Transient Spin Density Wave Order Induced in the Normal State of BaFe₂As₂ by Coherent Lattice Oscillations

¹ Department of Physics and Center for Applied Photonics, University of Konstanz, Universitätsstr. 10, 78464 Konstanz, Germany
² University of Fribourg, Department of Physics and Fribourg Center for Nanomaterials, Chemin du Musée 3, 1700 Fribourg, Switzerland
³ Department of Physics, Chungbuk National University, 52 Naesudong-ro, Cheongju 361-763, Korea
⁴ Department of Physics, University of Regensburg, Universitätsstr. 31, 93053 Regensburg, Germany
⁵ Karlsruhe Institute of Technology, Institute for Solid State Physics, 76021 Karlsruhe, Germany
⁶ Complex Matter Department, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

Abstract

We trace the ultrafast dynamics of the spin density wave gap of the pnictide system BaFe₂As₂ 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.