This crystal lattice consists of a large number of unit cells with an identical atomic arrangement. In the elementary adiabatic picture, the motions of electrons in the crystal follow the motion of the atomic nuclei instantly, i.e., atomic nuclei and electrons are moving as a single entity. While this physical picture is valid for the inner, so-called core electrons of an atom it fails for the valence electrons, which are shared by different atoms within its unit cell. A special type of phonons, the soft modes, can relocate electrons and, thus, change the electric properties of a crystal considerably. The properties of soft modes have been investigated for decades but are not sufficiently understood. A key prerequisite for a better understanding is to map atomic vibrations and charge motions simultaneously. This can be done by femtosecond X-ray diffraction.
