Phase diagram, metallization, black and shiny reflections of Hydrogen in the 3-7 Mbar range by quantum Monte-Carlo with anharmonic zero-point fluctuations

The interplay between electron correlation and nuclear quantum effects makes our understanding of elemental hydrogen a formidable challenge. Here, we present the phase diagram of hydrogen at low temperatures in high-pressure 300-700 GPa range, by accounting for highly accurate electronic and nuclear enthalpies. We evaluated internal electronic energies by diffusion quantum Monte Carlo, an nuclear quantum motion and anharmonicity by the stochastic self-consistent harmonic approximation [1]. Our results [2] show that the long-sought atomic shiny metallic hydrogen (Cs-IV), predicted to host room-temperature superconductivity, forms at 577(10) GPa. Indeed, anharmonicity pushes the stability of this phase towards pressures much larger than previous theoretical estimates or attained experimental values. Before atomization we predict a metal-insulator transition in the molecular phase III (C2/c-14) at 380 due to band overlap [3], in agreement with transport data [4]. This state is an unusual, non-reflecting, black metal, transparent in the infrared, as observed in [5]. At 412(40) GPa we predict a transition to phase VI (Cmca-12), another non-reflecting, black, metallic structure that is still molecular, but in contrast to phase III, totally opaque in the IR, as observed in [5]. We suggest that the III-VI boundary could be also detected by conductivity measurements, since we predict a significant drop of resistivity in correspondence of this phase transition [2].

[1] L. Monacelli, R. Bianco, M. Cherubini, M. Calandra, I. Errea, F. Mauri, Journal of Physics: Condensed Matter 33, 363001 (2021)

[2] L. Monacelli, M. Casula, K. Nakno, S. Sorella, F. Mauri, to be published

[3] L. Monacelli, I. Errea, M. Calandra, F. Mauri, Nat. Phys. 15, 1246–1249 (2019)

[4] M. I. Eremets, A. P. Drozdov, P.P. Kong, H. Wang, Nat. Phys. 17, 163 (2021)

[5] P. Loubeyre, F. Occelli, P. Dumas, Nature 577, 631 (2020)