12 / 2023-03-18 18:39:53

Colossal NQE in Metallic Hydrogen: Unexpected Prediction from FTDP and AI-PIMD

Anharmonicity; Self-consistent phonon; Path integral; Metallic hydrogen; High pressure

高压物理与材料科学 > 氢与高能量密度材料-I

Metallic hydrogen (MH) is a fascinating state of matter at extreme compression conditions, which had been predicted being a room-temperature superconductor with mobile solid state and dual superfluid-superconducting, thus is an ideal theoretical platform to explore extraordinary behavior of quantum solid. It has long been speculated that MH might be anharmonic, but lacks direct evidence. To this end, the currently wide-adopted temperature-dependent effective potential (TDEP) method for anharmonic phonon dispersion calculation is generalized to the full potential case (i.e., FTDP, full temperature dependent potential) by combining with path integral formalism. This extension naturally resolves the intrinsic difficulty in the original TDEP at low temperatures when below the Debye temperature. The new method is applied to solid metallic hydrogen at high pressure. A colossal nuclear quantum effect (NQE) and subsequent very strong anharmonicity are discovered, which not only leads to unexpectedly large drift of protons, but also slows down the size-effect convergence rate substantially when computing the phonon dispersions. By employing direct ab initio path integral molecular dynamics (AI-PIMD) simulations as the benchmark, a possible breakdown of phonon picture in metallic hydrogen due to colossal NQE is indicated for the first time, implying novel lattice dynamical phenomena unknown so far might exist in this quantum solid. This work establishes FTDP+PIMD as a viable method to probe colossal NQE and possible failure of phonon picture in anharmonic solid. Inspired by this observation, a general theoretical formalism for quantum lattice dynamics beyond phonon is presented, with the main features being discussed.