μSR study on the spin singlet state in theS= 1/2 cluster magnet GaNb₄S₈

GaNb4S8 is a S = 1/2 Nb-4-cluster magnet exhibiting a structural phase transition at T-s = 32 K. We have investigated the magnetic ground state of the compound by means of the zero-field (ZF) and the longitudinal-field (LF) mu SR techniques. In the ZF-mu SR spectra, no muon spin precession was observed below T-s, revealing an absence of magnetic long-range orders. Instead, a stretched exponential relaxation was observed, which remains under the LF up to 3950 G. The field dependence of the relaxation rate obeys the Redfield's equation, indicating that a dynamic component of electronic spin fluctuations is dominant below T-s. These mu SR results can be understood as sporadic dynamics of spin fluctuations in a spin singlet sea and support the spin singlet state of GaNb4S8 due to the formation of Nb-8 octamer from two Nb-4 tetramers.