Superconducting Sweet-Spot in Microcrystalline Graphite Revealed by Point-Contact Spectroscopy

In this letter we describe the observation of a magnetic field dependent electronic gap, suggestive of local superconductivity, in the point-contact spectrum of micro-crystalline graphite. Magnetic field dependent point-contact spectroscopy was carried out at a temperature of $1.8\mathrm{K}$ using an etched aluminium tip. At zero field a gap structure in the differential conductance is observed, showing a gap of $\mathrm{\Delta }=4.2\mathrm{m}\mathrm{e}\mathrm{V}$. On applying magnetic fields of up to $500\mathrm{m}\mathrm{T}$, this gap gradually closes, following the theoretical prediction by Ginzburg and Landau for a fully flux-penetrated superconductor. By applying BCS-theory, we infer a critical superconducting temperature of $14\mathrm{K}$.