Z${}_2$ topological number of local quantum clusters in the orthogonal dimer model

We have studied the $Z_2$ topological number defined by the Berry phase for the gapped frustrated systems including the orthogonal dimer model which has a direct product state of local quantum clusters as the exact ground state. The $Z_2$ topological number can clarify what kind of the local quantum clusters is formed to lift the macroscopic degeneracy due to frustration, even when the exact ground state is unknown. As a demonstration, the dimer-singlet and plaquette-singlet phase are identified by two kinds of Z${}_2$ topological numbers in the Shastry-Sutherland model and its generalization realized experimentally as SrCu${}_2$(BO${}_3$)${}_2$ and CaV${}_4$O${}_9$.