Evidence of maximum in the melting curve of hydrogen at megabar pressures

Hydrogen at high pressures of $\sim\,400\,$GPa might be in a zero-temperature liquid ground state (N. Ashcroft, J. Phys. Condens. Matter {A \bf12}, 129 (2000), E. G. Brovman et al., Sov. Phys. JETP 35, 783 (1972)). If metallic hydrogen is liquid, the melting $T_{\rm melt}$(P) line should possess a maximum. Here we report on the experimental evaluation of the melting curve of hydrogen in the megabar pressure range. The melting curve of hydrogen has been shown to reach a maximum with $T_{\rm melt}=1050\pm60\,$K at $P=106\,$GPa and the melting temperature of hydrogen decreases at higher pressures so that $T_{\rm melt}=880\pm50\,$K at $P=146\,$GPa. The data were acquired with the aid of a laser heating technique where diamond anvils were not deteriorated by the hot hydrogen. Our experimental observations are in agreement with the theoretical prediction of unusual behavior of the melted hydrogen [S. Bonev et al., Nature 431, 669 (2004)].