High-pressure physics: Testing one's metal

Eremets and Troyan achieved room-temperature compression and also sputtered electrodes into the diamond-anvil cell that allowed the conductivity of the sample to be measured, hence going directly to the heart of the process needed to confirm metallization. The authors passivated the diamond surface with a thin layer of sputtered gold or copper, which, while maintaining high transparency for visible light, prevented pressurized hydrogen from contacting bare diamond, a notoriously ill-suited combination in this type of set-up. Eremets and Troyan extrapolate a zero-bandgap state to 260-270 GPa. As pressure increases above 200GPa, they observe a rapid decrease in Raman vibron frequency ascribed to molecular hydrogen, in stark contrast with studies at lower temperature. A pronounced hysteresis is also observed on decreasing the pressure, the molecular Raman activity returns only at around 200 GPa indicating a first order transformation, as would be expected across a melting transition.