Two-dimensional weak localization in partially graphitic carbons

The weak-localization phenomenon for two-dimensional (2D) electronic systems is invoked to explain the negative magnetoresistance as well as the low-temperature dependence of the resistivity of pyrocarbon samples heat treated between 2000 and 2600 ^. The 2D character is found to originate from the random stacking of the graphene layers (turbostratic structure) characteristic of pregraphitic carbon materials. For a heat-treatment temperature (HTT) lower than 2200 ^, x-ray analysis reveals that the structure is almost turbostratic, while the material exhibits a pronounced negative magnetoresistance. For higher HTT, 3D order typical of crystalline graphite increases, leading to a 2D-to-3D crossover and to a vanishing negative magnetoresistance.