Transient Spin Density Wave Order Induced in the Normal State of BaFe₂As₂by Coherent Lattice Oscillations

Abstract

Abstract. We trace the ultrafast dynamics of the spin density wave gap of the pnictide system BaFe 2 As 2 probing resonantly with broadband multi-terahertz pulses. The photoexcitation in the low-temperature ground state leads to a fast suppression of the spin order followed by a slower recovery process. Surprisingly, in the normal state, we observe periodic oscillations of the spin density wave feature at a frequency as high as 5.5 THz. Our results indicate a transient development of a macroscopic order driven by a coherent lattice oscillation and attest to a pronounced spin-phonon coupling in pnictides.In recent years, the iron-based pnictides have been established as a new class of high-temperature superconductors. The superconducting state in these materials is achieved by chemical doping or application of external pressure to a parent compound with a spin density wave (SDW) ground state. Therefore, the underlying magnetic order has been at the heart of the discussion concerning the mechanism of high-temperature superconductivity in pnictides [1]. On the other hand, detailed theoretical and experimental studies have pointed out the intimate connection between the lattice structure and the magnetic ordering [2,3]. Therefore, unveiling the interplay among the lattice structure, magnetism and superconductivity is crucial for an understanding of the nature of hightemperature superconductivity in pnictides.Ultrafast pump-probe experiments provide access to the dynamics of various quantum degrees of freedom after perturbation by ultrashort optical pulses. Thus, direct information about the interplay between single-particle electronic states, collective modes, magnetization and lattice structure may be obtained. The time-resolved multi-THz techniques extending over the far-and mid-infrared spectral ranges with time resolution of a few tens of femtoseconds are particularly useful for resonant probing of single-particle and collective low-energy excitations in complex materials [4].Here, we employ few-cycle multi-THz pulses to resonantly probe the evolution of the SDW gap of BaFe 2 As 2 after excitation with a femtosecond optical pulse. This material represents the parent This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.