Surface spin-flop phases and bulk discommensurations in antiferromagnets

Phase diagrams as a function of anisotropy D and magnetic field H are obtained for discommensurations and surface states for a model antiferromagnet in which $H$ is parallel to the easy axis. The surface spin-flop phase exists for all $D$. We show that there is a region where the penetration length of the surface spin-flop phase diverges. Introducing a discommensuration of even length then becomes preferable to reconstructing the surface. The results are used to clarify and correct previous studies in which discommensurations have been confused with genuine surface spin-flop states.Comment: 4 pages, RevTeX, 2 Postscript figure

Causality and effects “beyond” strategy reinforcing organization: The process of decoupling decisions-actions towards ecologies

The "juxtaposition" between strategy and organization has seen in the last few decades a weakening of the former, while observing an increasing complexity and management of change as a modus operandi that has evolved the latter also to compensate for those weaknesses. A dynamic counter-balance process that therefore seems to improve the way people take decisions as well as their way of acting, decoupling them, to put them back together in better and more suitable organizational models (in terms of appropriateness and stability). Because they are able to cope with the complexities of permanent change and innovation to which (as in case of digital world) present and future organizations seem to be condemned. And yet also (but not only) as a substitute for an increasingly weaker concept of strategy and an alternative question to avoid being swallowed up by the logic of pure calculation of consequences (deterministic rationalism) or simple reactivity - more or less Pavlovian - (bound by external resources), or enclosed in the narrow spaces of a contingent intuitionism of the Bergsonian type (subjectivism and dependence on the constraints of internal resources). Which brings us back to the seminal contribution of Warren Weaver (1948) \u201cComplexity and Science\u201d, in which it deals with \u201cproblems of disorganized and organized complexity\u201d, claiming that the latter ones, \u201care all problems which involve dealing simultaneously with a sizable number of factors which are interrelated into an organic whole\u201d

Surface spin-flop and discommensuration transitions in antiferromagnets

Phase diagrams as a function of anisotropy $D$ and magnetic field $H$ are obtained for discommensurations and surface states for an antiferromagnet in which $H$ is parallel to the easy axis, by modeling it using the ground states of a one-dimensional chain of classical XY spins. A surface spin-flop phase exists for all $D$, but the interval in $H$ over which it is stable becomes extremely small as $D$ goes to zero. First-order transitions, separating different surface states and ending in critical points, exist inside the surface spin-flop region. They accumulate at a field $H'$ (depending on $D$) significantly less than the value $H_{SF}$ for a bulk spin-flop transition. For $H' < H < H_{SF}$ there is no surface spin-flop phase in the strict sense; instead, the surface restructures by, in effect, producing a discommensuration infinitely far away in the bulk. The results are used to explain in detail the phase transitions occurring in systems consisting of a finite, even number of layers.Comment: Revtex 17 pages, 15 figure

Dynamics of supercoiled DNA with complex knots: Large-scale rearrangements and persistent multi-strand interlocking

Knots and supercoiling are both introduced in bacterial plasmids by catalytic processes involving DNA strand passages. While the effects on plasmid organization has been extensively studied for knotting and supercoiling taken separately, much less is known about their concurrent action. Here, we use molecular dynamics simulations and oxDNA, an accurate mesoscopic DNA model, to study the kinetic and metric changes introduced by complex (five-crossing) knots and supercoiling in 2 kbp-long DNA rings. We find several unexpected results. First, the conformational ensemble is dominated by two distinct states, differing in branchedness and knot size. Secondly, fluctuations between these states are as fast as the metric relaxation of unknotted rings. In spite of this, certain boundaries of knotted and plectonemically-wound regions can persist over much longer timescales. These pinned regions involve multiple strands that are interlocked by the cooperative action of topological and supercoiling constraints. Their long-lived character may be relevant for the simplifying action of topoisomerases

Surface spin-flop transition in a uniaxial antiferromagnetic Fe/Cr superlattice induced by a magnetic field of arbitrary direction

