E. Altshuler

Symmetry Breaking in Escaping Ants

The phenomenon of herding is a very general feature of the collective behavior of many species in panic conditions, including humans. It has been predicted theoretically that panic‐induced herding in individuals confined to a room can produce a nonsymmetrical use of two identical exit doors. Here we demonstrate the existence of that phenomenon in experiments, using ants as a model of pedestrians. We show that ants confined to a cell with two symmetrically located exits use both exits in approximately equal proportions to abandon it in normal conditions but prefer one of the exits if panic is created by adding a repellent fluid. In addition, we are able to reproduce the observed escape dynamics in detail using a modification of a previous theoretical model that includes herding associated with a panic parameter as a central ingredient. Our experimental results, combined with theoretical models, suggest that some features of the collective behavior of humans and ants can be quite similar when escaping under panic.

Flux trapping in transport measurements of YBa2Cu3O7-x superconductors: A fingerprint of intragrain properties

Abstract A study of flux trapping in polycrystalline YBa 2 Cu 3 O 7- x was made through measurements of transport J c versus H characteristics. It was shown that most of the trapped flux which provokes hysteresis in such experiments comes from the superconducting grains, and not from persistent loops in the weak link network. The lower critical field of the grains and the intragrain critical current were calculated for oxygenated and partially deoxygenated samples, showing good coincidence with the commonly accepted values. Further evidence of the intragranular character of the flux trapping resulted from the study of the time variation of the voltage associated with flux creep, from which good estimates of the effective pinning energy of the grains were obtained.

Generation of Jc(He) hysteresis curves for granular YBa2Cu3O7-δ superconductors☆

Abstract The experimental hysteretic behaviour of the transport critical current density J c ( H e ) observed in granular YBa 2 Cu 3 O 7−δ has been compared with the analogous theoretical curves resulting from a well known model based on the effect of flux trapped by the superconducting grains at the intergranular junctions. The magnetization of the grains was calculated for different values of maximum applied field H m using the Bean model; at the same time the statistical distribution of the qualities of the junctions was taken into account. This approach was seen to predict the evolution of hysteretic J c ( H e ) curves for different values of H m with reasonable accuracy.

Hysteresis in the Jc(Ba) dependence of (Bi-Pb)-Sr-Ca-Cu-O polycrystalline superconductors

The hysteretic behavior in the Jc(Ba) dependence for (Bi1.64Pb0.36)Sr2Ca2Cu3Oy polycrystalline samples at low magnetic fields (Ba<30 mT) has been studied in detail. The standard four-probe technique was used to measure the critical current density. It was shown that the samples exhibit values of Jc (77 K) ranging from 100 to 400 A/cm2. In all the cases, a special behavior of the Jc(Ba) dependence in decreasing magnetic fields was observed as compared with the Y-Ba-Cu-O system. An alternative explanation is given using models based on intragranular flux trapping.

The resistive transition of (Hg0.85Re0.15)(Ba1-ySry)2Ca2Cu3O8+δ superconducting polycrystals

The resistive transition of (Hg0:85Re0:15)(Ba1� ySry)2Ca2Cu3O8þd superconducting polycrystals, with y ¼ 0:00, 0.17, 0.20 and 0.28 has been experimentally studied at different magnetic fields ( 6 500 Oe), being the Arrhenius plot char- acterized by two linear regions with different slope in the resistive tail. The crossover temperature between them depends on the Sr content. The dependence of the intergranular pinning energy with the magnetic field, for both regions, follows a power law, being the exponent dependent on the Sr content in a non-linear way. The intergranular irreversibility line benefits from the Sr doping, associated with the improvement of the transport properties (even in the normalstate) with the increment of Sr. The contribution of fluctuations to the conductivity is calculated from the measurement of re- sistivity in the interval of 130-300 K, and the interlayer coupling energy is obtained. From these calculations, an es- timation has been made of the interlayer distance, finding an increment with Sr content. Our study suggests that the Sr can improve the performance of Hg-based HTc polycrystals for transport applications, which are basically determined by intergranular properties. 2002 Elsevier Science B.V. All rights reserved.

