Effect of magnetic field on Motts variable-range hopping parameters in multiwall carbon nanotube mat

摘要

We report the temperature and magnetic field dependence of the conductivity of multiwall carbon nanotube mat in the temperature range 1.4150K and in magnetic fields up to 10T. It is observed that charge transport in this system is governed by Motts variable-range hopping of three-dimensional type in the higher temperature range and two-dimensional type in the lower temperature range. Motts various parameters, such as localization length, hopping length, hopping energy and density of states at the Fermi level are deduced from the variable-range hopping fit. The resistance of the sample decreases with the magnetic field applied in the direction of tube axis of the nanotubes. The magnetic field gives rise to delocalization of states with the well-known consequence of a decrease in Motts T _0 parameter in variable-range hopping. The application of magnetic field lowers the crossover temperature at which three-dimensional variable-range hopping turns to two-dimensional variable-range hopping. The conductivity on the lower temperature side is governed by the weak localization giving rise to positive magnetoconductance. Finally, a magnetic fieldtemperature diagram is proposed showing different regions for different kinds of transport mechanism.