Magnetic susceptibility, Cu NQR, and high-filed magnetization have been measured in polycrystalline SrCu 2 ͑BO 3 ͒ 2 having a two-dimensional (2D) orthogonal network of Cu dimers. This cuprate provides a new class of 2D spin-gap system ͑D 30 K͒ in which the ground state can be solved "exactly." Furthermore, in the magnetization, two plateaus corresponding to 1 4 and 1 8 of the full Cu moment were first observed for 2D quantum spin systems. [S0031-9007(99)08878-X]

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Multicellular organisms achieve greater complexity through cell divisions that generate different cell types. We engineered a simple genetic circuit that induces asymmetric cell division and subsequent cell differentiation in Escherichia coli . The circuit involves a scaffolding protein, PopZ, that is stably maintained at a single cell pole over multiple asymmetric cell divisions. PopZ was functionalized to degrade the signaling molecule, c-di-GMP. By regulating synthesis of functionalized PopZ via small molecules or light, we can chemically or optogenetically control the relative abundance of two distinct cell types, characterized by either low or high c-di-GMP levels. Differences in c-di-GMP levels can be transformed into genetically programmable differences in protein complex assembly or gene expression, which in turn produce differential behavior or biosynthetic activities. This study shows emergence of complex biological phenomena from a simple genetic circuit and adds programmable bacterial cell differentiation to the genetic toolbox of synthetic biology and biotechnology.

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The results of magnetic susceptibility, magnetization, electrical resistivity and specific heat measurements performed on Ho 3 Co single crystals show that this compound exhibits two different antiferromagnetic structures: AF II at 8 K < T < 22 K and AF I below T t ≈ 8 K. Below the Néel temperature T N = 22 K the application of a magnetic field along the main crystallographic directions induces magnetic phase transitions which are accompanied by giant magnetoresistance. At T < T t the field-induced phase transitions along the c-and b-axes are found to be irreversible, and a small ferromagnetic component is observed along the a-axis. These peculiarities are associated with the non-Kramers character of the Ho ion and with the presence of a complex incommensurate magnetic structure of Ho 3 Co below T N . The temperature coefficient of the electrical resistivity for Ho 3 Co above T N over a wide temperature range is found to differ from that observed for other R 3 Co compounds. Such a behaviour is attributed to the presence of an additional contribution to the conduction electron scattering by spin fluctuations induced by f-d exchange in the itinerant d-electron subsystem. The value of this extra contribution and its temperature range is suggested to depend on the spin value of the R ion. The excess of the effective magnetic moment per R ion, which is observed in Ho 3 Co and in other R 3 M type compounds, is also attributed to spin fluctuations induced by f-d exchange.

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