Anomalous low-temperature saturation effects and negative thermal expansion in the c-axis of highly oriented pyrolytic graphite at the magic angle

Abstract

We report a novel T-XRD and Rietveld-refinement investigation of pyrolytic-graphite samples with high degree of graphene-layer-orientation and misfit-rotational-angle of ∼0.8° in the T-range from 12 K to 298 K. An anomalous variation of the graphitic c-axis which involves firstly negative-thermal-expansion (from 12 K to ∼50 K), a saturation-effect (from 50 K to ∼160 K) and then a positive expansion (from ∼180 K to 298 K) is evidenced. The reported trend is significantly different with respect to that expected by considering the standard-thermal-expansion -parameter where no saturation-effect is present. SQUID-magnetometry revealed further presence of superconducting-like hysteresis which resemble those observed by Scheike et al.