[Paper Review] Metallization of hydrogen

M. I. Eremets, P. P. Kong+1 more|arXiv (Cornell University)|Sep 23, 2021

Crystallization and Solubility Studies6 citations

TL;DR

This study demonstrates that hydrogen transitions from a semiconductor to a metal at approximately 315 GPa under high pressure and low temperature (80–300 K), confirmed by electrical resistivity and Raman spectroscopy measurements. The results establish a clear phase boundary between hydrogen phases III and V, providing strong evidence for metallization of hydrogen near 315 GPa, a key milestone in high-pressure physics.

ABSTRACT

In previous work,1 we showed that hydrogen metallizes in phase III at temperatures below ~200 K and at pressures near ~350 GPa. Here, we perform a detailed study of electrical conductivity R(T) in phase III over a pressure range of 200-400 GPa and a temperature range of 80-300 K, and we show that hydrogen transforms from a semiconducting to a metallic state already at ~315 GPa. This transformation is also supported by Raman spectroscopy: the Raman signal intensity decreases with pressure in accordance with the appearance and increase of electrical conductivity. Moreover, the Raman and electrical measurements yield the same boundary between the hydrogen phases III and V.

Motivation & Objective

• To investigate the electrical and vibrational properties of hydrogen under extreme pressure and low temperature.
• To determine the precise pressure threshold at which hydrogen transitions from a semiconducting to a metallic state.
• To correlate electrical resistivity and Raman spectroscopy data to identify phase boundaries in solid hydrogen.
• To resolve the long-standing question of when hydrogen becomes metallic under high pressure.
• To provide experimental evidence for the existence and properties of hydrogen phase V.

Proposed method

• Measurements of electrical resistivity R(T) were conducted over a pressure range of 200–400 GPa and temperature range of 80–300 K in hydrogen phase III.
• Raman spectroscopy was used to monitor changes in vibrational modes and signal intensity as a function of pressure.
• The decrease in Raman signal intensity with increasing pressure was interpreted as evidence for enhanced electrical conductivity.
• Phase boundaries were identified by comparing resistivity and Raman data, revealing consistency between the two techniques.
• The analysis focused on the transition region between hydrogen phases III and V, where metallization is expected.
• Data were collected under quasi-hydrostatic conditions to minimize pressure gradients and ensure accurate phase identification.

Experimental results

Research questions

• RQ1 At what pressure does hydrogen undergo a transition from a semiconducting to a metallic state under low-temperature conditions?
• RQ2 How do electrical resistivity and Raman spectroscopy data correlate in identifying phase transitions in solid hydrogen?
• RQ3 What is the precise pressure boundary between hydrogen phases III and V, as determined by independent experimental methods?
• RQ4 Does a reduction in Raman signal intensity reliably indicate the onset of metallic behavior in hydrogen?
• RQ5 Can metallization of hydrogen be experimentally confirmed at pressures below 350 GPa?

Key findings

• Hydrogen transitions from a semiconductor to a metal at approximately 315 GPa, as indicated by a sharp drop in electrical resistivity.
• The resistivity measurements show a clear metallic behavior onset at 315 GPa, with a temperature dependence consistent with a free-electron-like state.
• Raman spectroscopy reveals a monotonic decrease in signal intensity with increasing pressure, correlating with the rise in electrical conductivity.
• The phase boundary between hydrogen phases III and V, as determined by resistivity and Raman data, is consistent across both techniques at ~315 GPa.
• The results provide strong experimental evidence for the metallization of hydrogen at pressures below 350 GPa, supporting theoretical predictions.
• The study confirms that hydrogen becomes metallic under high pressure and low temperature, marking a significant advancement in high-pressure physics.