Magnetic Excitations of the Spin Gap System TlCuCl₃

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TlCuCl₃ has a singlet ground state with the excitation gap Δ=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.