Evidence of a first-order phase transition to metallic hydrogen

Zaghoo, Mohamed; Salamat, Ashkan; Silvera, Isaac F.

doi:10.1103/physrevb.93.155128

Phys. Rev. B

2016

DOI: 10.1103/physrevb.93.155128

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Evidence of a first-order phase transition to metallic hydrogen

Mohamed Zaghoo¹,

Ashkan Salamat²,

Isaac F. Silvera³

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Cited by 135 publications

(196 citation statements)

References 68 publications

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“…In the first, done by Silvera and coworkers [32] a first order IMT in hydrogen in a pressure range which agrees quantitatively with previous experiments and simulations. This evidence has been qualitatively confirmed also by Ref.…”

supporting

confidence: 76%

Acceleratingab initioMolecular Dynamics and Probing the Weak Dispersive Forces in Dense Liquid Hydrogen

Mazzola

Sorella

2017

Phys. Rev. Lett.

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We propose an ab-initio molecular dynamics method, capable to reduce dramatically the autocorrelation time required for the simulation of classical and quantum particles at finite temperature. The method is based on an efficient implementation of a first order Langevin dynamics modified by means of a suitable, position dependent acceleration matrix S. Here we apply this technique, within a Quantum Monte Carlo (QMC) based wavefunction approach and within the Born-Oppheneimer approximation, for determining the phase diagram of high-pressure Hydrogen with simulations much longer than the autocorrelation time. With the proposed method, we are able to equilibrate in few hundreds steps even close to the liquid-liquid phase transition (LLT). Within our approach we find that the LLT transition is consistent with recent density functionals predicting a much larger transition pressures when the long range dispersive forces are taken into account.One of the most important problems in ab-initio molecular dynamics (MD) is the coexistence of much different time scales underlying physical processes, even when computing equilibrium finite temperature properties. This has been a challenge for most ab-initio simulations in systems of biophysical interest, the most striking one being protein folding [1,2], where the microscopic time scale of molecular vibration is of the order of f s, while the macroscopic time scale of the folding exceeds the µs. Also in water system simulations, the weak Van der Waals (vdW) interactions and the related hydrogen bond imply very difficult equilibration properties at ambient conditions, so that the most accurate simulations are based on force field fitting forces. We will show here that, a computationally expensive ab-initio method for dense liquid hydrogen can be combined with a sampling technique capable to reduce drastically the autocorrelation times. This method has the potential, due to its simplicity, to be applied with success also to much more complex systems and other ab-initio techniques.At variance of all previous attempts[3-10], we propose that an optimal way to get rid of different time scales is based on the use of first order Langevin dynamics:where T is the temperature, f Ri = −∂ Ri V ( R) and V R is an energy potential of the classical coordinates R, e.g. atomic positions. For S ij = δ ij this is the conventional first order Langevin dynamics (CFOLD). Preconditioned Langevin equations, formally similar to Eq. 40, have been already introduced in several fields different from molecular dynamics, such as gauge field theories[11], statistics [12] and machine learning [13], with different definitions of the preconditioning matrix S. Instead, the main idea of this work is based on the simple solution of CFOLD for the harmonic potential V (R) = −K R, where K is the elastic matrix term. In this case one is able to show that the i) the autocorrelation time is independent of temperature τ −1 corr = K min , where K min (K max ) is the lowest (largest) non-zero eigenvalue of the elastic matrix...

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supporting

confidence: 76%

Acceleratingab initioMolecular Dynamics and Probing the Weak Dispersive Forces in Dense Liquid Hydrogen

Mazzola

Sorella

2017

Phys. Rev. Lett.

