Microscopic Evidences of Bipolarons in the Quasi-One-Dimensional Conductor β-Na_0.33V₂O₅

Abstract

The electronic states of the V⁴⁺ ions in β-Na_0.33V₂O₅ are studied by NMR and EPR measurements. The anomalous temperature dependence of the Knight shift of the ⁵¹V nuclei in site I can be understood by the spin singlet ground state of the electron pairs of the adjacent V⁴⁺ ions (bipolarons). Recent X-ray diffraction measurements reveal that this paired state is accompanied with the small ionic displacement, as predicted by Chakraverty et al. and the long range ordering of the bipolaronic state in the real space occurs below 200 K. From analyses of the anisotropic Knight shift and the quadrupole interaction, the electron wave function of the V⁴⁺ ions in site I is found to be mainly made of the d_yz-orbitals. The EPR results show that there are two components of the spin states: the bipolaronic spin singlet and the dilute isolated paramagnetic states.