4,173 research outputs found
Emergence of active topological glass through directed chain dynamics and nonequilibrium phase segregation
Active matter states defy many notions that have been established for systems in thermodynamic equilibrium. Nevertheless, the lack of detailed balance might be utilized to design nonequilibrium materials with unique properties. Recently we have shown, employing a model of ring polymers containing segments with a larger mobility than given by equilibrium thermal fluctuations, that making polymers with intrinsic topology active can result in states that relax extremely slowly, the so-called active topological glass. In this paper, we focus on the role of nonequilibrium phase separation in the vitrification process. In particular, we detail the polymer dynamics and show that such activity-driven glassy states arise from heterogeneity of segmental dynamics that emerges on all scales. Provided that the activity quench is strong enough, the rings feature an oriented reptationlike motion, with the active segment serving as an effective chain's end, resulting into a dramatic increase of inter-ring treading that vitrifies the system. The scaling properties of the ensuing steady-state ring conformations, which are significantly elongated and usually possess a doubly folded structure, are discussed and compared to equilibrium counterparts. We further examine the connection between the glass formation and the nonequilibrium phase separation and we find that both appear to be initiated by the contrasting dynamics of ring segments. Finally, we consider the effect of nonequilibrium phase separation in other active copolymer architectures
Self-avoiding walks on scale-free networks
Several kinds of walks on complex networks are currently used to analyze
search and navigation in different systems. Many analytical and computational
results are known for random walks on such networks. Self-avoiding walks (SAWs)
are expected to be more suitable than unrestricted random walks to explore
various kinds of real-life networks. Here we study long-range properties of
random SAWs on scale-free networks, characterized by a degree distribution
. In the limit of large networks (system size ), the average number of SAWs starting from a generic site
increases as , with . For finite ,
is reduced due to the presence of loops in the network, which causes the
emergence of attrition of the paths. For kinetic growth walks, the average
maximum length, , increases as a power of the system size: , with an exponent increasing as the parameter is
raised. We discuss the dependence of on the minimum allowed degree in
the network. A similar power-law dependence is found for the mean
self-intersection length of non-reversal random walks. Simulation results
support our approximate analytical calculations.Comment: 9 pages, 7 figure
Heat capacity of -GaN: Isotope Effects
Until recently, the heat capacity of GaN had only been measured for
polycrystalline powder samples. Semiempirical as well as
\textit{first-principles} calculations have appeared within the past few years.
We present in this article measurements of the heat capacity of hexagonal
single crystals of GaN in the 20-1400K temperature range. We find that our data
deviate significantly from the literature values for polycrystalline materials.
The dependence of the heat capacity on the isotopic mass has also been
investigated recently for monatomic crystals such as diamond, silicon, and
germanium. Multi-atomic crystals are expected to exhibit a different dependence
of these heat capacities on the masses of each of the isotopes present. These
effects have not been investigated in the past. We also present
\textit{first-principles} calculations of the dependence of the heat capacities
of GaN, as a canonical binary material, on each of the Ga and N masses. We show
that they are indeed different, as expected from the fact that the Ga mass
affects mainly the acoustic, that of N the optic phonons. It is hoped that
these calculations will encourage experimental measurements of the dependence
of the heat capacity on isotopic masses in binary and more complex
semiconductors.Comment: 12 pages, 5 Figures, submitted to PR
Interacting Growth Walk - a model for hyperquenched homopolymer glass?
We show that the compact self avoiding walk configurations, kinetically
generated by the recently introduced Interacting Growth Walk (IGW) model, can
be considered as members of a canonical ensemble if they are assigned random
values of energy. Such a mapping is necessary for studying the thermodynamic
behaviour of this system. We have presented the specific heat data for the IGW,
obtained from extensive simulations on a square lattice; we observe a broad
hump in the specific heat above the -point, contrary to expectation.Comment: 4 figures; Submitted to PR
Active Topological Glass Confined within a Spherical Cavity
[Image: see text] We study active topological glass under spherical confinement, allowing us to exceed the chain lengths simulated previously and determine the critical exponents of the arrested conformations. We find a previously unresolved âtank-treadingâ dynamic mode of active segments along the ring contour. This mode can enhance activeâpassive phase separation in the state of active topological glass when both diffusional and conformational relaxation of the rings are significantly suppressed. Within the observational time, we see no systematic trends in the positioning of the separated active domains within the confining sphere. The arrested state exhibits coherent stochastic rotations. We discuss possible connections of the conformational and dynamic features of the system to chromosomes enclosed in the nucleus of a living cell
Field theory of self-avoiding walks in random media
Based on the analogy with the quantum mechanics of a particle propagating in
a {\em complex} potential, we develop a field-theoretical description of the
statistical properties of a self-avoiding polymer chain in a random
environment. We show that the account of the non-Hermiticity of the quantum
Hamiltonian results in a qualitatively different structure of the effective
action, compared to previous studies. Applying the renormalisation group
analysis, we find a transition between the weak-disorder regime, where the
quenched randomness is irrelevant, and the strong-disorder regime, where the
polymer chain collapses. However, the fact that the renormalised interaction
constants and the chiral symmetry breaking regularisation parameter flow
towards strong coupling raises questions about the applicability of the
perturbative analysis.Comment: RevTeX, 9 pages; accepted for publication in J. Phys.
Strong Attraction between Charged Spheres due to Metastable Ionized States
We report a mechanism which can lead to long range attractions between
like-charged spherical macroions, stemming from the existence of metastable
ionized states. We show that the ground state of a single highly charged
colloid plus a few excess counterions is overcharged. For the case of two
highly charged macroions in their neutralizing divalent counterion solution we
demonstrate that, in the regime of strong Coulomb coupling, the counterion
clouds are very likely to be unevenly distributed, leading to one overcharged
and one undercharged macroion. This long-living metastable configuration in
turn leads to a long range Coulomb attraction.Comment: REVTEX-published versio
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