Measurements of nonequilibrium interatomic forces using time-domain x-ray scattering

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.

Automated summary

In this study, an experimental approach was used to determine the excited-state interatomic forces in bismuth using femtosecond x-ray pulses. It was found that photoexcited carriers weaken nearest-neighbor forces, leading to a softening of transverse acoustic modes throughout the Brillouin zone. This demonstrates a bond-selective method for measuring electron-phonon coupling in a broad range of quantum materials undergoing photoinduced phase transitions and transient light-driven states.