Spin-gap behavior and charge ordering inα′−NaV2O5probed by light scattering

We present a detailed analysis of light scattering experiments performed on the quarter-filled spin ladder compound ${\alpha }^{\prime }-{\mathrm{NaV}}_2{\mathrm{O}}_5$ for the temperature range $5\hspace{0.5em}\mathrm{K}<~T<~300\hspace{0.5em}\mathrm{K}.$ This system undergoes a phase transition into a singlet ground state at $T=34\mathrm{}\hspace{0.5em}\mathrm{K}$ accompanied by the formation of a superstructure. For $T<~34\hspace{0.5em}\mathrm{K}$ several 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 $422\hspace{0.5em}{\mathrm{cm}}^{-1}$ for temperatures above 34 K. The smooth and crossoverlike onset of this ordering at $T_{\mathrm{CO}}=80\mathrm{}\hspace{0.5em}\mathrm{K}$ 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 ${\mathrm{La}}_2{\mathrm{NiO}}_{4+\delta }.$