Electrons Film Phonon Dynamics in Full

Atoms in solids are always in motion, vibrating about some average position. At a given temperature, the manner in which they jiggle depends on the available vibrational states—quantized modes called phonons [1]—and on how such states are populated. This simple picture does a good job of describing equilibrium states, but it leaves out an enormous amount of detail about how vibrational processes unfold in real time. On timescales as short as femtoseconds (fs), electrons can transfer energy to vibrations, and energy gets shuffled between phonons that are coupled to each other. These ultrafast processes play critical roles in many complex phenomena, including phase transitions, superconductivity, and other forms of emergent behavior. To date, unfortunately, our understanding of phonon dynamics relies mostly on theory, because of the difficulty of probing collective vibrational phenomena [2]. An experimental method capable of providing a full picture of phonon dynamics would be immensely useful. Now, a team led by Bradley Siwick at McGill University in Canada has demonstrated that extremely short packets of electrons can measure the dynamics of all the phonon modes of a material. After photoexciting electrons in a graphite sample, their experiments track how the energy from this excitation is transferred from electrons to phonons and then redistributed among specific phonon modes [3].