Femtosecond electron-phonon lock-in by photoemission and x-ray free-electron laser

Abstract

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.

Floquet analysis of excitations in materials

“…However, electron-phonon coupling is also known to result in observable distinct satellite, or shadow, bands in the photo-electron spectral function [86][87][88], also known as Polaron replicas [89][90][91][92][93]. The avenue to create a phonon-driven Floquet material is to selectively excite such strongly coupled phonon modes and thus to create new properties of the electronic structure or to better understand its properties, as for example realized in [94]. The crucial distinction to photon-driven Floquet states is that the systems needs to be excited first externally, but the dressing then is provided by an eigenmode of the system.…”

Role of matrix elements in the time-resolved photoemission signal

“…equation (1) can be extended in the time domain τ. Within this approximation, the temporal evolution of f (ω, k, τ) describes the intrinsic relaxation processes for fixed ω and k. In addition to f (ω, k, τ), much emphasis has also been placed on the evolution of A(ω, k, τ), which encodes information regarding the bare electronic dispersion [19,20] and many-body interactions [21]. Such dynamical analysis of A(ω, k, τ) has been applied successfully to, for example, ultrafast metal-insulator transitions [5,6,22] as well as the quenching of phase coherence in superconducting condensates [2,3].…”

Electron pairing: from metastable electron pair to bipolaron

“…However, the mechanism of electron pairing in unconventional superconductors remains one of the most challenging and unresolved problems in condensed matter physics [2][3][4]. Vast experimental evidences have shown that electron pairing and unconventional superconductivity occur in many different materials, such as cuprates [1][2][3][4], ironbased superconductors [5][6][7][8], and carbon-based superconductors [9,10], etc. Although there are many different theories for unconventional superconductivity, almost all theories follow the basic idea of the BCS theory [11].…”

“…Important progresses in recent investigations on the electron pairing mechanism in iron-based superconductors indicate small and preformed Cooper pairs [6][7][8]. For instance, using Bogoliubov quasiparticle interference imaging, Sprau et al [6] found that the superconducting energy gap in FeSe is extremely anisotropic and nodeless.…”

“…Their investigation discovered the existence of orbital-selective Cooper pairing in FeSe. Gerber et al [8] combined two time-domain experiments into a coherent lock-in measurement in the terahertz regime and was able to quantify the electron-phonon coupling strength in FeSe. Their study revealed a strong enhancement of the electron-phonon coupling strength in FeSe owing to electron correlations and highlighted the importance of the cooperative interplay between electron-electron and electron-phonon interactions.…”