Raman scattering in the inorganic spin-Peierls system α′-Na1-δV2O5

We have studied the spin-Peierls (SP) transition in alpha'-Na-1-deltaV2O5 (delta = 0, 0.01 and 0.1) by means of Raman scattering. At room temperature, we observe six Al and three Aa phonon modes and a broad Raman band. Below TSp several new peaks and a new broad hand appear. The new peak at 62 cm(-1) originates from the SP-gap excitation. The new peak at 128 cm(-1) and the new broad band between 130 and 400 cm(-1) come from two magnetic excitations. The new peaks at 102, 646 and 944 cm(-1) are assigned to the folded phonon modes and their Na+-ion deficiency dependence shows that the defect of Na+ ion suppresses the SP transition. The polarized Raman spectra below T-SP suggest that the possible crystal symmetry is C-s(2)(Pn) or C-1(1) (P1). The asymmetric lineshape of the 531-cm(-1) peak superimposed on the electronic Raman band from the d-d transition around 600 cm(-1) is interpreted in terms of the Fano resonance between the electronic continuous hand and the phonon with a finite lifetime. The defects of the Naf ions reduce the Fano effect because the lifetime of the phonon and the phonon-continuum interaction are decreased.