LAUSR

In order to describe helium atom motion through the disordered quartz structure the local chain approach is used. The number of local chains equals the number of nearest neighbours (i.e., the closest atoms to those selected). Each local chain includes a He-atom. Transition of a He-atom from its site to a neighbouring one generates a space-periodical field, which acts on the He-atom. This could take place in both cases, if (1) the local order is preserved, or (2) the local order is changed. The last situation means that the structure element of quartz, SiO4 , is removed / added with respect to a He-atom. Distortion of the local order creates a defect of the order and means appearance of an additional average field, which is proportional to the internal “surface” of the defect. The above approach shows that the problem of He-atom motion through the disordered quartz structure can be reduced to “a local version” of the Frenkel-Kontorova model.