In a laboratory at Harvard University, researchers have squeezed a hydrogen isotope to pressures approaching those found at the center of the Earth, revealing two previously unseen quantum states that could be stepping stones toward the long-sought metallic form of hydrogen. The work, submitted to the journal Physical Review Letters, pushes the boundaries of high-pressure physics and brings scientists closer to understanding how hydrogen behaves under extreme conditions.
Hydrogen, the simplest and most abundant element in the universe, consists of a single proton orbited by a single electron. Yet under certain conditions, this simplicity gives way to complex behavior. At temperatures near absolute zero, hydrogen exists as a neutral electron spin-polarized gas; at around 14 Kelvin, unpolarized hydrogen forms stable molecules that can readily solidify. But the real intrigue lies in what happens when hydrogen is subjected to pressures measured in millions of atmospheres.
Theorists have long predicted that under such megabar pressures, hydrogen would dissociate into an atomic metal, a state that could exhibit remarkable properties including high-temperature superconductivity and metastability. At these extreme pressures and low temperatures, hydrogen is expected to form a liquid, and its solid structure is thought to transform into planes resembling graphene.
In the new study, the Harvard team used a diamond anvil cell to compress solid hydrogen deuteride (HD) — a molecule composed of one hydrogen and one deuterium atom — to 3.4 megabars. That pressure is remarkably close to the 3.6 megabars estimated at the center of the Earth, making the experiment one of the most extreme ever performed in a laboratory setting.
The results showed that at around 2 megabars, HD underwent a transition into a phase the researchers labeled HD-IV*. With further pressure increases, it transformed into another phase, HD-PRE. At the HD-IV* transition, the scientists observed a rapid dissociation of the molecules, followed by recombination into a new isotopic solid, which they named DISREC.
Neither of the newly observed states exhibited metallic properties, but the researchers view them as important precursors. The discovery of these intermediate phases provides a clearer map of the pathway hydrogen takes as it approaches the metallic state, a goal that has eluded physicists for decades.
Why This Matters for Metallic Hydrogen Research
The pursuit of metallic hydrogen is not merely academic. If realized, it could lead to revolutionary advances in energy storage, superconductors, and other technologies. However, the extreme conditions required — pressures that rival those at the core of a planet — make experimentation incredibly challenging. Each new phase discovered, such as HD-IV* and HD-PRE, helps refine theoretical models and guides future experiments.
The Harvard team's findings also highlight the importance of studying hydrogen isotopes, which can behave differently under pressure than ordinary hydrogen. By using deuterium, the researchers were able to observe transitions that might not occur in pure hydrogen, offering a unique window into the quantum mechanics of these systems.
While the ultimate goal of creating metallic hydrogen remains out of reach, this research represents a tangible advance. The identification of these precursor states brings scientists one step closer to understanding how hydrogen transforms under the most extreme conditions imaginable.