1. Hydrogen at Ultra High Pressure Isaac F
Hydrogen at Ultra High Pressure Isaac F. Silvera, Harvard University, DMR
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.
The data shown in the top figure is published (V. Dzyabura, M. Zaghoo, and I. F. Silvera, "Evidence of a liquid–liquid phase transition in hot dense hydrogen," PNAS, vol. 110, pp , 2013.). How are the phase points measured? If one plots the temperature of the sample as a function of average laser power, the temperature rises monotonically with increasing laser power. At a phase transition power goes into heat of transformation rather than raising the temperature so that one finds a kink or plateau at the transition. This technique enabled determination of the melting line as well as a higher temperature plateau in agreement with theoretical predictions for a first order phase transition called the plasma phase transition (PPT), the liquid molecular to liquid atomic metallic transition. In order to show that this is metallic we developed techniques to measure the transmission of the sample during the ~100ns long laser pulse. The results are shown in the lower graph and the abrupt increased absorption (reduced transmission) at the PPT plateau is the expected behavior for a metal. The small change in transmission is because the layer of hydrogen that is heated is very thin, probably ~1 nm. There have been several claims of metallic hydrogen in the past that have been refuted and not reproduced. Before publishing these results (which are quite challenging to obtain) we want to expand our measurements in wavelength and pressure.
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.

2. Hydrogen at Ultra High Pressure Isaac F
Hydrogen at Ultra High Pressure Isaac F. Silvera, Harvard University, DMR
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.
Semi transparent film (green line) on diamond as absorber.
CW laser for transmission
Hole in the absorber
Optical measurements of transmission are made using a thin film laser absorber. Both the heating laser (grey beam at 1064 nm wavelength) and measurement laser (different wavelengths) pass through the absorber, with about 25% transmission at lower temperatures where only the film is absorbing, as the hydrogen Is in the insulating molecular phase. The powerful 1064 nm laser light is filtered out with a pair of notch filters that pass the variable frequency light. This technique should be very useful for the high pressure community. Typical metal absorbers have an absorption of less than 10%. By making a thin film and matching to the impedance of free space the absorption can be increased to 50%. Generally for these types of measurements, using pulsed lasers, one is limited by signal to noise. Increasing the absorption (emissivity) greatly increases the blackbody irradiance (proportional to the emissivity) and thus the signal. For hydrogen our biggest problem has been the deterioration of the film. Over time the absorber is repetitively heated (maybe 10^6 times during a run) and the hydrogen aggressively diffuses into the absorber and it deteriorates to end a run. We have been using tungsten films, covered with alumina to suppress the hydrogen diffusion, but at high temperatures we know that hydrogen slowly passes through the alumina diffusion barrier. We are now experimenting with other metallic films.
Transmission through a laser heated sample. Green cone is light for measuring transmission. Ruby is used to measure pressure.