Granular superconductivity at room temperature in bulk highly oriented pyrolytic graphite samples

TL;DR

This study provides evidence of granular superconductivity above room temperature in bulk graphite samples, showing anisotropic magnetic responses linked to internal interfaces, with irreversible changes occurring above 400 K.

Contribution

It demonstrates the existence of high-temperature granular superconductivity in bulk graphite, emphasizing the role of internal interfaces and magnetic field orientation.

Findings

Superconductivity observed above room temperature in graphite.

Hysteresis depends on magnetic field orientation relative to interfaces.

Irreversible magnetic behavior occurs above approximately 400 K.

Abstract

We have studied the magnetic response of two bulk highly oriented pyrolytic graphite (HOPG) samples with different internal microstructure. For the sample with well defined interfaces, parallel to the graphene layers, the temperature and magnetic field hysteresis are similar to those found recently in water-treated graphite powders. The observed behavior indicates the existence of granular superconductivity above room temperature in agreement with previous reports in other graphite samples. The granular superconductivity behavior is observed only for fields normal to the embedded interfaces, whereas no relevant hysteresis in temperature or field is observed for fields applied parallel to them. Increasing the temperature above $\sim 400$ K changes irreversibly the hysteretic response of the sample.