We have determined the magnetic structure of the low-temperature incommensurate phase of multiferroic YMn 2 O 5 using single-crystal neutron diffraction. By employing corepresentation analysis, we have ensured full compliance with both symmetry and physical constraints, so that the electrical polarization must lie along the b axis, as observed. The evolution of the spin components and propagation through the commensurateincommensurate phase boundary identifies the exchange-striction mechanism as the primary driving force for ferroelectricity.

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At large parallel magnetic field B , the ground state of bilayer quantum Hall system forms uniform soliton lattice phase. The soliton lattice will melt due to the proliferation of unbound dislocations at certain finite temperature leading to the Kosterlitz-Thouless (KT) melting. We calculate the KT phase boundary by numerically solving the newly developed set of Bethe ansatz equations, which fully take into account the thermal fluctuations of soliton walls. We predict that within certain ranges of B , the soliton lattice will melt at T KT . Interestingly enough, as temperature decreases, it melts at certain temperature lower than T KT exhibiting the reentrant behaviour of the soliton liquid phase. 75.10.-b, 73.43.-f, 64.60.-i Typeset using REVT E X 1

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