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Femtosecond electron-phonon lock-in by photoemission and x-ray free-electron laser
Gerber, S.,Yang, S.-L.,Zhu, D.,Soifer, H.,Sobota, J. A.,Rebec, S.,Lee, J. J.,Jia, T.,Moritz, B.,Jia, C.,Gauthier, A.,Li, Y.,Leuenberger, D.,Zhang, Y.,Chaix, L.,Li, W.,Jang, H.,Lee, J.-S.,Yi, M.,Dakovs American Association for the Advancement of Scienc 2017 Science Vol.357 No.6346
<P><B>A deeper look into iron selenide</B></P><P>In the past 10 years, iron-based superconductors have created more puzzles than they have helped resolve. Some of the most fundamental outstanding questions are how strong the interactions are and what the electron pairing mechanism is. Now two groups have made contributions toward resolving these questions in the intriguing compound iron selenide (FeSe) (see the Perspective by Lee). Gerber <I>et al.</I> used photoemission spectroscopy coupled with x-ray diffraction to find that FeSe has a very sizable electron-phonon interaction. Quasiparticle interference imaging helped Sprau <I>et al.</I> determine the shape of the superconducting gap and find that the electron pairing in FeSe is orbital-selective.</P><P><I>Science</I>, this issue p. 71, p. 75; see also p. 32</P><P>The interactions that lead to the emergence of superconductivity in iron-based materials remain a subject of debate. It has been suggested that electron-electron correlations enhance electron-phonon coupling in iron selenide (FeSe) and related pnictides, but direct experimental verification has been lacking. Here we show that the electron-phonon coupling strength in FeSe can be quantified by combining two time-domain experiments into a “coherent lock-in” measurement in the terahertz regime. X-ray diffraction tracks the light-induced femtosecond coherent lattice motion at a single phonon frequency, and photoemission monitors the subsequent coherent changes in the electronic band structure. Comparison with theory reveals a strong enhancement of the coupling strength in FeSe owing to correlation effects. Given that the electron-phonon coupling affects superconductivity exponentially, this enhancement highlights the importance of the cooperative interplay between electron-electron and electron-phonon interactions.</P>