Ultrafast Relaxation Dynamics of Spin-Density Wave Order in BaFe$_2$As$_2$ under High Pressures

BaFe$_2$As$_2$ is the parent compound for a family of iron-based high-temperature superconductors as well as a prototypical example of the spin-density wave (SDW) system. In this study, we perform an optical pump-probe study of this compound to systematically investigate the SDW order across the pressure-temperature phase diagram. The suppression of the SDW order by pressure manifests itself by the increase of relaxation time together with the decrease of the pump-probe signal and the pump energy necessary for complete vaporization of the SDW condensate. We have found that the pressure-driven suppression of the SDW order at low temperature occurs gradually in contrast to the thermally-induced SDW transition. Our results suggest that the pressure-driven quantum phase transition in BaFe$_2$As$_2$ (and probably other iron pnictides) is continuous and it is caused by the gradual worsening of the Fermi-surface nesting conditions.