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

How to fold intricately: using theory and experiments to unravel the properties of knotted proteins

Over the years, advances in experimental and computational methods have helped us to understand the role of thermodynamic, kinetic and active (chaperone-aided) effects in coordinating the folding steps required to achieving a knotted native state. Here, we review such developments by paying particular attention to the complementarity of experimental and computational studies. Key open issues that could be tackled with either or both approaches are finally pointed out

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

Shell and spatial structures: Between new developments and historical aspects

Advanced structural systems are more and more devoted to light, versatile, eco-sustainable structures. This goal can be achieved through the use of new materials and new approaches for structural optimization, form finding, design, and validation. Shell and spatial structures are representative of some of the most efficient structural systems in which the optimized use of materials is combined with effective structural forms and shapes. The ongoing development of analysis methods, design approaches and construction techniques of shell and spatial structures has resulted in an increasing interest from engineers, architects, and builders. This Special Issue is devoted to papers coming from a call principally addressed to the participants of the 1st Italian Workshop on Shell and Spatial Structures (https://sites.google.com/view/iwss2020/home) held online the last June 2020 after the lockdown restriction due to the Covid-19 pandemic (Figure 1). The experience of the first IWSS (IWSS2020) was particularly innovative. It brought together the interests of the Italian and the international community devoted to the study and applications of shell and spatial structures. The IWSS received two significant endorsements, from the IASS (www.iassstructures.org) and from the SISCO (www. siscoscienzadellecostruzioni.org)

Engineering characterization of the novel Bach impeller for bioprocessing applications requiring low power inputs

The choice of impeller design in stirred-tank reactors (STRs) affects important process characteristics such as the suspension of particulates, mixing of nutrients or gasses, and the distribution of turbulent kinetic energy and its dissipation rate. A novel impeller, the Bach impeller, has been characterized in this work and its novel geometry and mechanism of action are discussed in detail. Experimental measurements of the power consumption, mixing time, and microparticles suspension dynamics were carried out at the 1 L scale in an unbaffled reactor and the performance compared with two conventional up-pumping axial flow impellers, namely the 3-Blade segment (3BS, also called “elephant ear”) and marine impeller. In addition, the impeller flow number and pumping efficiency were obtained from Particle Image Velocimetry experiments. The Bach impeller exhibited a low power number of NP=0.36 and showed it can generate a uniform suspension of microcarriers even at relatively low impeller speeds (from N=25–30 rpm onwards). The impeller was found to effectively mix the reactor volume within tM=25 s at N>50 rpm (i.e. NtM=37±5) at a higher clearance when it was mounted closer to the free surface, C/T>0.5, while minimally affecting its suspension capabilities. Lastly, the Bach impeller displaced more fluid than the marine impeller and roughly an equal amount to the 3BS impeller, suggesting that the novel impeller can successfully be used for STR processes in the (bio)chemical engineering industry where low power inputs, but high mixing and particle suspension efficiencies are sought

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

Body composition symmetry in long-term active middle-aged and older individuals

This study aimed to analyze body composition and strength symmetry in a sample of 165 middle-aged and elderly Italian volunteers, which included 97 active (67 men and 30 women; 61.17 ± 7.56 years) individuals regularly engaged in Tai Chi Chuan, tennis, or running, and a control group of 59 age-matched sedentary (27 men and 32 women) individuals. Anthropometric and bioelectrical measurements and hand grip strength of both sides were collected. Segmental body composition was analyzed through specific bioelectrical impedance vector analysis. The body composition of the right and left limbs was similar among active individuals (arms: T2 = 6.3, n.s.; legs: T2 = 5.0, n.s.), with a similar pattern in the three different disciplines. By contrast, the control group showed bilateral asymmetry (arms: T2 = 6.8, p &lt; 0.001; legs: T2 = 8.8, p &lt; 0.001), mainly because of the higher values of specific reactance (t = 2.4; p = 0.018) and phase angle (t = 2.0; p = 0.054) in the dominant arm, and the higher specific vector length (t = −3.0; p = 0.027) in the left leg. All of the groups showed a higher hand grip strength in the dominant arm (active: t = 7.0, p &lt; 0.001; control: t = 2.9; p &lt; 0.01). In conclusion, the active individuals showed stronger body composition symmetry than the controls, thus indicating a previously undetected positive effect of sport in middle-aged and older adults

Peaked and low action solutions of NLS equations on graphs with terminal edges

We consider the nonlinear Schroedinger equation with focusing power-type nonlinearity on compact graphs with at least one terminal edge, i.e., an edge ending with a vertex of degree 1. On the one hand, we introduce the associated action functional, and we provide a profile description of positive low action solutions at large frequencies, showing that they concentrate on one terminal edge, where they coincide with suitable rescaling of the unique solution to the corresponding problem on the half-line. On the other hand, a Lyapunov-Schmidt reduction procedure is performed to construct one-peaked and multipeaked positive solutions with sufficiently large frequency, exploiting the presence of one or more terminal edges