Imaging nonequilibrium atomic vibrations with x-ray diffuse scattering

TL;DR

This paper demonstrates the first imaging of nonequilibrium phonons across the entire Brillouin zone in photoexcited semiconductors using time-resolved x-ray diffuse scattering, revealing sustained lattice non-equilibrium states.

Contribution

It introduces a novel method for visualizing nonequilibrium phonon populations in the Brillouin zone with picosecond resolution, advancing the study of lattice dynamics.

Findings

Lattice remains out of equilibrium for hundreds of picoseconds to nanoseconds after excitation.

Non-equilibrium phonon population is dominated by transverse acoustic phonons.

In InP, these phonons are aligned along high-symmetry directions.

Abstract

For over a century, x-ray scattering has been the most powerful tool for determining the equilibrium structure of crystalline materials. Deviations from perfect periodicity, for example due to thermal motion of the atoms, reduces the intensity of the Bragg peaks as well as produces structure in the diffuse scattering background. Analysis of the thermal diffuse scattering (TDS) had been used to determine interatomic force constants and phonon dispersion in relatively simple cases before inelastic neutron scattering became the preferred technique to study lattice dynamics. With the advent of intense synchrotron x-ray sources, there was a renewed interest in TDS for measuring phonon dispersion. The relatively short x-ray pulses emanating from these sources also enables the measurement of phonon dynamics in the time domain. Prior experiments on nonequilibrium phonons were either limited by time-resolution and/or to relatively long wavelength excitations. Here we present the first images of nonequilibrium phonons throughout the Brillouin zone in photoexcited III-V semiconductors, indium-phosphide and indium-antimonide, using picosecond time-resolved diffuse scattering. In each case, we find that the lattice remain out of equilibrium for several hundred picoseconds up to nanoseconds after laser excitation. The non-equilibrium population is dominated by transverse acoustic phonons which in InP are directed along high-symmetry directions. The results have wide implications for the detailed study of electron-phonon and phonon-phonon coupling in solids.