Magnetic excitations in weakly coupled classical and quantum spin systems

Magnetic excitations in the ordered state of Cu$_2$Fe$_2$Ge$_4$O$_{13}$ is studied by inelastic neutron scattering technique. Low energy excitations up to 10 meV is well explained by coupled classical chains of Fe$^{3+}$. Checkerboard like intensity patterns in wide reciprocal space is reproduced by spin-wave theory including magnetic non-Bravis lattice. The distinct narrow-band excitation at 25 meV is identified as quantum dimer excitation of Cu$^{2+}$. The cooperative ordering at low temperature is a result of weakly coupling between classical and quantum entities in this compound. This work was carried out under DOE Contract No. DE-AC05-00OR22725.