Molecular spin-orbit excitations in the J_eff = 1/2 frustrated spinel   GeCo$_2$O$_4$
  We describe powder and single-crystal inelastic neutron scattering experiments on a spinel-type antiferromagnet GeCo$_2$O$_4$, represented by an effective total angular momentum J_eff = 1/2. Several types of non-dispersive short-range magnetic excitations were discovered. The scattering intensity maps in $\vec{Q}$ space are well reproduced by dynamical structure factor analyses using molecular model Hamiltonians. The results of analyses strongly suggest that the molecular excitations below T_N arise from a hidden molecular-singlet ground state, in which ferromagnetic subunits are antiferromagnetically coupled. The quasielastic excitations above T_N are interpreted as its precursor. A combination of frustration and J_eff = 1/2 might induce these quantum phenomena. 
K. Tomiyasu, M. K. Crawford, D. T. Adroja, P. Manuel, A. Tominaga, S.   Hara, H. Sato, T. Watanabe, S. I. Ikeda, J. W. Lynn,...
cond-mat.str-el