Title:Spin gap behavior and charge ordering in α^{\prime}-NaV_2O_5 probed by light scattering

Abstract: We present a detailed analysis of light scattering experiments performed on the quarter-filled spin ladder compound $\alpha^\prime$-NaV$_{2}$O$_{5}$ for the temperature range 5 K$\le$T$\le$300 K. This system undergoes a phase transition into a singlet ground state at T=34 K accompanied by the formation of a super structure. For T$\leq$34 K several new modes were detected. Three of these modes are identified as magnetic bound states. Experimental evidence for charge ordering on the V sites is detected as an anomalous shift and splitting of a V-O vibration at 422 cm$^{-1}$ for temperatures above 34 K. The smooth and crossover-like onset of this ordering at T$_{\rm CO}$= 80 K is accompanied by pretransitional fluctuations both in magnetic and phononic Raman scattering. It resembles the effect of stripe order on the super structure intensities in La$_2$NiO$_{4+\delta}$.