Ultramicroscopy 147, 1–7 (2014)

Credit: © ELSEVIER

The energy absorbed or emitted by a phonon excitation is conventionally probed by spectroscopic techniques, such as infrared and Raman, or, more recently, by high-resolution electron energy-loss spectroscopy. The possibility of mapping phonon excitations in a lattice directly in real space has recently been theoretically demonstrated by Christian Dwyer (Phys. Rev. B 89, 054103; 2014), and now demonstrated experimentally by Ricardo Egoavil and co-workers at the University of Antwerp. By acquiring high-resolution spectrum images of a strontium titanate crystal and comparing the differences in electron energy-loss and energy-gain spectral signals around the elastically scattered peak of electrons transmitted by strontium, titanium and oxygen atoms, direct maps of multi-phonon excitations with atomic resolution were obtained. Resolving multi-phonon excitations with such high spatial resolution demonstrates that (differently to what happens for electronic transitions at low energy losses) due to the difference in mass between the fast beam-electrons and the lattice atoms, there is strong localization of the scattering of phonon excitations, allowing for atomic-resolution imaging of phonons.