Generation mechanism of terahertz coherent acoustic phonons in Fe

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Generation mechanism of terahertz coherent acoustic phonons in Fe

T. Henighan, M. Trigo, S. Bonetti, P. Granitzka, D. Higley, Z. Chen, M. P. Jiang, R. Kukreja, A. Gray, A. H. Reid, E. Jal, M. C. Hoffmann, M. Kozina, S. Song, M. Chollet, D. Zhu, P. F. Xu, J. Jeong, K. Carva, P. Maldonado, P. M. Oppeneer, M. G. Samant, S. S. P. Parkin, D. A. Reis, and H. A. Dürr

Phys. Rev. B 93, 220301(R) – Published 10 June 2016

Abstract

We use femtosecond time-resolved hard x-ray scattering to detect coherent acoustic phonons generated during ultrafast laser excitation of ferromagnetic bcc Fe films grown on MgO(001). We observe the coherent longitudinal-acoustic phonons as a function of wave vector through analysis of the temporal oscillations in the x-ray scattering signal. The width of the extracted strain wave front associated with this coherent motion is $\sim 100$ fs. An effective electronic Grüneisen parameter is extracted within a two-temperature model. However, ab initio calculations show that the phonons are nonthermal on the time scale of the experiment, which calls into question the validity of extracting physical constants by fitting such a two-temperature model.

Received 1 September 2015
Revised 11 May 2016

DOI:https://doi.org/10.1103/PhysRevB.93.220301

Authors & Affiliations

T. Henighan^1,2,*, M. Trigo^1,3, S. Bonetti³, P. Granitzka^3,4, D. Higley^3,5,6, Z. Chen^2,3, M. P. Jiang^1,2, R. Kukreja³, A. Gray³, A. H. Reid³, E. Jal³, M. C. Hoffmann⁶, M. Kozina^1,6, S. Song⁶, M. Chollet⁶, D. Zhu⁶, P. F. Xu^7,8, J. Jeong⁷, K. Carva^9,10, P. Maldonado¹⁰, P. M. Oppeneer¹⁰, M. G. Samant⁷, S. S. P. Parkin^7,8, D. A. Reis^1,3,5, and H. A. Dürr^3,†

¹Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
²Physics Department, Stanford University, Stanford, California 94305, USA
³Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
⁴Van der Waals-Zeeman Institute, University of Amsterdam, NL-1018 XE Amsterdam, The Netherlands
⁵Departments of Photon Science and Applied Physics, Stanford University, Stanford, California 94305, USA
⁶Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
⁷IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120, USA
⁸Max-Planck Institute for Microstructure Physics, D-06120 Halle (Saale), Germany
⁹Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Charles University in Prague, Ke Karlovu 5, CZ-12116 Prague 2, Czech Republic
¹⁰Department of Physics and Astronomy, Uppsala University, P. O. Box 516, S-75120 Uppsala, Sweden

^*henighan@slac.stanford.edu
^†hdurr@slac.stanford.edu

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Vol. 93, Iss. 22 — 1 June 2016

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