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M. P. Allen

Publications and source records attributed to M. P. Allen.

17 recordsLinked to original sources
arXiv2011

Folding Kinetics of a Polymer

By simulating the first order globule-crystal transition of a flexible homopolymer chain, both by collision dynamics and Monte Carlo with non-kinetic moves, we show that the effective and the thermodynamic transition temperatures are different and we propose a way of quantifying the kinetic hindering. We then also observe that the top eigenvalue in the spectrum of the dynamical (contact or adjacency) matrix provides insight into the ensembles of folding and unfolding trajectories, and may be a suitable additional reaction coordinate for the folding transition of chain molecules.

cond-mat.soft↗
arXiv2011

The supernova remnant CTB 37B and its associated magnetar CXOU J171405.7-381031: evidence for a magnetar-driven remnant

We discuss in this Letter the association of the candidate magnetar CXOU J171405.7-381031 with the supernova remnant CTB 37B. The recent detection of the period derivative of the object allowed an estimation of a young characteristic age of only $\sim 1000 yr$. This value is too small to be compatible even with the minimum radius of the remnant $\geq 10 pc$, the value corresponding to the {\it lower} limit of the estimated distance of $10.2 \pm 3.5 kpc$, unless the true distance happens to be even smaller than the lower limit. We argue that a consistent scenario for the remnant origin, in which the latter is powered by the energy injected by a young magnetar, is indeed more accurate to explain the young age, and points out to its non-standard (i.e. magnetar-driven) nature.

astro-ph.HE↗
arXiv2010

An assessment of the newest magnetar-SNR associations

Anomalous X-ray Pulsars and Soft-Gamma Repeaters groups are magnetar candidates featuring low characteristic ages ($τ= {P\over{2 {\dot P}}}$). At least some of them they should still be associated with the remnants of the explosive events in which they were born, giving clues to the type of events leading to their birth and the physics behind the apparent high value of the magnetar magnetic fields. To explain the high values of $B$, a self-consistent picture of field growth also suggests that energy injection into the SNR is large and unavoidable, in contrast with the evolution of {\it conventional} SNR. This modified dynamics, in turn, has important implications for the proposed associations. We show that this scenario yields low ages for the new candidates CXOU J171405.7-381031/CTB 37B and XMMU J173203.3-344518/G353.6-0.7, and predicted values agree with recently found ${\dot P}$, giving support to the overall picture.

astro-ph.HE↗
arXiv2009

Pion mass effects on axion emission from neutron stars through NN bremsstrahlung processes

The rates of axion emission by nucleon-nucleon bremsstrahlung are calculated with the inclusion of the full momentum contribution from a nuclear one pion exchange (OPE) potential. The contributions of the neutron-neutron (nn), proton-proton (pp) and neutron-proton (np) processes in both the nondegenerate and degenerate limits are explicitly given. We find that the finite momentum corrections to the emissivities are quantitatively significant for the non-degenerate regime and temperature-dependent, and should affect the existing axion mass bounds. The trend of these nuclear effects is to diminish the emissivities.

nucl-th↗
arXiv2003

Influence of an Internal Magnetar on Supernova Remnant Expansion

Most of the proposed associations between magnetars and supernova remnant suffer from age problems. Usually, supernova remnants ages are determined from an approximation of the Sedov-Taylor phase relation between radius and age, for a fixed energy of the explosion ~ 10^{51} erg. Those ages do not generally agree with the characteristic ages of the (proposed) associated magnetars. We show quantitatively that, by taking into account the energy injected on the supernova remnant by magnetar spin-down, a faster expansion results, improving matches between characteristic ages and supernova remnants ages. However, the magnetar velocities inferred from observations would inviabilize some associations. Since characteristic ages may not be good age estimators, their influence on the likelihood of the association may not be as important. In this work we present simple numerical simulations of supernova remnants expansion with internal magnetars, and apply it to the observed objects. A short initial spin period, thought to be important for the very generation of the magnetic field, is also relevant for the modified expansion of the remnant. We next analyze all proposed associations case-by-case, addressing the likelyhood of each one, according to this perspective. We consider a larger explosion energy and reasses the characteristic age issue, and conclude that about 50% of the associations can be true ones, provided SGRs and AXPs are magnetars.

astro-ph↗
arXiv2003

Forces between elongated particles in a nematic colloid

Using molecular dynamics (MD) simulations we study the interactions between elongated colloidal particles (length to breath ratio > 1) in a nematic host. The simulation results are compared to the results of a Landau-de Gennes elastic free energy. We find that depletion forces dominate for the sizes of the colloidal particles studied. The tangential component of the force, however, allows us to resolve the elastic contribution to the total interaction. We find that this contribution differs from the quadrupolar interaction predicted at large separations. The difference is due to the presence of nonlinear effects, namely th e change in the positions and structure of the defects and their annihilation at small separations.

cond-mat.soft↗
arXiv2002

Theory and simulation of the nematic zenithal anchoring coefficient

Combining molecular simulation, Onsager theory and the elastic description of nematic liquid crystals, we study the dependence of the nematic liquid crystal elastic constants and the zenithal surface anchoring coefficient on the value of the bulk order parameter.

cond-mat.soft↗
arXiv2002

Defect structures and torque on an elongated colloidal particle immersed in a liquid crystal host

Combining molecular dynamics and Monte Carlo simulation we study defect structures around an elongated colloidal particle embedded in a nematic liquid crystal host. By studying nematic ordering near the particle and the disclination core region we are able to examine the defect core structure and the difference between two simulation techniques. In addition, we also study the torque on a particle tilted with respect to the director, and modification of this torque when the particle is close to the cell wall.

