Magnetic Excitations of the Spin Gap System TlCuCl${}_3$

Abstract

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-$Q$ energy scan profiles were collected in the $a^{\ast }-c^{\ast }$ 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 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.

20 pages, 6 figures, 12 eps files, revtex style