November 08, 2016

Preparation of diamonds for diamond anvil key to success of creating metallic hydrogen

A Harvard research group announced the successful creation of solid metallic hydrogen. One of the keys for achieving the pressures needed was special treatment of the diamonds used in the diamond anvil.

A rigorous strategy to achieve the higher pressures needed to transform to SMH. The principal limitation for achieving the required pressures to observe SMH (solid metallic hydrogen) in a DAC (diamond anvil cell) has been failure of the diamonds. Hydrogen is very diffusive at room temperature or higher and it can disperse into the confining gasket or the diamonds (at high pressure); diamonds become embrittled and fail. Diffusion is an activated process and is suppressed at low temperatures. In both MH experimental runs, the sample was maintained at liquid nitrogen or liquid helium temperatures.

Diamond anvils used in DACs are generally polished with fine diamond powder on a polishing wheel. They believe that failure of diamonds can arise from microscopic surface defects created in the polishing process. We used type IIac conic synthetic diamonds (supplied by Almax Easy-Lab) with ~30 micron diameter culet flats. About 5 microns were etched off of the diamond culets using the technique of reactive ion etching, to remove defects from the surface. The diamonds were then vacuum annealed at high temperature to remove residual stress. Alumina is known to act as a diffusion barrier against hydrogen. The diamonds, with the mounted rhenium gasket, were coated with a 50 nm thick layer of amorphous alumina by the process of atomic layer deposition. They have had extensive experience with alumina coatings at high pressures and find that it does not affect or contaminate the sample, even at temperatures as high as ~2000 K. Finally, it is known that focused laser beams on samples at high pressures in DACs can lead to failure of the diamonds when the diamonds are highly stressed, even at laser powers as low as10 mW. This is especially true if the laser light is in the blue region of the spectrum and is possibly due to laser induced growth of defects; another possibility is thermal shock to the stressed culet region, resulting from inadvertent laser heating.

Another possibility is thermal shock to the stressed culet region, resulting from inadvertent laser heating. Moreover, a sufficiently intense laser beam, even at infra red (IR) wavelengths, can graphitize the surface of diamond. Thus, they studied the sample mainly with very low power incoherent IR radiation from a thermal source, and minimize illumination of the sample with lasers when the sample is at very high pressures.

The goal for this experiment was to go to higher pressures than in our previous run that ended at ~420 GPa. The determination of pressure in the multi-megabar regime is challenging (see the SM). The sample was cryogenically loaded at 15 K and included a grain of ruby for pressure determination. The pressure was initially determined to ~88 GPa by ruby fluorescence using the scale of Chijioke et al (20); the exciting laser power was limited to a few mW. At higher pressures we measured the IR vibron absorption peaks of hydrogen with a Fourier transform infrared spectrometer with a thermal IR source, using the known pressure dependence of the IR vibron peaks for pressure determination (see SM). This was done to a pressure of ~335 GPa, while the sample was still transparent. For higher pressures they used an alternate technique that did not require using a laser. Our DACs are equipped with strain gauges that measure the load on the sample. The load, or pressure, is increased by rotating a screw attached to the DAC in the cryostat by a long stainless steel tube, accessible at room temperature.

They had determined the pressure was proportional to the rotation of the scre. With the next rotations of the screw the sample transitioned to the phase H2-PRE and started turning black. This darkening was also observed in MH-run-1, meaning that the pressure was ~400 GPa. At this point the only traditional way available for determining the pressure was to use Raman scattering of the diamond phonon line in the highly stressed culet region. For fear of diamond failure due to laser illumination and possible heating of the black sample, they decided to use the rotation of the screw as an indication of pressure. After some turns the sample reflectance changed from black to high reflectivity, characteristic of a metal. They then studied the wavelength dependence of the reflectance of the sample at liquid nitrogen and liquid helium temperatures.

Liquid metallic hydrogen was made before

The solid metal hydrogen at 495 GPa is about 15-fold denser than zero-pressure hydrogen.

As of the writing of the research paper they are maintaining the first sample of the first element in the form of solid metallic hydrogen at liquid nitrogen temperature in a cryostat. This valuable sample may survive warming to room temperature and the DAC could be extracted from the cryostat for greatly enhanced observation and further study. Another possibility is to cool to liquid helium temperatures and slowly release the load to see if SMH is metastable. An important future measurement is to study this metal for high temperature superconductivity

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