Raman Scattering in the Inorganic Spin-Peierls System<i>α</i>^′-Na_{<b>1</b>-<i>δ</i>}V_<b>2</b>O_<b>5</b>

Abstract

We have studied the spin-Peierls (SP) transition in alpha'-Na_{1-delta}V_2O_5\n(delta = 0, 0.01 and 0.1) by means of Raman scattering. At room temperature, we\nobserve six A_1 and three A_2 phonon modes and a broad Raman band. Below\nT_{SP}several new peaks and a new broad band appear. The new peak at 62 cm^{-1}\noriginates from the SP-gap excitation. The new peak at 128 cm^{-1} and the new\nbroad band between 130 and 400 cm^{-1} come from two magnetic excitations. The\nnew peaks at 102, 646 and 944 cm^{-1} are assigned to the folded phonon modes\nand their Na^+-ion deficiency dependence shows that the defect of Na^+ ion\nsuppresses the SPtransition. The polarized Raman spectra below T_{SP} suggest\nthat the possible crystal symmetry is C_s^2(Pn) or C_1^1(P1). The asymmetric\nlineshape of the 531-cm^{-1} peak superimposed on the electronic Raman band\nfrom the d-d transition around 600 cm^{-1} is interpreted in terms of the Fano\nresonance between the electronic continuous band and the phonon with a finite\nlifetime. The defects of the Na^+ ions reduce the Fano effect because the life\ntime of the phonon and the phonon-continuum interaction are decreased.\n