High-field and high-temperature magnetoresistance reveals the superconducting behaviour of the stacking faults in multilayer graphene

In spite of 40 years of experimental studies and several theoretical proposals, an overall interpretation of the complex behavior of the magnetoresistance (MR) of multilayer graphene, i.e. graphite, at high fields ($B \lesssim 70~$T) and in a broad temperature range is still lacking. Part of the complexity is due to the contribution of stacking faults (SFs), which most of thick enough multilayer graphene samples have. We propose a procedure that allows us to extract the SF contribution to the MR we have measured at 0.48~K $\leq T \leq$ 250~K and 0~T$\leq B \lesssim$ 65~T. We found that the MR behavior of part of the SFs is similar to that of granular superconductors with a superconducting critical temperature $T_c \sim$ 350~K, in agreement with recent publications. The measurements were done on a multilayer graphene TEM lamella, contacting the edges of the two-dimensional SFs.Comment: 8 pages, 5 figure

Identification of a possible superconducting transition above room temperature in natural graphite crystals

Measuring with high precision the electrical resistance of highly ordered natural graphite samples from a Brazil mine, we have identified a transition at $\sim$350~K with $\sim$40~K transition width. The step-like change in temperature of the resistance, its magnetic irreversibility and time dependence after a field change, consistent with trapped flux and flux creep, and the partial magnetic flux expulsion obtained by magnetization measurements, suggest the existence of granular superconductivity below 350~K. The zero-field virgin state can only be reached again after zero field cooling the sample from above the transition. Paradoxically, the extraordinarily high transition temperature we found for this and several other graphite samples is the reason why this transition remained undetected so far. The existence of well ordered rhombohedral graphite phase in all measured samples has been proved by x-rays diffraction measurements, suggesting its interfaces with the Bernal phase as a possible origin for the high-temperature superconductivity, as theoretical studies predicted. The localization of granular superconductivity at these two dimensional interfaces prevents the observation of a zero resistance state or of a full Meissner state.Comment: 14 pages with 21 figure

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 $T_c \sim 350$ K in a natural graphite sample. The measurements were done in a restricted temperature 300 K $\leqslant T \leqslant 450$ K and magnetic field $B \leqslant 400$ 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 ${\sim}30$ min but it vanishes at a temperature of ${\sim}330$ K. The apparent transition is accompanied by a clear enhancement of the magnetoresistance at $T \lt T_c$ . 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

On the Localization of Persistent Currents Due to Trapped Magnetic Flux at the Stacking Faults of Graphite at Room Temperature

Granular superconductivity at high temperatures in graphite can emerge at certain two-dimensional (2D) stacking faults (SFs) between regions with twisted (around the c-axis) or untwisted crystalline regions with Bernal (ABA…) and/or rhombohedral (ABCABCA…) stacking order. One way to observe experimentally such 2D superconductivity is to measure the frozen magnetic flux produced by a permanent current loop that remains after removing an external magnetic field applied normal to the SFs. Magnetic force microscopy was used to localize and characterize such a permanent current path found in one natural graphite sample out of ∼50 measured graphite samples of different origins. The position of the current path drifts with time and roughly follows a logarithmic time dependence similar to the one for flux creep in type II superconductors. We demonstrate that a ≃10 nm deep scratch on the sample surface at the position of the current path causes a change in its location. A further scratch was enough to irreversibly destroy the remanent state of the sample at room temperature. Our studies clarify some of the reasons for the difficulties of finding a trapped flux in a remanent state at room temperature in graphite samples with SFs