FeSe is one of the simplest Fe-based unconventional superconductors yet the full impact of correlation effects remains subject of debate. Early DFT studies found negligible electron-phonon coupling and focused on spin fluctuations as mediator of pairing. However, more advanced DFT+DMFT calculations predicted that electronic correlations boost the electron-phonon coupling yet direct experimental evidence was lacking. Here, we combine time-resolved x-ray diffraction and time- and angle-resolved photoemission to study the coherent response of a A1g mode in FeSe. X-ray diffraction tracks the light-induced coherent lattice motion and photoemission monitors the coherent changes in the electronic band structure.
Focusing on the coherent lattice vibrations is analogous to electronic lock-in measurements, where a weak electronic signal is extracted by locking-in to a reference signal at the same frequency. Similarly, the coherent phonon mode provides an internal reference for measuring the electron-phonon coupling. ‘Locking-in’ on the phonon frequency avoids low-frequency contributions from other dynamical processes such as acoustic mode coupling or heat transport, which inevitably accompany optical excitation.
Without any a-priori assumptions we determine the deformation potential solely from experiment and find that correlation effects in FeSe enhance the electron-phonon coupling by an order of magnitude compared to DFT. Our results are in excellent agreement with DFT+DMFT calculations that explicitly account for electron-electron correlations and their impact on electron-phonon coupling. In the future, this combined time-domain approach may provide unambiguous results for more controversial materials like Cu- and Fe-based unconventional superconductors.
Patrick Kirchmann, "Coherent phonon lock-in using diffraction and time-resolved photo-emission (Conference Presentation)," Proc. SPIE 10530, Ultrafast Phenomena and Nanophotonics XXII, 105300O (Presented at SPIE OPTO: January 30, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2292415.5751504653001.
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