The effects of intra-dimer Dzyaloshinsky-Moriya interaction on the properties of SrCu2(BO3)2 in an external magnetic field

The two-dimensional spin-gap system SrCu2(BO3)(2) shows unique physical properties due to the strong quantum fluctuations that follow from its low-dimensionality and its very special topology. The magnetic properties of this material, e.g. dimer singlet ground state, an almost localized triplet and magnetization plateaux, are well described by a spin-1/2 antiferromagnetic Heisenberg model on the orthogonal dimer lattice. Recently, an unusual angular dependence of the shifts at the B sites with the external magnetic field rotated in the (110) plane, which indicates the presence of a field-induced staggered moment, has been observed by nuclear magnetic resonance. Such a behaviour cannot be explained by the Heisenberg model alone. We include an intra-dimer Dzyaloshinsky-Moriya interaction as well as an anisotropic gyromagnetic tensor in the orthogonal dimer model, and we show that the observed field-induced staggered moment can be very well described with physically reasonable values of the parameters.