Ground States of Double Spin Chain Systems TlCuCl₃, NH₄CuCl₃ and KCuBr₃

Abstract

The magnetic susceptibilities of S=1/2 double spin chain systems, TlCuCl₃, NH₄CuCl₃ and KCuBr₃, which are isomorphous with KCuCl₃ have been measured using single crystals and polycrystals. The exchange interactions in a double chain can be described by a Heisenberg spin ladder with an additional diagonal interaction. It is found for TlCuCl₃ and KCuBr₃ that with decreasing temperature the susceptibilities for different external field directions exhibit broad maxima at 38, K and 130, K, respectively, and decrease exponentially to zero as observed in KCuCl₃. This result indicates that their ground state is a spin singlet with an excitation gap. This behavior is attributed to the quantum effect characteristic of the spin ladder system. For NH₄CuCl₃, the susceptibility of polycrystals displays a round maximum at 4.5, K and decreases with a tendency toward zero, which implies the nonmagnetic ground state.