Anomalous magnetic field dependence of muon spin relaxation in S=12 double-chain system KCuCl3 and TlCuCl3

In mu(+)SR studies on the S = 1/2 double-chain systems KCuCl3 and TlCuCl3, which are thought to have a spin singlet ground state with finite spin gaps, we found that the muon spin becomes strongly depolarized below 20 and 10 K, respectively. We observed an increase in the relaxation rate with increasing longitudinal magnetic fields at 4.8 K in KCuCl3. At 1.8 K, we observed no decrease of the relaxation rate with increasing longitudinal fields. However, the shapes of these spectra cannot be represented by using usual formulae. Such an anomalous behavior was not seen in TlCuCl3, We discuss the origin of the muon spin relaxation and difference between the properties of KCuCl3 and TlCuCl3. (C) 2000 Elsevier Science B.V. All rights reserved.