ORIGIN OF THE TUNNELING STATES IN SIO2

Ultrasonic velocity change and attenuation measurements are performed in neutron-irradiated z-cut quartz, for three different doses. From numerical fits of the data using the tunneling model, the typical parameters of the tunneling states (TS) are determined and compared with those found in comparable x-cut samples (irradiated under the same conditions and with similar doses). This study reveals a remarkable anisotropy, which has to be attributed to the tunneling mechanism itself. In search of the microscopic origin of the tunneling states, these results give direct experimental evidence for a microscopic picture of the TS as a rotation of coupled SiO4 tetrahedra. At the same time, they provide important evidence for the missing link to obtain a common interpretation of all three low-frequency excitations in vitreous silica. © 1995 The American Physical Society.