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We present a numerical study of a topologically disordered, overdamped, array of Josephson junctions subjected to dc and ac currents. We find that vortices are nucleated by the ac current, apart from the ones produced at defects. After a complex transient, these vortices settle into a parity-broken timeperiodic, axisymmetric coherent vortex state characterized by rows of vortices lying along a tilted axis. This locked-in state leads to giant half-integer pseudosteps in the I-V characteristics that are hysteretic in nature. Possible connections of these results to recent experiments are discussed. PACS numbers: 74.50.+r, 74.60.Jg, 85.25.Dq Arrays of Josephson junctions (AJJ) have been the subject of considerable recent interest, both theoretically and experimentally [1]. Many of the early studies in AJJ were aimed at testing the predictions of the equilibrium long-range phase-coherent transitions expected to take place in these systems [2]. More recently, novel phenomena have been discovered in the dynamic responses of the arrays subject to time-dependent periodic probes. Giant Shapiro steps (GSS) in the I-V characteristics of periodic proximity-effect AJJ have been measured in zero [3] and rational frustration magnetic fields [4]. These results have led to a series of numerical and analytic analyses that have for the most part been successful in explaining the GSS results [5,6]. One exception is the recent report of half-integer pseudosteps seen in Nb-Au-Nb AJJ [7].In this paper we present results that yield half-integer pseudosteps that may be related to these experiments.Here we study the dynamics of defect and currentnucleated vortices as a result of a current / a = /dc + /ac xsin(2>rw), applied uniformly from the bottom of the AJJ along the y direction. The formation of vortex pairs by defects in a dc current at zero temperature was studied recently by Leath and Xia [8]. These authors emphasize the formation of these excitations as an example of breakdown phenomena. The new element in our investigation is that of adding an ac current to the study of the dynamics of the defect-generated vortices in AJJ. It turns out that adding an ac current changes the physics in a fundamental way as we shall discuss below. We have studied the dynamics of the AJJ when we have one, two, three, or all the lattice sites randomly displaced from their periodic positions. We have found that after a transient time that depends on the number of defects, the dynamics tends to a nonequilibrium time-periodic state with a well-defined vortex geometric structure that is qualitatively and to some extent quantitatively independent of the nature and number of defects in the system. This novel state leads to half-integer giant pseudosteps in the I-V characteristic. This state is in many respects different in nature from the GSS mentioned above and we shall call it an axisymmetric coherent vortex state (ACVS). Since the existence of the ACVS does not appear to depend qualitatively on the number of defects in the lattice, most of our discussion...

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This paper gives an overview of the experiments on-board the International Space Station (ISS) performed so far by the CETSOL team. Al-7 wt% Si alloys with and without grain refiners were solidified in microgravity. Detailed grain structure analysis showed columnar growth in case of non-refined alloy, but the existence of a columnar to equiaxed transition (CET) in refined alloy. One main result is a sharp CET when increasing the solidification velocity and a progressive CET for lowering the temperature gradient. Applying a front tracking model this behavior was confirmed numerically for sharp CET. Using a CAFE model both segregation and grain structures were numerically modeled and show a fair agreement with the experimental findings.

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