Metallic hydrogen

Metallic hydrogen is a phase in which pressure-induced electron delocalization gives hydrogen metallic electrical conductivity. It may occur as a dense fluid or a solid, depending on conditions.

Molecular hydrogen: Hydrogen in which atoms are paired into H₂ molecules. At lower pressures, metallic hydrogen’s molecular precursor remains electrically insulating.
Static compression: The application of sustained pressure to a sample, commonly with opposed diamond anvils. Diamond-anvil experiments seek metallic hydrogen while keeping samples under extreme pressure.
Eugene Wigner: A Hungarian-American physicist who made foundational contributions to quantum mechanics and solid-state physics. Wigner and Hillard Bell Huntington predicted pressure-induced metallic hydrogen in 1935.
Atomic metallic hydrogen: A proposed metallic phase of hydrogen composed of unpaired atoms rather than H₂ molecules. It is the atomic endpoint often envisioned after molecular hydrogen dissociates.
Molecular dissociation: The breaking of molecules into separate atoms. Hydrogen’s molecular bonds are expected to break as pressure drives metallization.
Dynamic compression: The rapid compression of matter by shocks, laser-driven waves, or other transient forces. Shock experiments create brief high-pressure hydrogen states and measure their conductivity.
Hillard Bell Huntington: An American physicist known for theoretical work on solids and high-pressure matter. His 1935 collaboration with Wigner introduced a landmark prediction of metallic hydrogen.
Molecular metallic hydrogen: A metallic phase in which hydrogen remains molecular while its electrons conduct collectively. Some proposed pathways allow metallization before hydrogen molecules fully dissociate.
Band gap: The energy range in a solid where no electronic states are available. Compression can close hydrogen’s electronic gap, enabling metallic conduction.
Diamond anvil cell: A device that compresses a tiny sample between the tips of two diamonds. It is the principal apparatus for searching for metallic hydrogen under static pressure.

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