Spin-gap behavior and charge ordering in α′-NaV2O5 probed by light scattering

We present a detailed analysis of light scattering experiments performed on the quarter-filled spin ladder compound alpha'-NaV2O5 for the temperature range 5 K less than or equal to T less than or equal to 300 K. This system undergoes a phase transition into a singlet ground state at T = 34 K accompanied by the formation of a superstructure, For T less than or equal to 34 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 cm(-1) for temperatures above 34 K. The smooth and crossoverlike onset of this ordering at T-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 La2NiO4+delta. [S0163-1829(99)02434-0].