Two Pathways to Metallic Hydrogen and Deuterium
Hydrogen and its isotopes can be insulators, semiconductors, metals, and possibly superconductors with very high critical temperatures, possibly room temperature, all depending on the density and the bonding, molecular or atomic. At zero pressure (P) and low temperature (T) (below 14 K) hydrogen crystallizes as a molecular solid. In 1935 Wigner and Huntington (WH) studied the dependence of the hydrogen phase diagram on density finding that at a high density or pressure, the molecular bond destabilizes and molecules dissociate to form atomic metallic hydrogen (MH). We have observed this transition at 495 GPa (4.95 megabars)  at liquid helium and liquid nitrogen temperatures. Prior to metallization a new phase we discovered called H2-PRE becomes black and is semiconducting.
It was also realized that at a lower, but still high pressure and very high temperatures, there is a temperature driven transition to liquid atomic metallic hydrogen (LMH). This liquid-liquid phase transition is sometimes called the Plasma Phase Transition or the PPT. We have determined the phase line for LMH and liquid atomic deuterium for several values of P and T, as well as optical properties and optical conductivity [2, 3]. We observe isotopic differences in the phase lines.
There are several interesting predictions for MH that we are studying: metastability, optical properties in the IR, and the possibility of room temperature superconductivity. MH at low temperatures has probably never before existed in the Universe, thus this is an exciting area of study on a unique material. LMH is the principle component of giant outer planets such as Jupiter and gives rise to its magnetic field via the dynamo. A number of new techniques and procedures were developed to achieve the conditions needed for this breakthrough to observe MH that has been sought since the WH prediction over 80 years ago.
 R. Dias and I. F. Silvera, Science, vol. 355, pp. 715-718, 2017. [pdf]
 M. M. Zaghoo, A. Salamat, and I. F. Silvera, Phys. Rev. B, vol. 93, p. 155128, 2016. [pdf]
 M. Zaghoo and I. F. Silvera, Proc. of the National Academy of Sciences, vol. 114, no. 11873, 2017. [pdf]