Three kinds of the response mechanisms to the external pressure have been
found for double-walled carbon nanotube (DWNT) bundle, depending strongly on
their average radius and symmetry. The small-diameter DWNT bundle undergoes a
small discontinuous volume change, and then deform continuously. The
intermediate-diameter DWNT bundle collapses completely after a structure phase
transition (SPT). Significantly, two SPTs exist for the larger-diameter DWNT
bundle if the outer tube has no C6β or C3β symmetry. It would be
interesting to search for signatures of these different structural
transformations by experimentally investigating mechanical, optical and thermal
response functions of DWNT bundle.Comment: 9 pages, 2 figure