Calculation of positron states in carbon-nanotube bundles

Positron distributions and lifetimes in carbon-nanotube bundles have been calculated using a combination of the superposed-neutral-atom model and the finite-difference method (SNA-FD) as well as with an ab initio method. The electron-positron correlation was described with the local density approximation (LDA) or the generalized gradient approximation (GGA). For the LDA, the SNA-FD and ab initio calculations give similar results. Positrons are predominantly distributed in interstitial regions for smaller-size nanotubes, while they are distributed inside nanotubes for larger sizes. The estimated positron lifetime ranges from 250 to 480 ps as a function of the nanotube diameter. In contrast to this, for the GGA, the SNA-FD and ab initio calculations give quite different results concerning the positron distribution and lifetime. (author)