight rigid rods on two-dimensional lattices: Theory and Monte Carlo simulations

The critical behavior of long straight rigid rods of length $k$ ($k$-mers) on
square and triangular lattices at intermediate density has been studied. A
nematic phase, characterized by a big domain of parallel $k$-mers, was found.
This ordered phase is separated from the isotropic state by a continuous
transition occurring at a intermediate density $\theta_c$. Two analytical
techniques were combined with Monte Carlo simulations to predict the dependence
of $\theta_c$ on $k$, being $\theta_c(k) \propto k^{-1}$. The first involves
simple geometrical arguments, while the second is based on entropy
considerations. Our analysis allowed us also to determine the minimum value of
$k$ ($k_{min}=7$), which allows the formation of a nematic phase on a
triangular lattice.Comment: 23 pages, 5 figures, to appear in The Journal of Chemical Physic

### Critical behavior of self-assembled rigid rods on triangular and honeycomb lattices

Using Monte Carlo simulations and finite-size scaling analysis, the critical
behavior of self-assembled rigid rods on triangular and honeycomb lattices at
intermediate density has been studied. The system is composed of monomers with
two attractive (sticky) poles that, by decreasing temperature or increasing
density, polymerize reversibly into chains with three allowed directions and,
at the same time, undergo a continuous isotropic-nematic (IN) transition. The
determination of the critical exponents, along with the behavior of Binder
cumulants, indicate that the IN transition belongs to the q=1 Potts
universality class.Comment: 6 pages, 5 figure

### Entropy-driven phase transition in a system of long rods on a square lattice

The isotropic-nematic (I-N) phase transition in a system of long straight
rigid rods of length k on square lattices is studied by combining Monte Carlo
simulations and theoretical analysis. The process is analyzed by comparing the
configurational entropy of the system with the corresponding to a fully aligned
system, whose calculation reduces to the 1D case. The results obtained (1)
allow to estimate the minimum value of k which leads to the formation of a
nematic phase and provide an interesting interpretation of this critical value;
(2) provide numerical evidence on the existence of a second phase transition
(from a nematic to a non-nematic state) occurring at density close to 1 and (3)
allow to test the predictions of the main theoretical models developed to treat
the polymers adsorption problem.Comment: 14 pages, 6 figures. Accepted for publication in JSTA

### Breaking the AMSP mould: the increasingly strange case of HETE J1900.1-2455

We present ongoing Rossi X-ray Timing Explorer (RXTE) monitoring observations
of the 377.3 Hz accretion-powered pulsar, HETE J1900.1-2455 Activity continues
in this system more than 3 years after discovery, at a mean luminosity of
4.4e36 erg/s (for d=5 kpc), although pulsations were present only within the
first 70 days. X-ray variability has increased each year, notably with a brief
interval of nondetection in 2007, during which the luminosity dropped to below
1e-3 of the mean level. A deep search of data from the intervals of
nondetection in 2005 revealed evidence for extremely weak pulsations at an
amplitude of 0.29% rms, a factor of ten less than the largest amplitude seen
early in the outburst.
X-ray burst activity continued through 2008, with bursts typically featuring
strong radius expansion. Spectral analysis of the most intense burst detected
by RXTE early in the outburst revealed unusual variations in the inferred
photospheric radius, as well as significant deviations from a blackbody. We
obtained much better fits instead with a comptonisation model.Comment: 8 pages, 3 figures, one table; to appear in the proceedings of the
workshop "A Decade of Accreting Millisecond X-ray Pulsars", Amsterdam, April
2008, eds. R. Wijnands et al. (AIP Conf. Proc.

### Mapping the QCD Phase Transition with Accreting Compact Stars

We discuss an idea for how accreting millisecond pulsars could contribute to
the understanding of the QCD phase transition in the high-density nuclear
matter equation of state (EoS). It is based on two ingredients, the first one
being a ``phase diagram'' of rapidly rotating compact star configurations in
the plane of spin frequency and mass, determined with state-of-the-art hybrid
equations of state, allowing for a transition to color superconducting quark
matter. The second is the study of spin-up and accretion evolution in this
phase diagram. We show that the quark matter phase transition leads to a
characteristic line in the Omega-M plane, the phase border between neutron
stars and hybrid stars with a quark matter core. Along this line a change in
the pulsar's moment of inertia entails a waiting point phenomenon in the
accreting millisecond X-ray pulsar (AMXP) evolution: most of these objects
should therefore be found along the phase border in the Omega-M plane, which
may be viewed as the AMXP analog of the main sequence in the
Hertzsprung-Russell diagram for normal stars. In order to prove the existence
of a high-density phase transition in the cores of compact stars we need
population statistics for AMXP's with sufficiently accurate determination of
their masses and spin frequencies.Comment: 8 pages, 3 figures, to appear in Proceedings of the Conference on "A
Decade of Accreting Millisecond X-Ray Pulsars, Amsterdam, April 14-18, 200

### Phase diagram of self-assembled rigid rods on two-dimensional lattices: Theory and Monte Carlo simulations

Monte Carlo simulations and finite-size scaling analysis have been carried
out to study the critical behavior in a two-dimensional system of particles
with two bonding sites that, by decreasing temperature or increasing density,
polymerize reversibly into chains with discrete orientational degrees of
freedom and, at the same time, undergo a continuous isotropic-nematic (IN)
transition. A complete phase diagram was obtained as a function of temperature
and density. The numerical results were compared with mean field (MF) and real
space renormalization group (RSRG) analytical predictions about the IN
transformation. While the RSRG approach supports the continuous nature of the
transition, the MF solution predicts a first-order transition line and a
tricritical point, at variance with the simulation results.Comment: 12 pages, 10 figures, supplementary informatio

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