cond-mat.soft↗
arXiv2002

Molecular simulation and theory of a liquid crystalline disclination core

Molecular simulations of a nematic liquid crystal confined in cylinder geometry with homeotropic anchoring have been carried out. The core structure of a disclination line defect of strength +1 has been examined, and comparison made with various theoretical treatments, which are presented in a unified way. It is found that excellent fits to the cylindrically-symmetrized order tensor profiles may be obtained with appropriate parameter choices; notwithstanding this, on the timescales of the simulation, the cylindrical symmetry of the core is broken and two defects of strength +1/2 may be resolved.

cond-mat.soft↗
arXiv2002

Computer simulation of topological defects around a colloidal particle or droplet dispersed in a nematic host

We use molecular dynamics to study the ordering of a nematic liquid crystal around a spherical particle or droplet. Homeotropic boundary conditions and strong anchoring create a hedgehog director configuration on the particle surface and in its vicinity; this topological defect is cancelled by nearby defect structures in the surrounding liquid crystal, so as to give a uniform director field at large distances. We observe three defect structures for different particle sizes: a quadrupolar one with a ring defect surrounding the particle in the equatorial plane; a dipolar one with a satellite defect at the north or south pole; and a transitional, non-equatorial, ring defect. These observations are broadly consistent with the predictions of the simplest elastic theory. By studying density and order-parameter maps, we are able to examine behaviour near the particle surface, and in the disclination core region, where the elastic theory is inapplicable. Despite the relatively small scale of the inhomogeneities in our systems, the simple theory gives reasonably accurate predictions of the variation of defect position with particle size.

cond-mat.soft↗
arXiv2002

Liquid crystal director fluctuations and surface anchoring by molecular simulation

We propose a simple and reliable method to measure the liquid crystal surface anchoring strength by molecular simulation. The method is based on the measurement of the long-range fluctuation modes of the director in confined geometry. As an example, molecular simulations of a liquid crystal in slab geometry between parallel walls with homeotropic anchoring have been carried out using the Monte Carlo technique. By studying different slab thicknesses, we are able to calculate separately the position of the elastic boundary condition, and the extrapolation length.

cond-mat.soft↗
arXiv2001

Dynamical precursor of nematic order in a dense fluid of hard ellipsoids of revolution

We investigate hard ellipsoids of revolution in a parameter regime where no long range nematic order is present but already finite size domains are formed which show orientational order. Domain formation leads to a substantial slowing down of a collective rotational mode which separates well from the usual microscopic frequency regime. A dynamic coupling of this particular mode into all other modes provides a general mechanism which explains an excess peak in spectra of molecular fluids. Using molecular dynamics simulation on up to 4096 particles and on solving the molecular mode coupling equation we investigate dynamic properties of the peak and prove its orientational origin.

cond-mat.dis-nn↗
arXiv2001

Surface tension of the isotropic-nematic interface

We present the first calculations of the pressure tensor profile in the vicinity of the planar interface between isotropic liquid and nematic liquid crystal, using Onsager's density functional theory and computer simulation. When the liquid crystal director is aligned parallel to the interface, the situation of lowest free energy, there is a large tension on the nematic side of the interface and a small compressive region on the isotropic side. By contrast, for perpendicular alignment, the tension is on the isotropic side. There is excellent agreement between theory and simulation both in the forms of the pressure tensor profiles, and the values of the surface tension.

cond-mat.mtrl-sci↗
arXiv2001

Molecular Dynamics Study of the Nematic-Isotropic Interface

We present large-scale molecular dynamics simulations of a nematic-isotropic interface in a system of repulsive ellipsoidal molecules, focusing in particular on the capillary wave fluctuations of the interfacial position. The interface anchors the nematic phase in a planar way, i.e., the director aligns parallel to the interface. Capillary waves in the direction parallel and perpendicular to the director are considered separately. We find that the spectrum is anisotropic, the amplitudes of capillary waves being larger in the direction perpendicular to the director. In the long wavelength limit, however, the spectrum becomes isotropic and compares well with the predictions of a simple capillary wave theory.

cond-mat.stat-mech↗
arXiv1997

Implications of a constant observed braking index for young pulsar's spindown

The observed braking index n_{obs} which had been determined for a few young pulsars, had been found to differ from the expected value for a rotating magnetic dipole model. Also, the observational jerk parameter, determined for two of these pulsars, disagree with the theoretical prediction m_{obs} = 15 in both cases. We propose a simple model able to account for these differences, based on a growth of the "torque function" K = -(dot Omega)/(Omega^{n}), under the constraint that n_{obs} is a constant. We show that there is observational evidence supporting the latter hypotesis, and derive initial values for several physical quantities for the four pulsars whose n_{obs} have been measured.

astro-ph↗
arXiv1997

Glitches, torque evolution and the dynamics of young pulsars

The Crab pulsar has suffered in 1975 and 1989 two glitches in which the frequency did not relaxed to the extrapolated pre-glitch value but rather spun up showing long-term changes in the frequency derivative dot Omega. This particular behaviour has been interpreted as evidence for an evolution of the torque acting upon the star. A variable torque may be related to non-canonical braking indexes, for which some determinations have been possible. We briefly analyse in this work the consistency of postulating a growth in the angle between the magnetic moment and the rotation axis as the cause of such events. We show that this hypothesis leads to the determination of the initial period, initial and present angles, according to the assumed angle growth, for young pulsars whose respective braking indices n_{obs} and jerk parameters m_{obs} are known, and some insights on the equation of state.

astro-ph↗