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-thermalexpansion (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 α-parameter where no saturation-effect is present. SQUID-magnetometry revealed further presence of superconducting-like hysteresis which resemble those observed by Scheike et al.

Unusual butterfly-shaped magnetization signals and spin-glass-like behaviour in highly oriented pyrolytic graphite

“…Instead, a highly oriented arrangement characterized by an intense 100 reflection could be found in HOPG2 (Fig. 1D) [12,16]. Presence of ferromagnetic hysteresis was revealed by magnetization vs field measurements of HOPG1 with the c-axis perpendicular to the applied field (Fig.…”

“…Granular superconductive features at room temperature have been demonstrated for commercially available HOPG samples and attributed to the high dislocation density within certain interfaces [7][8][9]. The stacking between the graphitic layers and the layer orientation with respect to the c-axis (rotational orientation) have been also indicated as some of the critical factors which can influence the formation of intrinsic magnetization features in HOPG [10][11][12] and bilayer-graphene [13][14][15]. Particularly, the recent observation of negative thermal expansion effects in dislocation-rich (HOPG) samples suggests that relative graphitic layers rotation has significant influence on the intrinsic physical properties of these systems [12].…”

“…Possible influences of rotational disorder and therefore non-uniform c-axis orientation within HOPG1, cannot be excluded (see ref. [12] for example of rotational disorder on lamellae extracted from surface regions of HOPG).…”