Effects of phonon dimensionality in the specific heat of multiwall carbon nanotubes

GUILLERMO A. JORGE; V. BEKERIS; C. ACHA; M. M. ESCOBAR; S. GOYANES; D. ZILLI; A. L. CUKIERMAN; R. J. CANDAL

Puerto Iguazú

Simposio; XIX Simposio Latinoamericano de Física del Estado Sólido; 2008

In this work, we measure the specific heat of a set of samples of multiwall Carbon nanotubes with different diameters and internal structures. The measurements were done by means of a microcalorimetric relaxation technique that allows the determination of the heat capacity of samples on the microgram scale. The specific heat of Carbon nanotubes is known to reflect only the vibrational degrees of freedom. It shows, for all samples investigated,  a clear change in dimensionality, from a two-three dimensional behavior at low temperatures to a one-dimensional-like behavior at temperatures above $T_D$ $approx$ 30-40 K. We interpret this result as a saturation of the transversal soft vibrational modes with a low Debye temperature, which leads to a pseudo-decoupled system of one-dimensional nanotubes above $T_D$. This dimensional change is related to the coupling energy between the tubes. Furthermore, a peak in the specific heat at 60 K is seen in some of the samples, with no signs of hysteresis. Although the origin of this peak is unknown, it is possible that it reflects a structural crossover or a suddenly activated vibrational mode.