Metallic Hydrogen

Isaac F. SilveraLyman Laboratory of Physics,

Harvard University

The SSAA program

Jing Song, Albert Derrick VanGennep Azza Elobeid Kiran Linsuain, postdoc postdoc grad student undergrad; missing pic.

Former Students now at the National Labs or SSAA support

Will Evans Livermore Hector Lorenzana LivermoreJon Eggert Livermore

Mohamed Zaghoo Laser Energetics-Omega, RochesterTodd Ditmire U. of Texas, Austin

Former Postdocs Training the next generation in High Density research

Ranga Dias U. of RochesterShanti Deemyad U. of UtahAshkan Salamat UNLV

We pressurize and study hydrogen in a Diamond Anvil Cell (DAC) at low temperatures

Density: 1 13-14 ~15

Pressure 0 400-450 GPa? 400-500 GPa?

High Tc Superconductivity

Liquid?High Tc Supercond.Metastable Metal?

E. Wigner and H. B. Huntington, On the Possibility of a Metallic Modification of Hydrogen, J. Chem. Phys. 3, 764 (1935).

1935 Prediction: 25 GPain the zero Temp. Limit

Hexagonal Close Packed Structure of Para-Molecular Hydrogenat Low Pressure and Temp.--LP or Phase I

The putative phase diagram of hydrogen showing Pathway I and II to the metallic phases. We have observed both transitions:• I The Wigner Huntington transition• II The liquid-liquid or PP Transition

to liquid atomic hydrogen

Our current project is to further understand Pathway I, but I shall start out with Pathway II as we have published results on on this last year.

When we started working on the liquid liquid transition to liquid atomic hydrogen, the only experimental measurement was by Weir et al (S. T. Weir, A. C. Mitchell, and W. J. Nellis, Metallization of Fluid Molecular Hydrogen at 140 GPa (1.4 Mbar), Phys. Rev. Lett. 76, 1860 (1996) using dynamic reverberating shock techniques. They observed liquid metallic hydrogenbut not the first-order phase transition line.

We wanted to observe the phase transition line and used static pressures in DACs.

To show that hydrogen is metallic we measure transmission and reflection using time resolved spectroscopy of heating curves, ie T vs heating power

Transmission and Reflection on the plateau or at higher temperatures than the plateau when the liquid metallic hydrogen film is thin.

We use pulsed laser heating in a DAC

Heating laser pulse

H2-PRE

Pathway II to liquid metallic hydrogen

Technique: pressurize to 100-200 GPa then heat with pulsed laser

M. M. Zaghoo, A. Salamat, and I. F. Silvera, Evidence of a first-order phase transition to metallic hydrogen, Phys. Rev. B 93, 155128 (2016).

Dynamic measurements use ramped compression to observe the transition line. They use the rise of the reflectance as the P,T point for the transition and calculate the temperature.

Static pressure: we measure P and T and used the onset of the plateau for the transition

So to clarify, we did a detailed Finite Element analysisM. Houtput, J. Tempere, and I. F. Silvera, Finite element simulation of the liquid-liquid transition to metallic hydrogen, Phys.Rev. B 100, 134106 (2019).

Phase transition and start of plateau

Caveat:The plateau arises from Latent Heat.

The theoretical value of the Latent heat of transformation is much too small to explain the plateau. Other mechanisms have been recently proposed.

Now we discuss the Wigner Huntington Transition

Problem encountered by researchers in trying to reach the pressures needed for MH: the diamonds fail at 300-400 GPa

Goal: achieve higher pressures to make Metallic Hydrogen

R. Dias and I. F. Silvera, Observation of the Wigner-Huntington Transition to Solid Metallic Hydrogen, Science 355, 715 (2017).

• We used synthetic diamonds (CVD grown)naturals tend to be inhomogeneous and have inclusions.

• Removed microscopic surface damage using Reactive Ion Etching and annealing to 1200 K for 3 days to remove residual stress in the diamonds

• Precision alignment so that the diamond culets remain opposed and parallel to the highest loads.

• Diamonds coated with alumina (Al2O3) that acts as a diffusion barrier against hydrogen into the diamonds. Inhibits diffusion and embrittlement.

• Maintain the hydrogen and diamonds at liquid nitrogen temperature (˜77K). Cryogenic loading. Inhibits diffusion.

• Never Shine significant laser power into the highly stressed diamonds. Known to cause diamond failure.

Transparent black opaque shiny metal

205 GPa 415 GPa 495 GPa

Pictures taken with an iphone camera at the ocular of a stereo microscope

Blow-up of MH

We recently published our observation of a new phase called H2-PRE, observed by FTIR spectroscopy.

R. Dias, O. Noked, and I.F.Silvera, A Quantum Phase Transition in Solid Hydrogen at High Pressure, Phys.Rev. B 100, 184112 (2019).

As pressure increases in the H2-PRE phase the transmission in the IR goes to zero.Semiconductor or Semi-metal?

Our current objective is to reproduce the WHMH

2018: reproduced reflecting sample of MH, but could not measure pressure due to high background.R. Husband and IFS, unpublished

Using data fromM. I. Eremets, A. P. Drozdov, P. P. Kong, and H. Wang, Molecular semimetallic hydrogen, arXiv:1708.05217 (2017).

They measured resistance as function of pressure.

P. Loubeyre, F. Occelli, and P. Dumas, Synchrotron infrared spectroscopic evidence of the probable transition to metal hydrogen, Nature 577, 631 (2020).

This paper seems to reproduce our observed black hydrogen in the H2-PRE phase and call it possible metallic hydrogen.

Future Plans

• Repeat the experiment• X-ray determination of ground state structure: liquid or solid• Measure conductivity vs T: superconductivity• Show if it is metastable• Produce MH at lower pressures

Thanks to all for your Attention

We have now studied the PPT in Deuterium, using static pressure in a DACWe measure both pressure and temperature!

At high pressure the molar densities of hydrogen and deuterium are the same at a given pressure, ie the HP EOS is the same (experiment)

At the same pressure (molar density) we find an isotope effect of ~700 K.

Hydrogen

Deuterium

Deuterium at the Z-Machine

What are phases I, II, III, IV, etc?These are not transitions to MH

They are molecular phases of orientational order of the molecules.

The low pressurePhase, LP or I

Reflectance and fit to the Drude free-electron model

No correction for diamond absorption.

Originally we fit the 4 reflectances (404, 732, 642.6, and 1555 nm), but due to uncertainty in diamond absorption in the blue we have thrown out the first two points.(erratum Science, Vol. 355 Aug.18,2017)

Fit of reflectance to red and IR line (using Drude Model) where diamond absorption is unimportant.

PeakTemperature

A kink or plateau in a heating curvecan signal a phase transition

Average pulse power

kink

plateau

Increased pulse energy goes into latent heat, not raising T.