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Ultrafast transient generation of spin-density-wave order in the normal state of BaFe2As2 driven by coherent lattice vibrations

Nature Materials volume 11, pages 497–501 (2012)Cite this article

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Abstract

The interplay among charge, spin and lattice degrees of freedom in solids gives rise to intriguing macroscopic quantum phenomena such as colossal magnetoresistance, multiferroicity and high-temperature superconductivity1,2,3. Strong coupling or competition between various orders in these systems presents the key to manipulate their functional properties by means of external perturbations such as electric and magnetic fields2 or pressure3. Ultrashort and intense optical pulses have emerged as an interesting tool to investigate elementary dynamics and control material properties by melting an existing order4,5,6. Here, we employ few-cycle multi-terahertz pulses to resonantly probe the evolution of the spin-density-wave (SDW) gap of the pnictide compound BaFe2As2 following excitation with a femtosecond optical pulse. When starting in the low-temperature ground state, optical excitation results in a melting of the SDW order, followed by ultrafast recovery. In contrast, the SDW gap is induced when we excite the normal state above the transition temperature. Very surprisingly, the transient ordering quasi-adiabatically follows a coherent lattice oscillation at a frequency as high as 5.5 THz. Our results attest to a pronounced spin–phonon coupling in pnictides that supports rapid development of a macroscopic order on small vibrational displacement even without breaking the symmetry of the crystal.

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Figure 1: Photo-induced transient changes of the MIR conductivity in the SDW state.
Figure 2: Photo-induced transient changes of the MIR conductivity in the normal state slightly above TSDW.
Figure 3: Schematic visualization of the A1g vibration and its effect on the band structure.
Figure 4: Temperature dependence of the relative modulation amplitude due to the coherent phonon at 5.5 THz.

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Acknowledgements

We acknowledge financial support by the Schweizer Nationalfonds (SNF) under Grants No. PA00P2_129091 and No. 200020-129484, by Deutsche Forschungsgemeinschaft through the Emmy Noether Program and SPP 1458, and by the Alexander von Humboldt Foundation.

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Authors and Affiliations

  1. Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz, Germany

    K. W. Kim, A. Pashkin, H. Schäfer, M. Beyer, M. Porer, J. Demsar, R. Huber & A. Leitenstorfer

  2. Department of Physics and Fribourg Center for Nanomaterials, University of Fribourg, Chemin du Musée 3, CH-1700 Fribourg, Switzerland

    K. W. Kim & C. Bernhard

  3. Department of Physics, Chungbuk National University, Cheongju 361-763, Korea

    K. W. Kim

  4. Department of Physics, University of Regensburg, D-93053 Regensburg, Germany

    M. Porer & R. Huber

  5. Karlsruhe Institute of Technology, Institute for Solid-State Physics, D-76021 Karlsruhe, Germany

    T. Wolf

  6. Complex Matter Department, Jozef Stefan Institute, SI-1000 Ljubljana, Slovenia

    J. Demsar

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Contributions

K.W.K., J.D., R.H. and A.L. planned the project; K.W.K. performed ellipsometry measurements; K.W.K., A.P. and M.P. performed terahertz measurements; H.S. and M.B. performed NIR/visible region measurements; K.W.K., A.P., H.S. and M.B. analysed data; T.W. grew samples; and K.W.K., A.P., C.B., J.D., R.H. and A.L. wrote the paper. All authors contributed to discussions and gave comments on the manuscript.

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Correspondence to K. W. Kim or R. Huber.

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Kim, K., Pashkin, A., Schäfer, H. et al. Ultrafast transient generation of spin-density-wave order in the normal state of BaFe2As2 driven by coherent lattice vibrations. Nature Mater 11, 497–501 (2012). https://doi.org/10.1038/nmat3294

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