Transient terahertz dynamics of spin density wave order in BaFe2As2

Research on iron-based superconductors has demonstrated the electronic band structure has high sensitivity to the modification of the anion height through optical excitation. A displacive excitation towards larger pnictogen height is seen in the transient state, which favors an enhanced magnetic moment [1-3]. Furthermore, an oscillatory response in the optical conductivity closely resembling the spin density wave (SDW) gap has been observed in BaFe2As2 at temperatures above the SDW transition [4]; however, it remains unclear whether there is a transient displacive change of the SDW gap.

We report a time-resolved broadband terahertz spectroscopic study of BaFe2As2 in the A1g phonon driven state. We observe a transient spectral weight redistribution towards the equilibrium SDW gap energy, which then evolves to a melting of the gap at later pump-probe delays. We discuss the origin of the transient gap from the lifetime of the state and its temperature and pump fluence dependence.

[1] L. Rettig, et al., Phys. Rev. Lett. 108 097002 (2012).

[2] I. Avigo, et al., J. Phys.: Condens. Matter 25 094003 (2013).

[3] L. X. Yang, et al., Phys. Rev. Lett. 112 207001 (2014).

[4] K. Kim, et al., Nature Mater. 11, 497–501 (2012).