Low-energy excitations in NaV2O5.

In the (ab) polarized Raman scattering spectra of NaV2O5 single crystals, measured with 647.1 nm laser line at T < Tc, we found two modes at 86, and 126 cm not previously reported. These two modes, together with 66, and 106 cm modes, make an array of four low-energy equidistant modes below the energy onset of the continuum at about 132 cm. All four modes are strongly suppressed by increasing Na deficiency, indicating their nonvibrational origin and the existence of a quantum phase transition at critical Na deficiency between 3 and 4%. These results question current understanding of NaV2O5 as quasi one-dimensional Heisenberg antiferromagnet. PACS: 78.30.-j, 75.50.Ee, 75.30.Ds, 75.30.Hx Sodium vanadate, NaV2O5, is a mixed valence compound (V 4+ : V 5+ = 1 : 1) with a structure consisting of vanadium-oxygen (V O5) pyramids, that are connected via common edges and corners to form layers in the the ab plane. The structure can be described as an array of parallel ladders (running along the b axis) that are coupled to form a trellis lattice, see Fig. 1. Each rung is made of V-O-V bond, and contains one valence electron donated by the sodium atoms which are situated between layers as intercalants. This compound exhibits a phase transition at Tc = 34K [1] that is usually referred to in the literature as a charge-ordering (CO) phase transition. From the mixed-valence point of view its existence is not unusual, since a common consequence of the mixed valence is the appearance of charge-ordering, which is a class of metal-insulator transitions [2]. However, the phases on both sides of the transition in NaV2O5 are insulators, and a very interesting interplay between charge and spin dynamics results in the phase-transition discovered in this compound. Indeed, despite the fact that the vanadium ions have uniform valence of +4.5 at room temperature [3,4] (indicating the quarter-filled structure of V-O-V rungs [5], and suggesting a metalic state!), the magnetic susceptibility of NaV2O5 is in excellent agreement with Bonner-Fisher curve for a one-dimensional antiferromagnet (AF) [1]. The one-dimensionality of the magnetic ordering is realized along the b axis which is perpendicular to the V-O-V rungs [1]. Furthermore, at low temperatures, the susceptibility decreases rapidly to zero in accord with a spin-liquid ground state, and suggesting the existence of a