Origin of high-Tc transitions in Li-alloy intercalated graphite

Determine the physical origin of the high critical-temperature (Tc) magnetization transitions observed at ambient pressure in expanded graphite sheets (SGL graphite) intercalated with lithium-based alloys, including Ca–Li and Sr–Ca–Li. Specifically ascertain whether these transitions arise from granular or local superconductivity and identify the structural or interfacial phases responsible for the effect.

Background

The paper reports reproducible high-Tc transitions between 240–350 K in graphite intercalated with lithium-based alloys, supported by magnetization, trapped flux, and field-dependent measurements indicative of type-II superconductivity. The authors compare these observations with alternative magnetic explanations and highlight features (Meissner-like behavior, trapped flux plateau, vortex creep) favoring a superconducting origin.

Despite this evidence, the authors explicitly state they cannot yet clarify the origin of the high-Tc transitions. They note theoretical predictions of enhanced superconductivity at interfaces between rhombohedral and hexagonal graphite due to flat bands, and their materials contain both phases, suggesting possible interfacial or granular superconductivity as a candidate mechanism.