Prof. Karlo Penc (Research Institute for Solid State Physics and Optics, Hungary)
"Role of the Dzyaloshsinkii-Moriya -interactions on the phase diagram and excitations in SrCu2(BO3)2"
Using bosons to describe the states of a two spin-1/2 dimer, we extend the usual spin-wave theory to get the excitation spectrum of SrCu$_2$(BO$_3$)$_2$, a spin--gap system constituted of orthogonal dimers. We are interested in the role of Dzyaloshinskii-Moriya (DM) interactions that are inherently present in this material. In particular, we find that in the absence of the intradimer DM interaction, the interdimer DM interaction perpendicular to the plane stabilizes a O(2) symmetric phase above the spin gap phase. Increasing the field, an additional $Z_2$ symmetry breaking appears. The introduction of the intradimer DM interaction lifts the O(2) degeneracy and introduces a small gap and, depending on the details, a $Z_2$ broken phase even at low fields. Concerning the excitation spectrum, we compare our results with the neutron scattering measurements of the dispersion at zero magnetic field, and with the low field behavior of the ESR spectra.