We present a detailed analysis of light scattering experiments performed on the quarter-filled spin ladder compound α ′ -NaV 2 O 5 for the temperature range 5 K≤T≤300 K. This system undergoes a phase transition into a singlet ground state at T=34 K accompanied by the formation of a super structure.

Observation of three-magnon light scattering in CuGeO3 Els, G.; Loosdrecht, P.H.M. van; Lemmens, P.; Vonberg, H.; Güntherodt, G.; Uhrig, G.S.; Fujita, O.; Akimitsu, J.; Dhalenne, G.; Revcolevschi, A.

Dedicated to Prof. J. Zittartz, on the occasion of his 60th birthday Abstract. -Polarized inelastic light scattering experiments on Cu1−xZnxGeO3 (0 ≤ x ≤ 0.045) single crystals show for x = 0 a new distinct mode at nearly half the energy of the singlet response below the spin-Peierls transition. The temperature, magnetic field, polarization, and doping dependences of this mode are similar to those of the singlet bound state. The data are interpreted in terms of a spinon-assisted light scattering process. Position and form of the peak provide strong evidence for the presence of dopant-bound spinons in Cu1−xZnxGeO3.

Raman scattering in the quarter-filled spin ladder system α ′ -NaV2O5 shows in the dimerized singlet ground state (T≤TSP =35K) an unexpected sequence of three magnetic bound states. Our results suggest that the recently proposed mapping onto an effective spin chain for T>TSP has to be given up in favor of the full topology and exchange paths of a ladder in the dimerized phase for T < TSP. As the new ground state we propose a dynamic superposition of energetically nearly degenerate dimer configurations on the ladder.Low-dimensional quantum spin systems like spin chains, ladders or plaquettes received considerable attention from both theoretical and experimental points of view due to their manifold of unconventional spin excitation spectra. Of particular interest is the so-called spin-Peierls transition. The degeneracy of the ground state of an isotropic one-dimensional (1d) spin-1/2 system is lifted due to the coupling to the lattice, leading to a dimerized singlet ground state and the opening of a singlet-triplet gap.The magnetic excitation spectrum of a dimerized spin chain consists of a triplet branch at ∆ 01 , a corresponding two-particle continuum of triplet excitations starting at 2∆ 01 , and well defined magnetic bound states 1-3 . The latter consist of strongly interacting triplet excitations with a high energy cutoff at 2∆ 01 . The existence of a singlet bound state at √ 3∆ 01 is predicted by a 1d model with frustrated next-nearest neighbor interaction J 2 = 0.24J 1 , acting as a binding potential. A further triplet bound state should appear for higher values of J 2 2 . A study of magnetic bound states in quantum spin systems therefore gives valuable insight into the low energy spin excitations which govern the physics of these systems.Half-filled spin ladder systems attracted enormous interest recently due to the surprising changes of the ground state and excitation spectrum interpolating between one-and two-dimensional quantum spin systems 4 . For even-leg ladders the doping by holes is found to lead to superconductivity. On the other hand, for the quarterfilled ladder system as the corresponding diluted system the ground state and spin excitation spectrum are not well understood until now. However it is expected that they show a similar rich excitation scheme as the halffilled system 5 . The inorganic compound α ′ -NaV 2 O 5 with double chains of edge-sharing distorted tetragonal VO 5 and a spin-Peierls like transition at T SP =35K 6,7 has been interpreted as a quarter-filled spin ladder system 8 . Recent X-ray diffraction data at room temperature are in favor of a centrosymmetric (D 13 2h ) structure with only one type of V site of average valence +4.5. The spins are therefore attached to a V-O-V molecular orbital on the rungs 8-10 . The exchange interaction across the rung is by a factor of five larger compared to that along the legs 8 . Hence the ladder can be described by an effective spin chain.

Many low dimensional spin systems with a dimerized or ladder-like antiferromagnetic exchange coupling have a gapped excitation spectrum with magnetic bound states within the spin gap. For spin ladders with an even number of legs the existence of spin gaps and within the t-J model a tendency toward superconductivity with d-wave symmetry is predicted. In the following we will characterize the spin excitation spectra of different low dimensional spin systems taking into account strong spin phonon interaction (CuGeO3), charge ordering (NaV2O5) and doping on chains and ladders (Sr14−xCaxCu24O41 ). The spectroscopic characterization of the model systems mentioned above has been performed using magnetic inelastic light scattering originating from a spin conserving exchange scattering mechanism. This is also bound to yield more insight into the interrelation between these spin gap excitations and the origin of the pseudo gap in high temperature superconductors.