Quantum spin chain systems with singlet ground state, ACu2(PO4)2 (A=Sr and Ba)

The magnetic susceptibilities of SrCu2(PO4)2 and BaCu2(PO4)2 with Cu2+-ion chains composed of four-spin zigzag units approach zero at 0 K after correction for paramagnetic components which is suggestive of a singlet ground state. A four-spin cluster model and an alternating chain model were applied to the Sr compound and the Ba compound, respectively. Muon spin relaxation varies little with temperature. On cooling, the resonance field of 31P NMR shifts with temperature in the Sr compound, whereas in the Ba compound the resonance peak does not shift with temperature but broadens at low temperatures. The energy gap was evaluated to be 47 K for the Sr compound from the temperature dependence of T1.The difference in magnetism of the two compounds may be attributed to the difference in the intercluster interaction