Measurements of nonequilibrium interatomic forces using time-domain x-ray scattering
Samuel W. Teitelbaum, Thomas Henighan, Hanzhe Liu, Mason Jiang, Diling Zhu, Matthieu Chollet, Takahiro Sato, Éamonn Murray, Stephen Fahy, Shane O’Mahony, Trevor P. Bailey, Ctirad Uher, Mariano Trigo, David A. Reis
Abstract
We demonstrate an experimental approach to determining the excited-state interatomic forces using femtosecond x-ray pulses from an x-ray free-electron laser. We determine experimentally the excited-state interatomic forces that connect photoexcited carriers to the nonequilibrium lattice dynamics in the prototypical Peierls-distorted material, bismuth. The forces are obtained by a constrained least-squares fit of a pairwise interatomic force model to the excited-state phonon dispersion relation as measured by the time- and momentum-resolved x-ray diffuse scattering. We find that photoexcited carriers weaken predominantly the nearest-neighbor forces, which drives the measured softening of the transverse acoustic modes throughout the Brillouin zone as well as the zone-center ${A}_{1g}$ optical mode. This demonstrates a bond-selective approach to measuring electron-phonon coupling relevant to a broad range of photoinduced phase transitions and transient light-driven states in quantum materials.