Magnetic Excitations in the Spin Gap System KCuCl 3

Single-crystal neutron inelastic scattering was performed in order to investigate the magnetic excitations of KCuCl 3 , which has a singlet ground state with an excitation gap. The constant- Q energy scan profiles are collected at T =8 K for selected reciprocal points in the \(a^{}\)–\(c^{}\) plane which includes the double-chain direction ( a -axis). Dispersion relations along the \(a^\)- and \(c^{}\)-axes are determined. It is suggested that the present system can be magnetically described as a coupled antiferromagnetic dimer system rather than a double spin chain. The lowest excitation energy is determined to be 2.70 meV at Q =(0,0,-1) and (1,0, ±1), the value of which is consistent with our previous evaluation of the excitation gap from the high-field magnetization measurement.