Magnetic bound states in the quarter-filled ladder system   $\alpha'-NaV_{2}O_{5}$}
  Raman scattering in the quarter-filled spin ladder system alpha'-NaV_2O_5 shows in the dimerized singlet ground state ($T \leq T_{SP}=35K$) an unexpected sequence of three magnetic bound states. Our results suggest that the recently proposed mapping onto an effective spin chain for $T > T_{SP}$ has to be given up in favor of the full topology and exchange paths of a ladder in the dimerized phase for $T < T_{SP}$. As the new ground state we propose a dynamic superposition of energetically nearly degenerate dimer configurations on the ladder. 
P. Lemmens, M. Fischer, G. Els, G. G\"untherodt, A.S. Mishchenko, M.   Weiden, R. Hauptmann, C. Geibel, F. Steglich