Low-energy excitations in NaV2O5 -: art. no. 012404

In the (ab)-polarized Raman scattering spectra of NaV2O5 single crystals, measured with a 647.1 mn laser line at T< T-c, we found two modes at 86 and 126 cm(-1) not previously reported, These two modes, together with 66 and 106 cm(-1) modes, make an array of four low-energy equidistant modes below the energy onset of the continuum at about 132 cm-1. All four modes are strongly suppressed by increasing Na deficiency, suggesting a nonvibrational origin of the modes and the existence of a quantum phase transition at the critical Na deficiency between 3% and 4%. These results question our current understanding of NaV2O5 as a quasi-one-dimensional Heisenberg antiferromagnet.