Striking isotope effect on the metallization phase lines of liquid hydrogen and deuterium

Abstract

Liquid atomic metallic hydrogen is the simplest, lightest, and most abundant of all liquid metals 1,2 . The role of nucleon motions or ion dynamics has been somewhat ignored in relation to the dissociative insulator-metal transition. Almost all previous experimental high-pressure studies have treated the fluid isotopes, hydrogen and deuterium, with no distinction 3-8 .Studying both hydrogen and deuterium at the same density, most crucially at the phase transition line, can experimentally reveal the importance of ion dynamics. We use static compression to study the optical properties of dense deuterium in the pressure region of 1.2-1.7 Mbar and measured temperatures up to ~3000 K. We observe an abrupt increase in reflectance, consistent with dissociation-induced metallization, at the transition. Here we show that at the same pressure (density) for the two isotopes, the phase line of this transition reveals a prominent isotopic shift, ~700 K. This shift is lower than the isotopic difference in the free-molecule dissociation energies 9 , but it is still large considering the high density of the liquid and the complex many-body effects. Our work reveals the importance of quantum nuclear effects in describing the metallization transition and conduction properties in dense hydrogen systems at conditions of giant planetary interiors, and provides an invaluable benchmark for ab-initio calculations.As the lightest atoms, hydrogen and its isotopes exhibit the largest mass ratios of the elements, giving rise to large differences in their properties. The binding energies of the homonuclear freemolecules H 2 and D 2 differ by ~900 K (4.477eV for H 2 and 4.556 eV for D 2 ) 9 . The isotopic shift in the binding energy is related to the different zero-point energies (ZPE) arising from the fundamental vibrational mode of the molecules. At low temperatures (T), where both hydrogen and deuterium form quantum solids due to their ZPE, the large isotopic effects are manifested in phonon, vibrational, and rotational excitations, as well as differences in their equations of state and melting temperatures 10 . In 1935, Wigner and Huntington first discussed the role of density in destabilizing

Thermodynamic anomalies and three distinct liquid-liquid transitions in warm dense liquid hydrogen

“…As mentioned in the main text, there might be an isotope effect on the location of dissociation. A recent reflectance measurement also indicated a similar effect on the metallization of liquid hydrogen and deuterium, a further confirmation of an early dynamic compression experimental observations [72]. Our AIMD simulations did not include nuclear quantum effects (NQE), thus might overestimate the dissociation temperature and pressure.…”

“…S25 that is a simplified version of Fig. 6 in the main text but including the recent DAC experimental data on hydrogen and deuterium [72], vdW-DF functional fits data for deuterium, whereas the cancellation in PBE functional for hydrogen still has some residual. Though the PBE results are in good agreement with the experimental data obtained by the "temperature vs laser heating power" curves…”

Finite-element simulation of the liquid-liquid transition to metallic hydrogen

“…They found a phase line that differed from that of Knudson et al [18], and was in much better agreement with earlier measurements, the phase line of ref. [37], and theory [11,12]. Just as in [18] they presented a semi-empirical phase line, as temperatures were calculated, not measured.…”

“…At this point in the study of the PPT there is now reasonable agreement between recently published experiments. There is a smaller disagreement for the isotope effect measured in static experiments [16,17,37,38] and theory [11]. Yet there is disagreement as to the method of determining the P-T points of the transition: either the onset of the plateau or the abrupt rise in reflectance.…”

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

“…The nature of the molecular-to-atomic transition in fluid hydrogen and deuterium is a fundamental problem 1,2 that has been drawing increasing attention for more than two decades since the first reliable experiments on electrical conductivities of fluid H 2 and D 2 at shock pressures 3 . In the subsequent dynamic and static experiments a large amount of experimental data has been collected on the revealed insulator-tometal transition (IMT) (e.g., see [4][5][6][7] and [8][9][10][11][12] ). However at the moment, there are inconsistencies between different experimental results and there is no complete theoretical understanding of this transition.…”