14–16 Sept 2020
Europe/Berlin timezone

Direct observation of a transient fluctuation state mediating a picosecond topological phase transition

15 Sept 2020, 15:05
30m
https://desy.zoom.us/j/94012379937

https://desy.zoom.us/j/94012379937

Speaker

Dr Felix Büttner (Helmholtz-Zentrum Berlin)

Description

Topological phases of matter are highly non-trivial configurations of the electronic or spin system, which often result in exotic and previously unimaginable material properties. The topology endows these states with a remarkable stability that often allows them to be realized at room temperature and with lifetimes exceeding decades. A key challenge, however, is the creation of topological phases since long lifetimes usually imply that switching events are rare. Surprisingly, we find that picosecond nucleation of an extended topological phase, comprising a dense array of nanometer-scale magnetic skyrmions, can be induced by a single femtosecond laser pulse. In this talk, I will discuss the nucleation dynamics of this topological phase transition, which we were able to follow in real time during the early user operation of beamline SCS at the European XFEL [1]. Using time-resolved small angle x-ray scattering, we discovered that rapid, homogeneous nucleation of the skyrmion phase is mediated by a previously undisclosed transient fluctuation state. This state, which is characterized by high spatial frequency magnetic fluctuations, persists for approximately 100 ps after exciting our magnetic multilayer with a femtosecond, infrared laser pulse. The topological phase emerges from these fluctuations by nucleation and coalescence, a mechanism that goes well beyond existing theories of topological phase transitions such as the Kibble–Zurek mechanism and the Berezinskii–Kosterlitz–Thouless transition. The process is completed on a time scale of 300 ps. Using atomistic spin dynamics simulations, we confirm that the fluctuation state is key to the ultrafast increase of the global topological charge, enabled by an almost complete elimination of the topological energy barrier in this transient state of matter. Figure 1: Time-trace of the measured magnetization of skyrmions (total intensity) as well as their distance (correlation length) and size (diameter), at a time delay relative to the incident infrared pulse. Inset: Representative real space image of skyrmion state long after the excitation. Scale bar 200 nm. [1] F. Büttner, B. Pfau, M. Böttcher, M. Schneider, G. Mercurio, C. M. Günther, P. Hessing, C. Klose, A. Wittmann, K. Gerlinger, L.-M. Kern, C. Strüber., C. von Korff Schmising, J. Fuchs , D. Engel, A. Churikova, S. Huang, D. Suzuki, I. Lemesh, M. Huang, L. Caretta, D. Weder, J. H. Gaida, M. Möller, T. R. Harvey, S. Zayko, K. Bagschik, R. Carley, L. Mercadier, J. Schlappa, A. Yaroslavtsev, L. Le Guyarder, N. Gerasimova , A. Scherz, C. Deiter, R. Gort, D. Hickin, J. Zhu, M. Turcato, D. Lomidze, F. Erdinger, A. Castoldi, S. Maffessanti, M. Porro, A. Samartsev, J. Sinova, C. Ropers, J. H. Mentink, B. Dupé, G. S. D. Beach, and S. Eisebitt (unpublished).

Primary author

Dr Felix Büttner (Helmholtz-Zentrum Berlin)

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