Superconducting Behavior of Interfaces in Graphite: Transport Measurements of Micro-constrictions

We have studied the magnetoresistance (MR) of thin highly oriented pyrolytic graphite mesoscopic samples without and with micro-constrictions of different widths between the voltage electrodes. The MR for fields parallel to the c-axis shows an anomalous hysteresis loop compatible with the behavior expected for granular superconductors. The smaller the constriction width the larger is the anomalous hysteresis and the higher the temperature for its observation. Our results support the existence of granular superconductivity probably embedded at interfaces between crystalline graphite regions.