Ultrafast Pump-Probe Spectroscopy of BaFe₂As₂ under High Pressures

An important member of the iron-based high-temperature superconductor family, BaFe₂As₂ undergoes a transition to a spin-density wave (SDW) state on cooling below 137 K. Under application of external pressure the SDW transition temperature gradually decreases, and the SDW phase gets completely suppressed above 3 GPa, enabling the onset of superconductivity. Such phenomenon is often referred to as quantum phase transition. Optical pump-probe spectroscopy has been used previously to investigate the dynamics of the SDW and the superconducting order at various temperatures and different doping levels. However, a direct study of the quantum phase transition induced by external high pressure has not been carried out until now.
In our study, pump and probe pulses with the central wavelength of 800 nm were focused onto the sample inside the diamond anvil cell, mounted inside a cryostat. The reflected probe signal was collected using a confocal microscopy scheme. From the measured pump-probe traces the quasiparticle lifetimes and the condensation energy of the SDW state were obtained at various pressures up to 4.4 GPa and the fixed temperature of 8 K. The SDW condensation energy decreases with pressure, and already below 4.4 GPa the SDW state is completely suppressed. At the same time, the decrease of the condensation energy is accompanied by the increase of the quasiparticle lifetimes. Since the lifetimes should be inversely proportional to the SDW gap energy, this critical slowing down of the quasiparticle relaxation dynamics confirms the vanishing of the SDW gap at the quantum phase transition.