Exact ground states of frustrated quantum spin systems consisting of   spin-dimer units
  We study frustrated quantum spin systems consisting of dimers of spin-1/2 spins. We derive several models that have the exact ground state of the form of the direct product of dimer states. The ground states realized include the product state of dimer singlets, that of dimer triplets with zero magnetization, those of dimer-spin-nematic states, and those of a mixture of the dimer states. Pseudo spin-1/2 operators emerging in each dimer are also introduced. 
Toshiya Hikihara, Takashi Tonegawa, Kiyomi Okamoto, T\^oru Sakai
cond-mat.stat-mech