Dynamics of an anisotropic spin dimer system in a strong magnetic field

Recently measured high-field ESR spectra of the spin dimer material ${\mathrm{TlCuCl}}_3$ are described within the framework of an effective field theory. A good agreement between the theory and experiment is achieved, for all geometries and in a wide field range, under the assumption of a weak anisotropy of the interdimer as well as intradimer exchange interaction.