1,721,012 research outputs found
New results on the melting thermodynamics of a circular DNA chain
No full textWe investigate the impact of supercoil period and nonzero supercoil
formation energy on the thermal denaturation of a circular DNA. Our
analysis is based on a recently proposed generalization of the Poland
Scheraga model that allows the DNA melting to be studied for plasmids
with circular topology, where denaturation is accompanied by formation
of supercoils. We find that the previously obtained first-order melting
transition persists under the generalization discussed. The dependence
of the size of the order-parameter jump at the transition point and the
associated melting temperature are obtained analytically
Supercoil formation in DNA denaturation
No full textWe generalize the Poland-Scheraga model to the case of a circular DNA,
taking into account the twisting of the two strains around each other.
Guided by recent single-molecule experiments on DNA strands, we assume
that the torsional stress induced by denaturation enforces the formation
of supercoils whose writhe absorbs the linking number expelled by the
loops. Our model predicts that when the entropy parameter of a loop
satisfies c <= 2, denaturation transition does not take place. On the
other hand, for c>2, a first-order denaturation transition is consistent
with our model and may take place in the actual system, as in the case
with no supercoils. These results are in contrast with other treatments
of circular DNA melting where denaturation is assumed to be accompanied
by an increase in twist rather than writhe on the bound segments
Inequivalence of ensembles in a system with long-range Interactions
No full textWe study the global phase diagram of the Infinite-range Blume-Emery-Griffiths model both in the canonical and in the microcanonical ensembles. The canonical phase diagram shows first-order and continuous transition lines separated by a tricritical point. We find that below the tricritical point, when the canonical transition is first order, the phase diagrams of the two ensembles disagree. In this region the microcanonical ensemble exhibits energy ranges with negative specific heat and temperature jumps at transition energies. These results can be extended to weakly decaying nonintegrabl
Breaking of ergodicity and long relaxation times in systems with long-range interactions
No full textThe thermodynamic and dynamical properties of an Ising model with both short range and long range, mean field like, interactions are studied within the microcanonical ensemble. It is found that the relaxation time of thermodynamically {\it unstable} states diverges logarithmically with system size. This is in contrast with the case of short range interactions where this time is finite. Moreover, at sufficiently low energies, gaps in the magnetization interval may develop to which no microscopic configuration corresponds. As a result, in {\it local microcanonical dynamics} the system cannot move across the gap, leading to breaking of ergodicity even in finite systems. These are general features of systems with long range interactions and are expected to be valid even when the interaction is slowly decaying with distance
Ensemble inequivalence in the Blume-Emery-Griffiths model near a fourth-order critical point
No full textThe canonical phase diagram of the Blume-Emery-Griffiths model with infinite-range interactions is known to exhibit a fourth-order critical point at some negative value of the biquadratic interaction K<0. Here we study the microcanonical phase diagram of this model for K<0, extending previous studies which were restricted to positive K. A fourth-order critical point is found to exist at coupling parameters which are different from those of the canonical ensemble. The microcanonical phase diagram of the model close to the fourth-order critical point is studied in detail revealing some distinct features from the canonical counterpart
Long-range effects in layered spin structures
No full textWe study theoretically layered spin systems where long-range dipolar interactions play a relevant role. By choosing a specific sample shape, we are able to reduce the complex Hamiltonian of the system to that of a much simpler coupled rotator model with short-range and mean-field interactions. This latter model has been studied in the past because of its interesting dynamical and statistical properties related to exotic features of long-range interactions. It is suggested that experiments could be conducted such that within a specific temperature range the presence of long-range interactions crucially affects the behavior of the system
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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