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 $\hat{I}{\pm }^{\prime }$-NaV${}_2$O${}_5$ for the temperature range 5 K$\le$T$\le$300 K. This system undergoes a phase transition into a singlet ground state at T=34 K accompanied by the formation of a super structure. For T$\le$34 K several new modes were detected. Three of these modes are identified as magnetic bound states. Experimental evidence for charge ordering on the V sites is detected as an anomalous shift and splitting of a V-O vibration at 422 cm${}^{-1}$ for temperatures above 34 K. The smooth and crossover-like onset of this ordering at T${}_{\mathrm{CO}}$= 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${}_{4+\hat{I}´}$.

36 pages, 11 figures, accepted for publication in PRB (sept.99)