Signatures of Mode-Resolved, Nonlocal Electron-Phonon Coupling in Two-Dimensional Spectroscopy

Abstract: Electron-phonon coupling (EPC) is foundational in condensed matter physics, determining intriguing phenomena and properties in both conventional and quantum materials. In this manuscript, we propose and demonstrate a novel two dimensional (2D) EPC spectroscopy which allows for direct extraction of EPC matrix elements for specific phonon modes and different electron energies, simultaneously. Using this technique, we are able to measure the electron-energy dependence of the EPC strength for individual phonon modes. This capability allows us to identify unique signatures distinguishing nonlocal Su-SchriefferHeeger (SSH)-type couplings from local Holstein-type couplings. In application to a methylammonium lead iodide (MAPI) perovskite, we find that two pronounced phonon modes at room temperature exhibit highly distinctive EPC behaviors, concerning strength, anisotropy, and temperature-dependence across the structural phase transition. Our approach paves the way for unraveling the microscopic origin of EPC, the change of the phonon-mode-specific EPC with external conditions, and phonon-mediated ultrafast control of condensed materials.