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High Pressure Metallic Hydrogen Metallic Hydrogen Isaac F. Silvera Lyman 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 Livermore Jon Eggert Livermore Mohamed Zaghoo Laser Energetics-Omega, Rochester Todd Ditmire U. of Texas, Austin Former Postdocs Training the next generation in High Density research Ranga Dias U. of Rochester Shanti Deemyad U. of Utah Ashkan Salamat UNLV We pressurize and study hydrogen in a Diamond Anvil Cell (DAC) at low temperatures E. Wigner and H. B. Huntington, On the Possibility of a Metallic Modification of Hydrogen, J. Chem. Phys. 3, 764 (1935). 1935 Prediction: 25 GPa in the zero Temp. Limit Liquid? High Tc High Tc Supercond. Superconductivity Metastable Metal? Density: 1 13-14 ~15 Pressure 0 400-450 GPa? 400-500 GPa? Hexagonal Close Packed Structure of Para-Molecular Hydrogen at 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 hydrogen but 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 Pathway II to liquid metallic hydrogen 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). H2-PRE Technique: pressurize to 100-200 GPa then heat with pulsed laser 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 analysis M. 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 Caveat: start of The plateau arises from Latent Heat. plateau 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 R. Dias and I. F. Silvera, Observation of the Wigner-Huntington Transition to Solid Metallic Hydrogen, Science 355, 715 (2017). 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 • 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 Blow-up of MH Pictures taken with an iphone camera at the ocular of a stereo microscope As pressure increases in the H2-PRE phase the transmission in the IR goes to zero. Semiconductor or Semi-metal? 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). Our current objective is to reproduce the WHMH 2018: reproduced reflecting P. Loubeyre, F. Occelli, and P. Dumas, sample of MH, but could not Synchrotron infrared spectroscopic evidence of the probable transition to metal hydrogen, measure pressure due to high Nature 577, 631 (2020). background. R. Husband and IFS, unpublished This paper seems to reproduce our observed black hydrogen in the H2-PRE phase and call it possible metallic hydrogen. Using data from M. 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. 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 DAC We 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) Deuterium At the same pressure (molar density) we find an isotope effect of ~700 K. Hydrogen 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 pressure Phase, 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. A kink or plateau in a heating curve can signal a phase transition Increased pulse energy goes into latent heat, not raising T. plateau Peak Temperature kink Average pulse power.
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