Microscopic Evidences of Bipolarons in the Quasi-One-Dimensional Conductor β-Na 0.33 V 2 O 5

The electronic states of the V 4+ ions in β-Na 0.33 V 2 O 5 are studied by NMR and EPR measurements. The anomalous temperature dependence of the Knight shift of the 51 V nuclei in site I can be understood by the spin singlet ground state of the electron pairs of the adjacent V 4+ 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 V 4+ ions in site I is found to be mainly made of the d y z -orbitals. The EPR results show that there are two components of the spin states : the bipolaronic spin singlet and the dilute isolated paramagnetic states.