μsR study on quantum spin system NH₄CUCl₃

The quantum spin dimer system NH4CuCl3 has a gapless ground state with TN 1.3 K, and shows plateaus in the high-field magnetization at 1/4 and 3/4 of the saturation magnetization, the mechanism of which has not yet been resolved until now. The three dimer model recently proposed by Matsumoto, which seems well accounting for the emergence of plateaus, predicts the existence of three magnetically-inequivalent dimers. In order to test the model, we have measured transverse-field (TF) muon depolarization curves. In fourier transform of TF muon depolarization curves, we observed homogeneous single peak at T > 100 K. Below 70 K, the peak showed rapid broadening, and split into multiple peaks in still lower temperatures, supporting the idea of the three dimer subsystems.