Metallic hydrogen confined by graphene.

The metallization of hydrogen is a prerequisite for the emergence of superconducting properties, along with those of hydrides. Chemical precompression has enabled a class of high-transition-temperature superconductors based on hydrogen-rich hydrides, whereas direct metallization of molecular hydrides remains challenging. In this study, we demonstrate the metallization of hydrogen confined within fixed two-dimensional graphene channels, which provide a nanoscale environment for effective compression. The confined hydrogen forms compressed molecular hydrides whose intermolecular H-H separations are significantly reduced compared to those in ambient molecular hydrogen. Electronic structure calculations reveal that all the hydrogens in both stacks exhibit metallic properties. The Fermi surface of both stacks exhibits a peculiar nesting pattern, suggesting enhanced electron-phonon coupling. These results identify graphene-confined hydrogen (graphene-H6) systems as a promising platform for achieving metallic hydrogen and potentially low-pressure, high-Tc superconductors.