Radio-frequency muon spin resonance of Haldane-gap systems, Ni(C2H8N2)2NO2(ClO4) and NiC2O42·(2-methylimidazole)

In quantum spin systems with a singlet ground state, an enhancement of the muon spin relaxation was frequently observed at low temperatures, though the ground state is non-magnetic. In order to clarify the origin of this enhancement, we have measured the muon spin resonance with radio-frequency field (RF-mu SR) for Haldane-gap systems, Ni(C2H8N2)(2) NO2(ClO4) (NENP) and NiC(2)O(4)2 .(2-methylimidazole). Possible origins of an enhancement of the muon spin relaxation at low temperatures are discussed. (C) 2000 Elsevier Science B.V. All rights reserved.