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

The transition of molecular hydrogen to atomic ionized state with the increase of temperature and pressure poses still unresolved problems for experimental methods and theory. Here we analyze the dynamics of this transition and show its nonequilibrium nonadiabatic character overlooked in both interpreting experimental data and in theoretical models. The nonadiabatic mechanism explains the strong isotopic effect [M. Zaghoo, R. J. Husband, and I. F. Silvera, Phys. Rev. B 98, 104102 (2018).] and the large latent heat [M. Houtput, J. Tempere, and I. F. Silvera, Phys. Rev. B 100, 134106 (2019).] reported recently. We demonstrate the possibility of the formation of intermediate excitonlike molecular states at heating of molecular hydrogen that can explain the puzzling experimental data on reflectivity and conductivity during the insulator-to-metal transition.

Figure 1

Temperatures and composition in the eFF model of fluid ${\mathrm{H}}_2$ under isochoric heating $(\rho =0.3$ g/cc).

Figure 2

The scheme of the Kohn-Sham orbital occupations in the DFT models for the finite-temperature case, for the ground-state case, and for the ROKS case.

Figure 3

Electron density distributions in the ROKS DFT model for the $S0$-equilibrated fluid structures at two densities (480 particles at 1300 K).

Figure 4

The parameters along the example ROKS DFT SH-MD trajectory for fluid ${\mathrm{H}}_2$ (the state of the electronic subsystem, the $S1\text{−}S0$ energy gap, the concentrations of molecules and single atoms/ions, and the temperature of the ion subsystem).