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

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.

Electronic energy gap closure and metal-insulator transition in dense liquid hydrogen

“…Using long pulsed-laser heating, another experimental group observed a similar two-stage transition: an anomalous temperature behaviour and the onset of absorption followed by the rapid increase of the reflectivity [18,19]. However, the P-T conditions ascribed to this transitions are in disagreement with the previous DAC experiments [ [14][15][16][17]20].…”

“…Dynamically, hydrogen can be compressed with shock waves, following the time-varying changes in pressure, the metallic states can be detected via electrical, optical and density measurements [5][6][7][8][9][10][11][12][13]. Metallic liquid hydrogen can also be investigated in diamond anvil cell (DAC), using controlled laser heating at constant volume [14][15][16][17][18][19]. A rapid change in the reflectivity has been observed with both techniques, but inconsistencies between different experimental results remain.…”

Nonadiabatic effects and excitonlike states during the insulator-to-metal transition in warm dense hydrogen

“…The eFF calculations show the isotopic effect ∼ 400 K 33 that is close to experimental data 12 and much larger than the isotopic effect based on nuclear quantum effects in equilibrium FPMD and QMC. The absence of the isotopic effect in the experimental results in DAC of Goncharov et al 44 can be explained by measuring fluid H 2 /D 2 properties during µs-long cooling contrary the DAC experiments of Silvera et al 12 that reports measurements done during heating. The relaxation of atomic fluid to the state with molecules in their electronic ground state proceeds during cooling differently and non-adiabatic radiationless internal conversion can not be expected to play a major role.…”

Phases of the hydrogen isotopes under pressure: metallic hydrogen

“…It was pointed out that Goncharov et al's experiment had a flaw in the design of the sample absorber used for heating and in the use of spatial filters that would give spurious results [126]. This group has again studied hydrogen and deuterium in a DAC, this time observing the first-order phase line, reporting substantially higher temperatures for the phase line and no isotope effect [127]. Their supplementary Figure 1 shows that they are still using the same experimental design that was flawed, so we do not plot their arXiv results in Figure 13.…”