The coupling of phonons to electrons, excitons and other phonons plays a defining role in material properties, including charge and energy transport, light emission, and superconductivity. In atomic solids such as Si or GaAs, phonons are delocalized over the three-dimensional (3D) lattice and are determined by bonding and crystal symmetry. In molecular materials, by contrast, localized molecular vibrations couple to electrons to produce, for example, high temperature superconductivity, as in A3C60. Here we describe a hierarchical semiconductor that expands the phonon space by combining localized 0D phonon modes with delocalized phonon manifolds. The structure of this material consists of superatomic building blocks (Re6Se8) covalently linked into 2D sheets that are stacked into a layered van der Waals lattice.
Using transient reflectance spectroscopy and theoretical calculations, we identify three different types of coherent phonons produced by optical excitation: localized 0D breathing modes of isolated superatom, 2D synchronized twisting of superatoms in layers, and 3D acoustic interlayer deformation. We show that these phonons are coupled to the electronic degrees of freedom to varying extents. The presence of local phonon modes in an extended crystal opens the door to controlling material properties from hierarchical phonon engineering.
