Magnetic excitations and exchange interactions in the spin-gap system TlCuCl3

The magnetic excitations from the gapped ground state in TlCuCl3 have been investigated by means of inelastic neutron scattering experiments. The excitation data were collected along four different directions in the a*-c* plane. A well-defined single magnetic excitation mode was observed. The lowest excitation occurs at Q=(h,0,l) with integer h and odd l, as observed in KCuCl3. The dispersion relations were analyzed by the cluster-series expansion up to the sixth order, so that the individual exchange interactions were evaluated. It was demonstrated that TlCuCl3 is a strongly coupled spin-dimer system. (orig.)