Hydrogen at Ultra High PressureIsaac F. Silvera, Harvard University, DMR 1308641

We present recent optical data in support of our experiments to produce metallic hydrogen (MH) in the laboratory. Last year we observed a phase transition in hydrogen at high pressure and temperature, believed to be the Plasma Phase Transition to liquid metallic hydrogen. Below the phase transition hydrogen is transparent in the visible. We now show that in the new phase the transmission abruptly decreases and the reflectivity increases, providing evidence that we have produced metallic hydrogen in the laboratory.As seen in the phase diagram, MH can be produced at high pressure either isothermally at low temperature or high temperature at high pressure. We have focused on the latter. Using pulsed laser heating of hydrogen at megabar pressures in a diamond anvil cell, we have also extended the melting line to the highest pressures yet. Current efforts are aimed at refining the optical data and extending measurements to higher pressures.

Top: A P/T phase diagram of hydrogen showing our data for the proposed liquid-liquid phase transition to metallic hydrogen (MH) based on heating curves. Bottom: Transmission as a function of temperature supporting the proposed metallic behavior at and above the proposed phase transition to MH.

If metallic hydrogen is metastable, i.e., remains metallic when the pressure is lifted, it could revolutionize society as a room temperature superconductor and be a game-changing rocket propellant that would revolutionize rocketry. Below I address a technical development.

A large number of of high-pressure researchers are studying phase diagrams of transparent materials at high pressures and temperatures of thousands of degrees. A laser absorber embedded in the sample, when heated (grey beam in figure), also heats the sample. To measure optical properties a hole can be made in the absorber. There are two serious problems: 1. most absorbers have low emissivity, so for example, Pt with emissivity of ~0.1 only absorbs 10% of the light, and the graybody radiation used to determine the temperature is only 10% of that of a blackbody; thus the signal is low; 2. An absorber with a, hole as in the upper figure, can be deformed and the hole closes at high pressures.

A thin film absorber deposited on diamond will break the diamond when heated. We have developed a technique in which a thin insulating layer of alumina is deposited on the diamond and on top of that a thin absorbing metallic layer. The emissivity can be designed to be ~0.5 and transmission can be measured through the thin layer. This important development will will have a broad impact on the community. This new technique enabled optical measurements of of a thin film of hydrogen.

Hydrogen at Ultra High PressureIsaac F. Silvera, Harvard University, DMR 1308641

Semi transparent film (green line) on diamond as absorber.

CW laser for transmission

Hole in the absorber

Transmission through a laser heated sample. Green cone is light for measuring transmission. Ruby is used to measure pressure.