  TlCuCl$_3$ has a singlet ground state with the excitation gap $\Delta =0.65$
meV. The magnetic excitations in TlCuCl$_3$ have been investigated by means of
neutron inelastic scattering experiments. The constant-$\it{\pmb Q}$ 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
${\it{\pmb Q}}=(h, 0, l)$ with integer $h$ and odd $l$, as observed in
KCuCl$_3$. 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 TlCuCl$_3$ is a strongly coupled spin-dimer
system.

Strongly Correlated Electrons