Flow-controlled densification and anomalous dispersion of E. coli through a constriction

Dispersion and migration of bacteria under flow in tortuous and confined structures such as porous or fractured materials is related to a large spectrum of practical interest, but is still poorly understood. Here, we address the question of transport and dispersion of an E. coli suspension flowing through a micro-fluidic channel with a funnel-like constriction in its center. We show a counter-intuitive symmetry breaking of the bacterial concentration, which increases significantly past the funnel. This concentration enhancement persists over large distances from the funnel and disappears at large flow rate values. We map our results onto a one dimensional convection–diffusion equation predicting quantitatively the experimental results, without free parameters, when a conservative non-local source term is introduced. This last term, measured experimentally, represents a long range memory effect due to the unbalance of wall adsorption and desorption processes past the constriction. Our model experiment points out the generic importance of considering such constriction effects in the description of transport properties of active matter in porous media. It also opens the possibility to control the concentration of bacterial suspensions in micro-fluidic channels by simply tuning the flow intensity or direction.

Hysteresis of the critical current density in YBCO, HBCCO and BSCCO superconducting polycrystals: a comparative study

Abstract The hysteresis of the transport critical current of YBa 2 Cu 3 O 7− δ (Hg,Re)Ba 2 Ca 2 Cu 2 O 8+ δ and (BiPb) 2 Sr 2 Ca 2 Cu 3 O 10+ δ polycrystals was studied. While the first two systems display qualitatively similar hysteretical patterns, the third one behaves differently. The experimental results are interpreted in terms of a phenomenological model which considers the effects of the magnetization in the grains on the intergranular weak links as responsible for the observed hysteresis. The hysteresis of the transport critical current is discussed on the light of the microstructural features of the samples, concluding that the shape and disposition of the grains do not account in a straightforward fashion for the differences in the hysteretical behavior.

Magnetic hysteresis of the zero-resistance critical temperature in YBaCuO, BiSrCaCuO and HgBaCaCuO superconducting polycrystals

Abstract A comparative study of the hysteretical behavior of the zero resistance critical temperature for YBa 2 Cu 3 O 7− δ , (BiPb) 2 Sr 2 Ca 2 Cu 3 O 10+ δ and (HgRe)Ba 2 Ca 2 Cu 3 O 8+ δ superconducting ceramics was performed at low magnetic fields (under 400 Oe). The results were compared with those obtained through the magnetic hysteresis of the critical current density, showing good agreement in most cases. The qualitative explanation of the observed hysteresis in terms of intragranular flux trapping of the superconducting grains satisfies YBaCuO and BiSrCaCuO materials, but modifications might be necessary in the case of HgBaCaCuO.

Vortex avalanches with robust statistics observed in superconducting niobium

In 1993 Tang proposed [1] that vortex avalanches should produce a self organized critical state in superconductors, but conclusive evidence for this has heretofore been lacking. In the present paper, we report extensive micro-Hall probe data from the vortex dynamics in superconducting niobium, where a broad distribution of avalanche sizes scaling as a power-law for more than two decades is found. The measurements are combined with magneto-optical imaging, and show that over a widely varying magnetic landscape the scaling behaviour does not change, hence establishing that the dynamics of superconducting vortices is a SOC phenomenon.

Sandpile formation by revolving rivers.

Experimental observation of a new mechanism of sandpile formation is reported. As a steady stream of dry sand is poured onto a horizontal surface, a pile forms which has a thin river of sand on one side flowing from the apex of the pile to the edge of its base. The river rotates about the pile, depositing a new layer of sand with each revolution, thereby causing the pile to grow. For small piles the river is steady and the pile formed is smooth. For larger piles, the river becomes intermittent and the surface of the pile becomes undulating. The essential features of the system that produce the phenomenon are discussed, and the robustness of the phenomena is demonstrated with experiments using different boundary conditions and sands.