Ultrafast structure dynamics probed by surface electron diffraction

Surfaces and interfaces break the translational symmetry of a crystal lattice, giving rise to electronic and structural states that can differ dramatically from those of the bulk. These surface-specific states, often originating from unsaturated bonds, reconstructions, or reduced dimensionality, govern key aspects of chemical reactivity and device functionality. Low-dimensional and, in particular, 2D materials represent the most extreme case, where strong interactions and correlations can stabilize superconductivity or charge- and spin-density-wave phases. A microscopic understanding of the elementary processes that underlie such phenomena requires experimental access to their intrinsic dynamics on relevant time and length scales. To this end, an array of advanced techniques has been developed, providing direct insight into ultrafast dynamics in both the electronic and structural subsystems. Among these, time-resolved electron diffraction is uniquely suited to capture laser-induced atomic displacements, with dedicated implementations optimized for surface sensitivity. This review highlights recent advances in ultrafast surface-sensitive electron diffraction, tracing the methodological developments and showcasing applications to selected material systems. Particular attention is given to prototypical surface phenomena including charge-density-wave phase transitions, nanoscale heat transfer, and non-thermal phonon dynamics.