We studied the transition between the antiferromagnetic and the surface spin-flop phases of a uniaxial antiferromagnetic [Fe(14 \AA)/Cr(11 \AA]$_{\rm x20}$ superlattice. For external fields applied parallel to the in-plane easy axis, the layer-by-layer configuration, calculated in the framework of a mean-field one-dimensional model, was benchmarked against published polarized neutron reflectivity data. For an in-plane field $H$ applied at an angle $\psi \ne 0$ with the easy axis, magnetometry shows that the magnetization $M$ vanishes at H=0, then increases slowly with increasing $H$. At a critical value of $H$, a finite jump in $M(H)$ is observed for $\psi<5^{\rm o}$, while a smooth increase of $M$ $vs$ $H$ is found for $\psi>5^{\rm o}$. A dramatic increase in the full width at half maximum of the magnetic susceptibility is observed for $\psi \ge 5^{\rm o}$. The phase diagram obtained from micromagnetic calculations displays a first-order transition to a surface spin-flop phase for low $\psi$ values, while the transition becomes continuous for $\psi$ greater than a critical angle, $\psi_{\rm max} \approx 4.75^{\rm o}$. This is in fair agreement with the experimentally observed results.Comment: 24 pages, 7 figure

Directional translocation resistance of Zika xrRNA

xrRNAs from flaviviruses survive in host cells because of their exceptional dichotomic response to the unfolding action of different enzymes. They can be unwound, and hence copied, by replicases, and yet can resist degradation by exonucleases. How the same stretch of xrRNA can encode such diverse responses is an open question. Here, by using atomistic models and translocation simulations, we uncover an elaborate and directional mechanism for how stress propagates when the two xrRNA ends, 5 \u2032 and 3 \u2032, are driven through a pore. Pulling the 3 \u2032 end, as done by replicases, elicits a progressive unfolding; pulling the 5 \u2032 end, as done by exonucleases, triggers a counterintuitive molecular tightening. Thus, in what appears to be a remarkable instance of intra-molecular tensegrity, the very pulling of the 5 \u2032 end is what boosts resistance to translocation and consequently to degradation. The uncovered mechanistic principle might be co-opted to design molecular meta-materials

ψ(2S) production in p-Pb collisions at √sNN = 8.16 TeV

The inclusive production of the charmonium state ψ(2S) was studied at √sNN = 8.16 TeV in proton-lead collisions, using the ALICE detector at the CERN LHC. The measurement is performed in the two centre of mass rapidity ranges, 2.03 < ycms < 3.53 and −4.46 < ycms < −2.96, by reconstructing the ψ(2S) decay to a muon pair. The results are compared to those obtained for the J/ψ at the same centre of mass energy by showing the ratios between the J/ψ and ψ(2S) production cross sections and by studying the nuclear modification factor (RpA) as a function of transverse momentum (pT < 12 GeV/c) and rapidity. The comparison with ψ(2S) results in proton-lead collisions at √sNN = 5.02 TeV and theoretical predictions is also shown

Role of Secondary Motifs in Fast Folding Polymers: A Dynamical Variational Principle

A fascinating and open question challenging biochemistry, physics and even geometry is the presence of highly regular motifs such as alpha-helices in the folded state of biopolymers and proteins. Stimulating explanations ranging from chemical propensity to simple geometrical reasoning have been invoked to rationalize the existence of such secondary structures. We formulate a dynamical variational principle for selection in conformation space based on the requirement that the backbone of the native state of biologically viable polymers be rapidly accessible from the denatured state. The variational principle is shown to result in the emergence of helical order in compact structures.Comment: 4 pages, RevTex, 4 eps figure

Pandemic Data Quality Modelling: A Bayesian Approach = Modellazione della qualit`a dei dati pandemici: un approccio bayesiano

When dealing with pandemics like COVID-19, it is crucial for policymakers to constantly monitor the emergency. Correct data reporting is a hard task during pandemics, and errors affect the overall mortality, resulting in excess deaths in official statistics. In this work, we provide tools for evaluating the quality of pandemic mortality data. We accomplish this through a spatio-temporal Bayesian approach accounting for the bias implicitly contained in the data