Hints of granular superconductivity in natural graphite verified by trapped flux transport measurements

This study describes electrical transport measurements as a function of magnetic field and temperature that provide hints of the existence of superconductivity with a critical temperature of T-c similar to 350 K in a natural graphite sample. The measurements were done in a restricted temperature 300 K <= 450 <= K and magnetic field B <= 400 mT range. Electrical resistance measurements at remanence (zero field), after the application of a magnetic field, indicate the existence of trapped flux, which remains nearly unchanged within similar to 30 min but it vanishes at a temperature of similar to 330 K. The apparent transition is accompanied by a clear enhancement of the magnetoresistance at T < T-c. Raman measurements on the bulk sample reveal the existence of the rhombohedral stacking order, which interfaces with the usual Bernal phase were predicted to lead to high temperature superconductivity due to the formation of robust flat bands in the electron dispersion relation.