Electron - phonon and spin - phonon coupling in NaV 2 O 5 : charge fluctuations effects

(received ; accepted) PACS. 63.20Kr – Phonon-electron and phonon-phonon interactions. PACS. 75.30Et – Exchange and superexchange interactions. PACS. 78.30−j – Infrared and Raman spectra. Abstract. – We show that the asymmetric crystal environment of the V site in the ladder compound NaV2O5 leads to a strong coupling of vanadium 3d electrons to phonons. This coupling causes fluctuations of the charge on the V ions, and favors a transition to a charge-ordered state at low temperatures. In the low temperature phase the charge fluctuations modulate the spin-spin superexchange interaction, resulting in a strong spin-phonon coupling. Introduction. – The spin-Peierls transition in one-dimensional spin systems is one of the best known and investigated cooperative phenomena in low-dimensional physics. This transition comprises a dimerization of the lattice of a Heisenberg spin chain, leading to an alternation of the exchange parameter J. As a result, nearest-neighbor spins form a singlet state accompanied by the opening of a spin gap. Consequently, the static spin susceptibility drops drastically below the transition temperature (T SP). This effect, predicted by Pytte [1], was first observed in one-dimensional organic compounds [2]. The discovery was followed by theoretical work of Cross and Fisher [3], who showed a possibility to calculate T SP from the coupling of spins to the lattice. The discovery of the spin-Peierls transition in the inorganic spin-chain compound CuGeO 3 [4] renewed interest in this phenomenon. In this half-filled chain compound each Cu site holds one " hole " with spin s = 1/2. The spin-spin exchange interaction occurs via oxygen orbitals. Lattice vibrations cause a modulation of the spin-spin superexchange due to changes in the inter-ionic distances and bond angles. This leads to a spin-lattice coupling which plays an important role as a driving force of the spin-Peierls transition. The ladder system NaV 2 O 5 was proposed to be a second inorganic spin-Peierls compound [5]. It contains VO 5 pyramids which form V-O ladders extended along the b−axis [6], see Fig. 1a. For half the ladders the apical oxygen O3 is above the V ions while for the other half they are below them. From the chemical formula one finds an average vanadium valence of