Ground States of Double Spin Chain Systems TlCuCl_<b>3</b>, NH_<b>4</b>CuCl_<b>3</b>and KCuBr_<b>3</b>

Abstract

The magnetic susceptibilities of S =1/2 double spin chain systems, TlCuCl 3 , NH 4 CuCl 3 and KCuBr 3 , which are isomorphous with KCuCl 3 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 TlCuCl 3 and KCuBr 3 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 KCuCl 3 . 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 NH 4 CuCl 3 , 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.