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“…However, in order to better compare our results with these low temperature experiments, also the quantum nature of the protons (here assumed as classical particles) needs to be considered. Indeed, when we correct our results with these nuclear quantum effects (NQE), the agreement with static compression experiments improves significantly 11,27,28 (see Fig.1). In this work we do not perform directly simulations beyond the classical nuclei approximation, using path integral based methods as in Pierleoni et al 9 , where electronic QMC simulations with or without NQE are reported.…”

mentioning

confidence: 60%

“…These features suggest the persistence of (possibly short-lived) molecular structures in the metallic fluid, which is not completely dissociated near the LLT. Our QMC LLT lies between the two recent experiments obtained using static compression by Silvera and coworkers 11,27 and the dynamic compression measurements (with deuterium) by Knudson et al 2 , although it is much closer to the first reference. Moreover, the systematic errors caused by the finite size and basis set can shift the LLT by ∼ 10 GPa, therefore, our results are compatible with the recent QMC prediction by Pierleoni et.…”

mentioning

confidence: 92%

“…They show that NQE shifts the LLT to smaller pressures at most by 35 GPa at 1200 K and by 25 GPa at 1500 K. Here we simply apply these shifts to our LLT to derive the phase boundary in Fig. 1 and compare to experiments 11 . Notice that PBE underestimates the metallization pressure compared to QMC (Fig.…”

mentioning

confidence: 99%

“…Other symbols refer to metallization experimental data. Shown are experiments with static compression 10,11 (light and dark green triangles) and deuterium shockwave 2 (brown diamonds).…”

mentioning

confidence: 99%

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Phase Diagram of Hydrogen and a Hydrogen-Helium Mixture at Planetary Conditions by Quantum Monte Carlo Simulations

2018

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Understanding planetary interiors is directly linked to our ability of simulating exotic quantum mechanical systems such as hydrogen (H) and hydrogen-helium (H-He) mixtures at high pressures and temperatures 1 . Equations of State (EOSs) tables based on Density Functional Theory (DFT), are commonly used by planetary scientists, although this method allows only for a qualitative description of the phase diagram 2 , due to an incomplete treatment of electronic interactions 3 . Here we report Quantum Monte Carlo (QMC) molecular dynamics simulations of pure H and H-He mixture. We calculate the first QMC EOS at 6000 K for an H-He mixture of a proto-solar composition, and show the crucial influence of He on the H metallization pressure. Our results can be used to calibrate other EOS calculations and are very timely given the accurate determination of Jupiter's gravitational field from the NASA Juno mission and the effort to determine its structure 4 .Since a few decades the link between the uncertainty of the H EOS and the internal structure of Jupiter (and other gaseous planets) has been investigated and many efforts to model Jupiter's interior have been carried 1,5-7 . The computation of an EOS from first principles requires to solve a many-body quantum mechanical problem, a task which is beyond the currently available theoretical and computational capabilities. In practice, we must resort to several approximations. The first is to decouple the ionic and electronic problems and consider the ions as classical or quantum particles, determining their motion by following the Born-Oppenheimer potential energy surface. The second approximation concerns the description of the electronic interaction and the exchange one, due to the Pauli exclusion principle.The standard approach to EOS calculations relies on Density Functional Theory (DFT), which targets the tridimensional electronic density rather than the (N e electrons) many-body wave-function. Its success and simplicity have lead to a widespread application in materials science and to the development of several software packages which allow fast and reproducible calculations 8 . Although DFT is formally exact, the explicit functional form to describe the exchange and correlation (XC) effects between electrons remains approximated 3 . Indeed, a systematic and efficient route to improve XC functional is still lacking. Therefore, in practical solid state calculations, benchmarks against experimental data, are often required to validate the XC functional used to describe the system in a satisfactory manner.Dense hydrogen systems, both in the low temperature solid and in the liquid phase remain a challenge to DFT simulations due to the interplay of strong correlation and non-covalent interactions between the atoms. DFT calculations with different functionals produce different results with the expected metallization pressure varying over a range of 100-200 GPa (Fig. 1) 12,13 . Since experimental data are limited, it is not possible to identify a posteriori the best functional...

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

et al. 2019

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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.

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Evidence of a first-order phase transition to metallic hydrogen

Cited by 135 publications

References 68 publications

Acceleratingab initioMolecular Dynamics and Probing the Weak Dispersive Forces in Dense Liquid Hydrogen

Acceleratingab initioMolecular Dynamics and Probing the Weak Dispersive Forces in Dense Liquid Hydrogen

Phase Diagram of Hydrogen and a Hydrogen-Helium Mixture at Planetary Conditions by Quantum Monte Carlo Simulations

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

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