Magnetic excitations in the spin-gap system TlCuCl3

TlCuCl3 has a singlet ground state with the excitation gap Delta = 0.65 meV. The magnetic excitations in TlCuCl3 have been investigated by means of neutron inelastic-scattering experiments. The constant-Q energy scan profiles were collected in the a*-c* plane. A well-defined single magnetic excitation mode was observed. The dispersion relations along four different directions were determined. The lowest excitation occurs at Q = (h,0,l) with integer h and odd l, as observed in KCuCl3. A cluster series expansion up to the sixth order was applied to analyze the dispersion relations, and the individual exchange interactions were evaluated. It was demonstrated that TlCuCl3 is a strongly coupled spin-dimer system.