Singlet ground state and energy gap in double spin chain system KCuCl₃ family

Magnetic properties of S = 1/2 double spin chain systems KCuCl₃ and TlCuCl₃ have been investigated by susceptibility, high-field magnetization process and ESR measurements using single crystals. The present systems can be described as a Heisenberg spin ladder with an additional interaction. It is found that the susceptibilities of both the compounds exhibit broad maxima and decrease exponentially to zero with decreasing temperature. This result indicates that the ground state of the present system is a spin singlet and there is a finite gap in the excitation spectrum. The transition to the magnetic state is clearly observed by the high-field magnetization process. ESR absorption due to the excitation from the singlet ground state to the excited state is observed for TlCuCl₃.