Phonons are ubiquitous in condensed matter, but detecting them can be difficult—especially with the momentum and time resolution necessary to follow their dynamics on the fundamental time and length scales of electron-phonon and phonon-phonon interactions. In a new paper published this week in Physical Review B, physicists from the Stanford PULSE Institute report on first images of nonequilibrium phonon distributions using time-resolved x-ray diffuse scattering. Here they were able to capture the transient redistribution of energy from photoexcited electrons into a nonequilibrium phonon population and eventually into heat. Surprisingly, they found that the semiconductor crystals they used remain out of equilibrium much longer than anticipated (several hundred picoseconds to a few nanoseconds), with a delayed emission of high wavevector (short wavelength) acoustic modes.
The experiments were carried out at the Advanced Photon Source synchrotron in Chicago, in collaboration with scientists from the University of Michigan and The Center for Advanced Radiation Sources, University of Chicago.