When an electron moves through a polar crystalline solid, it attracts the positively charged atomic cores around it, denting the lattice as it goes. Electron and distortion then travel together as a single composite object, the polaron, a quasiparticle described theoretically as early as 1933. Polarons shape charge transport in many semiconductors and energy materials, yet the fleeting moment of their formation had never been directly observed.
A team led by Jochen Feldmann at LMU's Faculty of Physics has now captured that moment. Using an ultrafast imaging method, the researchers tracked the formation process as it unfolded, watching the electron couple to the deforming lattice around it. The measurements allowed the team to determine both how quickly the polaron forms and how far its lattice distortion extends through the crystal.
The observations match the long-standing theory: as the polaron assembles, the electron loses energy and gains effective mass, exactly as predicted nine decades ago. Beyond settling a venerable question in solid-state physics, direct access to polaron formation gives materials scientists a sharper handle on the processes that govern how charges move through the perovskites and other polar semiconductors now central to solar cells and light-emitting technologies.