A specific heat anomaly in multiwall carbon nanotubes as a possible sign of orientational order-disorder transition

We present micro-calorimetric specific heat measurements on different multiwall carbon nanotubes of large outer diameter, as a function of temperature in the range 10-120 K. A clear anomaly at 60 K with the shape of a peak is present in two of the samples, and both the height and the characteristic temperature of the peak are independent of magnetic field and do not exhibit thermal hysteresis, discarding magnetic degrees of freedom related to Fe seeds or contributions from adsorbed gases. These features suggest that the anomaly may be caused by a structural change. As the anomaly is also unaffected by induced intertube disorder, it may be related with a melting of orientational dislocations of individual tubes within multiwall nanotubes, an effect that was theoretically predicted to occur in carbon nanotubes and represents a distinctive feature of asymmetric molecular systems.