A Spectroscopic Study of the Insulator-Metal Transition in Liquid Hydrogen and Deuterium.

R. Stewart McWilliams,Alexander F. Goncharov,Nicholas Holtgrewe,Sergey S. Lobanov,Mohammad F. Mahmood,Zachary M. Geballe,Shuqing Jiang

doi:10.1002/advs.201901668

Abstract

The insulator‐to‐metal transition in dense fluid hydrogen is an essential phenomenon in the study of gas giant planetary interiors and the physical and chemical behavior of highly compressed condensed matter. Using direct fast laser spectroscopy techniques to probe hydrogen and deuterium precompressed in a diamond anvil cell and laser heated on microsecond timescales, an onset of metal‐like reflectance is observed in the visible spectral range at P >150 GPa and T ≥ 3000 K. The reflectance increases rapidly with decreasing photon energy indicating free‐electron metallic behavior with a plasma edge in the visible spectral range at high temperatures. The reflectance spectra also suggest much longer electronic collision time (≥1 fs) than previously inferred, implying that metallic hydrogen at the conditions studied is not in the regime of saturated conductivity (Mott–Ioffe–Regel limit). The results confirm the existence of a semiconducting intermediate fluid hydrogen state en route to metallization.

Highlights

The insulator-to-metal transition (IMT) in physical and chemical behavior of highly compressed condensed matter
The metallic state exhibits a plasma edge in the visible spectral range, implying a plasma frequency and electronic scattering time that contrasts with previous inferences,[14,20,21] mainly based on the Mott–Ioffe– Regel (MIR) limit approximation in which the electronic meanfree-path reaches the interatomic spacing, and in stark disagreement with the prior static experiments probing hydrogen at few laser wavelengths.[13]
The overall reflectance value increases with the laser heating pulse energy (Figure 1c). These transient changes at high temperatures are reversible (Figure 1a), sometimes occurring with relatively smaller changes to the background attributed to laser absorber movement; they must manifest a transition in the state of hydrogens at these extreme P–T conditions

Summary

Introduction phenomenon in the study of gas giant planetary interiors and the

The insulator-to-metal transition (IMT) in physical and chemical behavior of highly compressed condensed matter. Static diamond anvil cell (DAC) experiments combined with laser heating probing similar low temperature fluid states have yielded controversial results on the electronic properties of hydrogen and the location of the phase lines.[13,28,29,30,31,32] The difficulty of interpreting these optical DAC experiments is due to indirect probing of the state of hydrogen,[30,31] or detection of reflectance signals superimposed with those of other materials in the DAC cavity and interpreted assuming a priori a direct transformation from insulator to metal.[13,29,32] The latter results, reporting transient reflectance and transmission at a few laser wavelengths, have been found inconsistent with the proposed IMT, while an indirect transformation via intermediate-conductivity states is a plausible alternative.[12,14,15,28,33,34] One of the major drawbacks of the majority of preceding dynamic and static experiments is an extreme paucity of robust spectroscopic observations, which are critical for assessing the material electronic properties. The metallic state exhibits a plasma edge in the visible spectral range, implying a plasma frequency and electronic scattering time that contrasts with previous inferences,[14,20,21] mainly based on the Mott–Ioffe– Regel (MIR) limit approximation in which the electronic meanfree-path reaches the interatomic spacing, and in stark disagreement with the prior static experiments probing hydrogen at few laser wavelengths.[13]

Experimental Section

Discussion

Findings

Conclusions

Conflict of Interest