Speaker
Description
The nature of the couplings within and between lattice and charge degrees of freedom is central our understanding of material properties. These interactions are essential to phenomena as diverse as thermoelectricity, superconductivity, charge density waves, and carrier and phonon transport. Despite their fundamental role in a broad range of processes, detailed momentum-dependent information on the strength of electron-phonon coupling (EPC) and phonon-phonon coupling (PPC) across the entire Brillouin zone has proved to be very difficult to obtain.
This talk will describe an emerging pump-probe technique, ultrafast electron diffuse scattering (UEDS), that directly provides such information from the perspective of the phonon system by measuring the time-dependence of phonon-diffuse scattering from single-crystal samples [1,2]. Recent examples and proposals for the application of UEDS to a range of phonon-related phenomena in layered and monolayer materials will be discussed. Specifically, the direct connection that can be made between UEDS measurements and ab-initio computations for inelastic carrier scattering [3], phonon transport [4] and polaron formation [5] processes will be emphasized. We will show that UEDS can reveal signatures of chiral electron-phonon coupling [3] and phonon hydrodynamic transport, including second sound oscillations [4]. Such signatures are provided by the time, momentum, and branch resolved information on the nonequilibrium state-of-excitation of the phonon system that UEDS provides. We will also show that phonon diffuse scattering signatures of polarons in materials are directly emblematic of the underlying polaron wavefunction [5].
The combination of new time and momentum resolved experimental probes of nonequilibrium phonons with novel computational methods promises to complement the qualitative results obtained via model Hamiltonians with a first principles, material-specific quantitative understanding of polarons and their properties.
[1] M. J. Stern et al, Phys. Rev. B 97 (2018) 165416.; Waldecker, L. et al. Phys. Rev. Lett. 119, 036803 (2017). Chase, T. et al Appl. Phys. Lett. 108, 041909 (2016).
[2] T. Britt et al, NanoLett , 22 (2022) 4718.
[3] T. L Britt and B. J. Siwick, Phys. Rev. B 107 (2023) 214306.
[4] L. Kremeyer, J. Haibeh, B. J. Siwick and S. C. Huberman, Structural Dynamics, 11 (2024) 024101.
[5] T. L Britt, F. Caruso and B. J. Siwick, Computational Materials, 10 (2024) 178
