Topology by Design in Magnetic nano-Materials: Artificial Spin Ice

Artificial Spin Ices are two dimensional arrays of magnetic, interacting nano-structures whose geometry can be chosen at will, and whose elementary degrees of freedom can be characterized directly. They were introduced at first to study frustration in a controllable setting, to mimic the behavior of spin ice rare earth pyrochlores, but at more useful temperature and field ranges and with direct characterization, and to provide practical implementation to celebrated, exactly solvable models of statistical mechanics previously devised to gain an understanding of degenerate ensembles with residual entropy. With the evolution of nano--fabrication and of experimental protocols it is now possible to characterize the material in real-time, real-space, and to realize virtually any geometry, for direct control over the collective dynamics. This has recently opened a path toward the deliberate design of novel, exotic states, not found in natural materials, and often characterized by topological properties. Without any pretense of exhaustiveness, we will provide an introduction to the material, the early works, and then, by reporting on more recent results, we will proceed to describe the new direction, which includes the design of desired topological states and their implications to kinetics.Comment: 29 pages, 13 figures, 116 references, Book Chapte

Study of kinetic parameters for the production of recombinant rabies virus glycoprotein

Gene expression in insect cells is an advantageous system for recombinant protein production, mainly because of its capacity to produce complex proteins with correct post-translational modifications. Recently, we identified and purified a protein from Lonomia obliqua hemolymph able to increase the production of rabies virus glycoprotein, expressed in Drosophila melanogaster cells, by about 60%. In this work, the kinetic parameters for cell growth and recombinant rabies virus glycoprotein production were determined in cultures of transfected Drosophila melanogaster Schneider 2 (S2) cells expressing recombinant rabies virus glycoprotein (rRVGP), enriched and non-enriched with the hemolymph of Lonomia obliqua (Hb). The highest concentration of rRVGP was achieved at the beginning of the culture enriched with Hb, indicating that the cells produce greater amounts of rRVGP per cell (specific rRVGP concentration) at the early exponential growth phase. After day 8, a decrease in the concentration of rRVGP (ng/mL) was observed, probably due to protein decomposition. The average specific rRVGP production rate (μrRVGP) was 30 ng rRVGP/107cell.day, higher than that observed in the non-enriched culture

Classical topological order in the kinetics of artificial spin ice

Systems of interacting nanomagnets known as artificial spin ice 1-4 have allowed the design, realization and study of geometrically frustrated exotic collective states 5-10 that are absent in natural magnets. We have experimentally measured 11,12 the thermally induced moment fluctuations in the Shakti geometry of artificial spin ice. We show that its disordered moment configuration is a topological phase described by an emergent dimer-cover model 13 with excitations that can be characterized as topologically charged defects. Examination of the low-energy dynamics of the system confirms that these effective topological charges have long lifetimes associated with their topological protection, that is, they can be created and annihilated only as charge pairs with opposite sign and are kinetically constrained. This manifestation of classical topological order 14-19 demonstrates that geometrical design in nanomagnetic systems can lead to emergent, topologically protected kinetics that can limit pathways to equilibration and ergodicity