Metallic hydrogen

Hydrogen is the simplest and most abundant element in the Universe. There are two pathways for creating metallic hydrogen under high pressures. Over 80 years ago Wigner and Huntington predicted that if solid molecular hydrogen was sufficiently compressed in the T  =  0 K limit, molecules would dissociate to form atomic metallic hydrogen (MH). We have observed this transition at a pressure of 4.95 megabars. MH in this form has probably never existed on Earth or in the Universe; it may be a room temperature superconductor and is predicted to be metastable. If metastable it will have an important technological impact. Liquid metallic hydrogen can also be produced at intermediate pressures and high temperatures and is believed to make up ~90% of the planet Jupiter. We have observed this liquid-liquid transition, also known as the plasma phase transition, at pressures of ~1-2 megabar and temperatures ~1000-2000 K. However, in this paper we shall focus on the Wigner-Huntington transition. We shall discuss the methods used to observe metallic hydrogen at extreme conditions of static pressure in the laboratory, extending our understanding of the phase diagram of the simplest atom in the periodic table.