Scanning tunneling microscopy identification of van Hove singularities and negative thermal expansion in highly oriented pyrolytic graphite with hexagonal moiré superlattices

Abstract

We report a novel investigation on the structural, electronic and magnetic properties of highly-oriented-pyrolytic-graphite (HOPG) by employing STM/S, TEM and SQUID. Van Hove singularities (vHs) were identified by STS-acquisition of local-density-of-states spectra (LDOS) acquired from moiré-superlattice-periodicities D ~0.5 nm (θrot ~28.6°), ~2 nm (θrot of ~7.08°) and ~ 4 nm (θrot ~3.6°). A possible interlayer-coupling- or moiré-thickness-induced variation of the vHs-separation-parameter was identified. Investigation of the spatial-distribution of the moiré-superlattices by exfoliation revealed also D ~13.87 nm (θrot ~1.02°), ~13.0 nm (θrot ~1.09°), ~12.65 nm (θrot ~1.12°) and ~2.03 nm (θrot ~7.0°). These observations were further supported by additional LDOS spectra acquired on HOPG from moiré-superlattice-periodicities D ~8 nm (θrot ~1.8°). In the latter, the unusual presence of four vHs peaks evidenced the existence of multiple rotational effects between internal sublattices. Extended analyses were further performed by T-XRD from 12 K to 298 K. Unknown peak-features exhibiting unusual T-dependent shifts were analysed at 2θ ~ 21° (Fig. 6A-C) and ~ 43° (Fig. 6D). ZFC and FC-signals from the exfoliated lamellae further evidenced an anisotropic ferromagnetic behaviour, possibly involving the coexistence of (1) disorder induced percolative ferromagnetism and (2) additional magnetic components arising from the locally twisted sublattices, which were found to exhibit the moiré superlattices.