Raman scattering in the quarter-filled spin ladder system ${\alpha }^{\prime }-{\mathrm{NaV}}_2{\mathrm{O}}_5$ shows in the dimerized singlet ground state $(T<~T_{\mathrm{SP}}=35\mathrm{}\hspace{0.5em}\mathrm{K})$ an unexpected sequence of three magnetic bound states. Our results suggest that the recently proposed mapping onto an effective spin chain for $T>\mathrm{}{T}_{\mathrm{SP}}$ has to be given up in favor of the full topology and exchange paths of a ladder in the dimerized phase for $T<\mathrm{}{T}_{\mathrm{SP}}.$ As the new ground state we propose a dynamic superposition of energetically nearly degenerate dimer configurations on the ladder.

• Received 8 September 1998

DOI:https://doi.org/10.1103/PhysRevB.58.14159