Direct observation of the energy gap generating the 1/3 magnetization plateau in the spin- 1/2 trimer chain compound Cu3 (P2 O6 OD)2 by inelastic neutron scattering measurements

We studied magnetism of Cu3 (P2 O6 OD)2 using inelastic neutron scattering measurements on a powder sample. This cuprate has spin- 1/2 trimer chains with J1 - J2 - J2 interactions, where J1 and J2 denote two antiferromagnetic (AF) exchange interaction parameters, showing a 1/3 magnetization plateau. We observed an energy gap of 9.8 meV (114 K), which generated the magnetization plateau. The gap corresponds to singlet-triplet-like excitation of an AF dimer formed by the dominant J1 interaction. We determined the AF exchange interaction parameters J1 =111 K and J2 =30 K, which can reproduce the gap value as well as the magnetization.