Magnetic excitations in the spin-gap systemTlCuCl3

${\mathrm{TlCuCl}}_3$ has a singlet ground state with the excitation gap $\Delta =0.65\mathrm{}\hspace{0.5em}\mathrm{meV}.$ The magnetic excitations in ${\mathrm{TlCuCl}}_3$ have been investigated by means of neutron inelastic-scattering experiments. The constant-$\mathit{Q}$ 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 $\mathit{Q}=(h,0,l)\mathrm{}$ with integer h and odd l, as observed in ${\mathrm{KCuCl}}_3.$ 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 ${\mathrm{TlCuCl}}_3$ is a strongly coupled spin-dimer system.