Positron annihilation in a single TiO2 crystal

Positron annihilation in a single crystal of VO2 is investigated by analysing the Doppler broadening spectra in the insulator and metal phases. The anisotropy in the energy distribution, for emission parallel and perpendicular to the crystal c-axis, at room temperature is 2.4+or-0.1% for k=0, and it vanishes at k=4.0+or-0.1 mRad. The changes in the spectra on passing from the insulating to the metal phase are found to be gradual and spread over tens of degrees. The change in the direction parallel to the c-axis is within the statistical error, while the changes in the two perpendicular directions are about 0.5% and have opposite signs.